National Ambient Air Quality Standards for Particulate Matter

ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 50

[AD-FRL-5725-2]

RIN 2060-AE66

National Ambient Air Quality Standards for Particulate Matter

AGENCY: Environmental Protection Agency (EPA).

ACTION: Final rule.

SUMMARY: This document describes EPA's decision to revise the national

ambient air quality standards (NAAQS) for particulate matter (PM) based

on its review of the available scientific evidence linking exposures to

ambient PM to adverse health and welfare effects at levels allowed by

the current PM standards. The current primary PM standards are revised

in several respects: Two new PM2.5 standards are added, set

at 15 g/m3 , based on the 3-year average of annual

arithmetic mean PM2.5 concentrations from single or multiple

community-oriented monitors, and 65 g/m 3 , based

on the 3-year average of the 98th percentile of 24-hour

PM2.5 concentrations at each population-oriented monitor

within an area; and the current 24-hour PM10 standard is

revised to be based on the 99th percentile of 24-hour

PM10 concentrations at each monitor within an area. The new

suite of primary standards will provide increased protection against a

wide range of PM-related health effects, including premature mortality

and increased hospital admissions and emergency room visits, primarily

in the elderly and individuals with cardiopulmonary disease; increased

respiratory symptoms and disease, in children and individuals with

cardiopulmonary disease such as asthma; decreased lung function,

particularly in children and individuals with asthma; and alterations

in lung tissue and structure and in respiratory tract defense

mechanisms. The current secondary standards are revised by making them

identical in all respects to the new suite of primary standards. The

new secondary standards, in conjunction with a regional haze program,

will provide appropriate protection against PM-related public welfare

effects including soiling, material damage, and visibility impairment.

In conjunction with the new PM2.5 standards, a new reference

method has been specified for monitoring PM as PM2.5 .

EFFECTIVE DATE: This action is effective September 16, 1997.

ADDRESSES: A docket containing information relating to the EPA's review

of the PM primary and secondary standards (Docket No. A-95-54) is

available for public inspection in the Central Docket Section of the

U.S. Environmental Protection Agency, South Conference Center, Rm. 4,

401 M St., SW., Washington, DC. This docket incorporates the docket

established for the air quality Criteria Document (Docket No. ECAO-CD-

92-0671). The docket may be inspected between 8 a.m. and 3 p.m., Monday

through Friday, except legal holidays, and a reasonable fee may be

charged for copying. The information in the docket constitutes the

complete basis for the decision announced in this document. For the

availability of related information, see ``SUPPLEMENTARY INFORMATION.''

FOR FURTHER INFORMATION CONTACT: John H. Haines, MD-15, Air Quality

Strategies and Standards Division, Office of Air Quality Planning and

Standards, U.S. Environmental Protection Agency, Research Triangle

Park, NC 27711; telephone: (919) 541-5533; e-mail:

[email protected].

SUPPLEMENTARY INFORMATION:

Related Final Rules on PM Monitoring

In a separate document published elsewhere in this issue of the

Federal Register, EPA is amending its ambient air quality surveillance

requirements (40 CFR part 58) and its ambient air monitoring reference

and equivalent methods (40 CFR part 53) for PM.

Availability of Related Information

Certain documents are available from the U.S. Department of

Commerce, National Technical Information Service, 5285 Port Royal Road,

Springfield, VA 22161. Available documents include:

(1) Air Quality Criteria for Particulate Matter (Criteria Document)

(three volumes, EPA/600/P-95-001aF thru EPA/600/P-95-001cF, April 1996,

NTIS #PB-96-168224, $234.00 paper copy).

(2) Review of the National Ambient Air Quality Standards for

Particulate Matter: Policy Assessment of Scientific and Technical

Information (Staff Paper) (EPA-452/R-96-013, July 1996, NTIS #PB-97-

115406, $47.00 paper copy and $19.50 microfiche). (Add a $3.00 handling

charge per order.)

A limited number of copies of other documents generated in

connection with this standard review, such as technical support

documents pertaining to air quality, monitoring, and health risk

assessment, can be obtained from: Environmental Protection Agency

Library (MD-35), Research Triangle Park, NC 27711, telephone (919) 541-

2777. These and other related documents are also available for

inspection and copying in the EPA docket at the address under

``ADDRESSES,'' at the beginning of this document.

Electronic Availability

The Staff Paper and human health risk assessment support documents

are available on the Agency's Office of Air Quality Planning and

Standards' (OAQPS) Technology Transfer Network (TTN) Bulletin Board

System (BBS) in the Clean Air Act Amendments area, under Title I,

Policy/Guidance Documents. To access the bulletin board, a modem and

communications software are necessary. To dial up, set your

communications software to 8 data bits, no parity and one stop bit.

Dial (919) 541-5742 and follow the on-screen instructions to register

for access. After registering, proceed to choice `` Gateway to TTN

Technical Areas'', then choose `` CAAA BBS''. From the main menu,

choose `` Title I: Attain/Maint of NAAQS'', then `` Policy

Guidance Documents.'' To access these documents through the World Wide

Web, click on ``TTN BBSWeb'', then proceed to the Gateway to TTN

Technical areas, as above. If assistance is needed in accessing the

system, call the help desk at (919) 541-5384 in Research Triangle Park,

NC.

Implementation Strategy For Revised Air Quality Standards

On Wednesday, July 16, 1997, President Clinton signed a memorandum

to the Administrator specifying his goals for the implementation of the

O3 and PM standards. Attached to the President's memorandum

is a strategy prepared by an interagency Administration group outlining

the next steps that would be necessary for implementing these

standards. The EPA will prepare guidance and proposed rules consistent

with the President's memorandum. Copies of the Presidential document

are available in paper copy by contacting the U.S. Environmental

Protection Agency Library at the address under ``Availability of

Related Information'' and in electronic form as discussed above in

``Electronic Availability.''

The following topics are discussed in this preamble:

I. Background

A. Legislative Requirements

B. Related Control Requirements

C. Review of Air Quality Criteria and Standards for PM

D. Summary of Proposed Revisions to the PM Standards

II. Rationale for the Primary PM Standards

A. Introduction

B. Need for Revision of the Current Primary PM Standards

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C. Indicators of PM

D. Averaging Time of PM2.5 Standards

E. Form of PM2.5 Standards

F. Levels for the Annual and 24-Hour PM2.5 Standards

G. Conclusions Regarding the Current PM10 Standards

H. Final Decisions on Primary PM Standards

III. Rationale for the Secondary Standards

A. Need for Revision of the Current SecondaryStandards

B. Decision on the Secondary Standards

IV. Other Issues

A. Consideration of Costs

B. Margin of Safety

C. Data Availability

D. 1990 Amendments

V. Revisions to 40 CFR Part 50, Appendix K--Interpretation of the PM

NAAQS

A. PM2.5 Computations and Data Handling Conventions

B. PM10 Computations and Data Handling Conventions

C. Changes that Apply to Both PM2.5 and

PM10 Computations

VI. Reference Methods for the Determination of Particulate Matter as

PM10 and PM2.5 in the Atmosphere

A. Revisions to 40 CFR Part 50, Appendix J--Reference Method for

PM10

B. 40 CFR Part 50, Appendix L--New Reference Method for

PM2.5

VII. Effective Date of the Revised PM Standards and Applicability of

the Existing PM10 Standards

VIII. Regulatory and Environmental Impact Analyses

A. Executive Order 12866

B. Regulatory Flexibility Analysis

C. Impact on Reporting Requirements

D. Unfunded Mandates Reform Act

E. Environmental Justice

F. Submission to Congress and the Comptroller General

IX. Response to Petition for Administrator Browner's Recusal

X. References

I. Background

A. Legislative Requirements

Two sections of the Clean Air Act (Act) govern the establishment,

review, and revision of NAAQS. Section 108 of the Act (42 U.S.C. 7408)

directs the Administrator to identify certain pollutants which ``may

reasonably be anticipated to endanger public health and welfare'' and

to issue air quality criteria for them. These air quality criteria are

to ``accurately reflect the latest scientific knowledge useful in

indicating the kind and extent of all identifiable effects on public

health or welfare which may be expected from the presence of [a]

pollutant in the ambient air * * *.''

Section 109 of the Act (42 U.S.C. 7409) directs the Administrator

to propose and promulgate ``primary'' and ``secondary'' NAAQS for

pollutants identified under section 108 of the Act. Section 109(b)(1)

of the Act defines a primary standard as one ``the attainment and

maintenance of which in the judgment of the Administrator, based on

[the] criteria and allowing an adequate margin of safety, are requisite

to protect the public health.'' The margin of safety requirement was

intended to address uncertainties associated with inconclusive

scientific and technical information available at the time of standard

setting, as well as to provide a reasonable degree of protection

against hazards that research has not yet identified. Both kinds of

uncertainties are components of the risk associated with pollution at

levels below those at which human health effects can be said to occur

with reasonable scientific certainty. Thus, by selecting primary

standards that provide an adequate margin of safety, the Administrator

is seeking not only to prevent pollution levels that have been

demonstrated to be harmful but also to prevent lower pollutant levels

that she finds may pose an unacceptable risk of harm, even if the risk

is not precisely identified as to nature or degree. The Act does not

require the Administrator to establish a primary NAAQS at a zero-risk

level, but rather at a level that reduces risk sufficiently so as to

protect public health with an adequate margin of safety. The selection

of any particular approach to providing an adequate margin of safety is

a policy choice left specifically to the Administrator's judgment. Lead

Industries Ass'n v. EPA, 647 F.2d 1130, 1161-1162 (D.C. Cir.1980).

A secondary standard, as defined in section 109 (b)(2) of the Act,

must ``specify a level of air quality the attainment and maintenance of

which in the judgment of the Administrator, based on [the] criteria,

[are] requisite to protect the public welfare from any known or

anticipated adverse effects associated with the presence of [the]

pollutant in the ambient air.'' Welfare effects as defined in section

302(h) of the Act (42 U.S.C. 7602(h)) include, but are not limited to,

``effects on soils, water, crops, vegetation, manmade materials,

animals, wildlife, weather, visibility, and climate, damage to and

deterioration of property, and hazards to transportation, as well as

effects on economic values and on personal comfort and well-being.''

Section 109(d)(1) of the Act requires periodic review and, if

appropriate, revision of existing air quality criteria and NAAQS.

Section 109(d)(2) of the Act requires appointment of an independent

scientific review committee to review criteria and standards and

recommend new standards or revisions of existing criteria and

standards, as appropriate. The committee established under section

109(d)(2) of the Act is known as the Clean Air Scientific Advisory

Committee (CASAC), a standing committee of EPA's Science Advisory

Board.

B. Related Control Requirements

States are primarily responsible for ensuring attainment and

maintenance of ambient air quality standards once EPA has established

them. Under section 110 of the Act (42 U.S.C. 7410) and related

provisions, States are to submit, for EPA approval, State

implementation plans (SIP's) that provide for the attainment and

maintenance of such standards through control programs directed to

sources of the pollutants involved. The States, in conjunction with

EPA, also administer the prevention of significant deterioration

program (42 U.S.C. 7470-7479) for these pollutants. In addition,

Federal programs provide for nationwide reductions in emissions of

these and other air pollutants through the Federal Motor Vehicle

Control Program under Title II of the Act (42 U.S.C. 7521-7574), which

involves controls for automobile, truck, bus, motorcycle, nonroad

engine, and aircraft emissions; the new source performance standards

under section 111 of the Act (42 U.S.C. 7411); and the national

emission standards for hazardous air pollutants under section 112 of

the Act (42 U.S.C. 7412).

C. Review of Air Quality Criteria and Standards for PM

Particulate matter is the generic term for a broad class of

chemically and physically diverse substances that exist as discrete

particles (liquid droplets or solids) over a wide range of sizes.

Particles originate from a variety of anthropogenic stationary and

mobile sources as well as from natural sources. Particles may be

emitted directly or formed in the atmosphere by transformations of

gaseous emissions such as sulfur oxides (SOx), nitrogen

oxides (NOx), and volatile organic compounds (VOC). The

chemical and physical properties of PM vary greatly with time, region,

meteorology, and source category, thus complicating the assessment of

health and welfare effects.

The last review of PM air quality criteria and standards was

completed in July 1987 with notice of a final decision to revise the

existing standards published in the Federal Register (52 FR 24854, July

1, 1987). In that decision, EPA changed the indicator for PM from total

suspended particles (TSP) to

[[Page 38654]]

PM10.1 Identical primary and secondary

PM10 standards were set for two averaging times: 50

g/m3, expected annual arithmetic mean, averaged

over 3 years, and 150 g/m3, 24-hour average, with

no more than one expected exceedance per year.2

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1 PM10 refers to particles with an aerodynamic

diameter less than or equal to a nominal 10 micrometers. Technical

details further specifying the measurement of PM10 are

contained in 40 CFR part 50, Appendices J and M.

2 A more complete history of the PM NAAQS is presented in

section II.B of the OAQPS Staff Paper, Review of National Ambient

Air Quality Standards for Particulate Matter: Assessment of

Scientific and Technical Information (U.S. EPA, 1996b).

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The EPA initiated this current review of the air quality criteria

and standards for PM in April 1994 by announcing its intention to

develop a revised Air Quality Criteria Document for Particulate Matter

(henceforth, the ``Criteria Document''). Thereafter, the EPA presented

its plans for review of the criteria and standards for PM under a

highly accelerated, court-ordered schedule3 at a public

meeting of the CASAC in December 1994. Several workshops were held by

EPA's National Center for Environmental Assessment (NCEA) to discuss

important new health effects information in November 1994 and January

1995. External review drafts of the Criteria Document were made

available for public comment and were reviewed by CASAC at public

meetings held in August and December 1995 and February 1996. The CASAC

came to closure in its review of the Criteria Document, advising the

Administrator in a March 15, 1996 closure letter (Wolff, 1996a) that

``although our understanding of the health effects of PM is far from

complete, a revised Criteria Document which incorporates the Panel's

latest comments will provide an adequate review of the available

scientific data and relevant studies of PM.'' CASAC and public comments

from these meetings, and from subsequent written comments and the

closure letter, were incorporated as appropriate in the final Criteria

Document (U.S. EPA, 1996a).

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3 A court order entered in American Lung Association v. Browner,

CIV-93-643-TUC-ACM (D. Ariz.,October 6, 1994), as subsequently

modified, requires publication of EPA's final decision on the review

of the PM NAAQS by July 19, 1997.

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External review drafts of a Staff Paper prepared by the Office of

Air Quality Planning and Standards (OAQPS), Review of the National

Ambient Air Quality Standards for Particulate Matter: Assessment of

Scientific and Technical Information (henceforth, the ``Staff Paper''),

were made available for public comment and were reviewed by CASAC at

public meetings in December 1995 and May 1996.4 The CASAC

came to closure in its review of the Staff Paper, advising the

Administrator in a June 13, 1996 closure letter (Wolff, 1996b) that

``the Staff Paper, when revised, will provide an adequate summary of

our present understanding of the scientific basis for making regulatory

decisions concerning PM standards.'' CASAC and public comments from

these meetings, subsequent written comments, and the CASAC closure

letter were incorporated as appropriate in the final Staff Paper (U.S.

EPA, 1996b).

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4 The Staff Paper evaluates policy implications of the key

studies and scientific information in the Criteria Document,

identifies critical elements that EPA staff believes should be

considered, and presents staff conclusions and recommendations of

suggested options for the Administrator's consideration.

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On November 27, 1996, EPA announced its proposed decision to revise

the NAAQS for PM (61 FR 65638, December 13, 1996) (hereafter

``proposal'') as well as its proposed decision to revise the NAAQS for

ozone (O3)(61 FR 65716, December 13, 1996). In the proposal,

EPA identified proposed revisions, based on the air quality criteria

for PM, and solicited public comments on alternative primary standards

and on the proposed forms of the standards.

To ensure the broadest possible public input on the PM and

O3 proposals, EPA took extensive and unprecedented steps to

facilitate the public comment process beyond the normal process of

providing an opportunity to request a hearing and receiving written

comments submitted to the rulemaking docket. The EPA established a

national toll-free telephone hotline to facilitate public comments on

the proposed revisions to the PM and O3 NAAQS, and on

related notices dealing with the implementation of revised PM and

O3 standards, as well as a system for the public to submit

comments on the proposals electronically via the Internet. Over 14,000

calls and over 4,000 electronic mail messages were received through

these channels. The public could also access key supporting documents

(including the Criteria Document, Staff Paper, related technical

documents and fact sheets) via the Internet.

The EPA also held several public hearings and meetings across the

country to provide direct opportunities for public comment on the

proposed revisions to the PM and O3 NAAQS and to disseminate

information to the public about the proposed standard revisions. On

January 14 and 15, 1997, EPA held concurrent, 2-day public hearings in

Boston, MA, Chicago, IL, and Salt Lake City, UT. A fourth public

hearing, which focused primarily on PM monitoring issues, was held in

Durham, NC on January 14, 1997. Over 400 citizens and organizations

testified during these public hearings. EPA also held two national

satellite telecasts to answer questions on the standards and

participated in meetings sponsored by the Air and Waste Management

Association on the proposed revisions to the standards at more than 10

locations across the country. Beyond that, several EPA regional offices

held public meetings and workshops and participated in hearings that

States and cities held around the country.

As a result of this intensive effort to solicit public input, over

50,000 written and oral comments were received on the proposed

revisions to the PM NAAQS by the close of the public comment period on

March 12, 1997. Major issues raised in the comments are discussed

throughout the preamble of this final decision. A comprehensive summary

of all significant comments, along with EPA's response to such comments

(hereafter ``Response to Comments''), can be found in the docket for

this rulemaking (Docket No. A-95-54).

The principal focus of this current review of the air quality

criteria and standards for PM is on recent epidemiological evidence

reporting associations between ambient concentrations of PM and a range

of serious health effects. Particular attention has been given to

several size-specific classes of particles, including PM10

and the principal fractions of PM10, referred to as the fine

(PM2.5)5 and coarse

(PM10-2.5)6 fractions. As discussed in the

Criteria Document, fine and coarse fraction particles can be

differentiated by their sources and formation processes and their

chemical and physical properties, including behavior in the atmosphere.

Detailed discussions of atmospheric formation, ambient concentrations,

and health and welfare effects of PM, as well as quantitative estimates

of human health risks associated with exposure to PM, can be found in

the Criteria Document and in the Staff Paper.

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5 PM2.5 refers to particles with an aerodynamic

diameter less than or equal to a nominal 2.5 micrometers, as further

specified in 40 CFR part 50, Appendix L in this document.

6 PM10-2.5 refers to those particles with an

aerodynamic diameter less than or equal to a nominal 10 micrometers

but greater than 2.5 micrometers. In other words, it refers to the

inhalable particles that remain if fine (PM2.5) particles

are removed from a sample of PM10 particles.

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[[Page 38655]]

D. Summary of Proposed Revisions to the PM Standards

For reasons discussed in the proposal, the Administrator proposed

to revise the current primary standards for PM (as indicated by

PM10), by adding two new primary PM2.5 standards

set at 15 g/m3, annual mean, and 50 g/

m3, 24-hour average. The proposed annual PM2.5

standard would be based on the 3-year average of the annual arithmetic

mean PM2.5 concentrations, spatially averaged across an

area. The proposed 24-hour PM2.5 standard would be based on

the 3-year average of the 98th percentile of 24-hour

PM2.5 concentrations at each population-oriented monitor

within an area. The proposal solicited comment on two alternative

approaches for selecting the levels of PM2.5 standards. The

Administrator also proposed to revise the current 24-hour primary

PM10 standard of 150 g/m3 by replacing

the 1-expected-exceedance form with a 98th percentile form,

averaged over 3 years at each monitor within an area, solicited comment

on an alternative proposal to revoke the 24-hour PM10

standard, and proposed to retain the current annual primary

PM10 standard of 50 g/m3. The proposal

also solicited comment on proposed revisions to 40 CFR part 50,

Appendix K to establish new data handling conventions for calculating

98th percentile values and spatial averages, revisions to 40

CFR part 50, Appendix J to modify the reference method for monitoring

PM as PM10, and a proposed new reference method for

monitoring PM as PM2.5 (40 CFR part 50, Appendix L).

With regard to the secondary standards, the Administrator proposed

to revise the current secondary standards by making them identical to

the suite of proposed primary standards, in conjunction with the

establishment of a regional haze program under section 169A of the Act.

II. Rationale for the Primary Standards

A. Introduction

1. Overview. This document presents the Administrator's final

decisions regarding the need to revise the current primary ambient air

quality standards for PM, and, more specifically, regarding the

establishment of new annual and 24-hour PM2.5 primary

standards and revisions to the form of the current 24-hour

PM10 primary NAAQS. These decisions are based on a thorough

review, in the Criteria Document, of the latest scientific information

on known and potential human health effects associated with exposure to

PM at levels typically found in the ambient air. These decisions also

take into account:

(1) Staff Paper assessments of the most policy-relevant information

in the Criteria Document, upon which staff recommendations for new and

revised primary standards are based.

(2) CASAC advice and recommendations, as reflected in discussions

of drafts of the Criteria Document and Staff Paper at public meetings,

in separate written comments, and in the CASAC's closure letters to the

Administrator.

(3) Public comments received during the development of these

documents, either in connection with CASAC meetings or separately.

(4) Extensive public comments received on the proposed decisions

regarding the primary PM standards.

After taking this information and comments into account, and for

the reasons discussed below in this unit, the Administrator concludes

that revisions to the current primary standards to provide increased

public health protection against a variety of health risks are

appropriate. More specifically, the Administrator has determined that

it is appropriate to establish new annual and 24-hour PM2.5

standards, to revise the current 24-hour PM10 standard, and

to retain the current annual PM10 standard. As discussed

more fully below in this unit, the rationale for the final decisions

regarding the PM primary NAAQS includes consideration of:

(1) Health effects information, and alternative views on the

appropriate interpretation and use of the information, as the basis for

judgments about the risks to public health presented by population

exposures to ambient PM.

(2) Insights gained from a quantitative risk assessment conducted

to provide a broader perspective for judgments about protecting public

health from the risks associated with PM exposures.

(3) Specific conclusions regarding the need for revisions to the

current standards and the elements of PM standards (i.e., indicator,

averaging time, form, and level) that, taken together, would be

appropriate to protect public health with an adequate margin of safety.

As with virtually any policy-relevant scientific research, there is

uncertainty in the characterization of health effects attributable to

exposure to ambient PM. As discussed in the proposal, however, there is

now a greatly expanded body of health effects information as compared

with that available during the last review of the PM standards.

Moreover, the recent evidence on PM-related health effects has

undergone an unusually high degree of scrutiny and reanalysis over the

past several years, beginning with a series of workshops held early in

the review process to discuss important new information. A number of

opportunities were provided for public comment on successive drafts of

the Criteria Document and Staff Paper, as well as for intensive peer

review of these documents by CASAC at several public meetings attended

by many knowledgeable individuals and representatives of interested

organizations. In addition, there have been a number of important

scientific conferences, symposia, and colloquia on PM issues, sponsored

by the EPA and others, in the U.S. and abroad, during this period.

While significant uncertainties exist, the review of the health effects

information has been thorough and deliberate. In the judgment of the

Administrator, this intensive evaluation of the scientific evidence has

provided an adequate basis for regulatory decision making at this time,

as well as for the comprehensive research needs document recently

developed by EPA, and reviewed by CASAC and others, for improving our

future understanding of the relationships between ambient PM exposures

and health effects.

The health effects information and human risk assessment were

summarized in the proposal and are only briefly outlined below in this

unit. Subsequent units provide a more complete discussion of the

Administrator's rationale, in light of key issues raised in public

comments, for concluding that it is appropriate to revise the current

primary standards (Unit II.B. of this preamble) and to revise the

specific elements of the standards including indicator (Unit II.C. of

this preamble); averaging time, form, and level of new PM2.5

standards (Units II.D., II.E., and II.F. of this preamble); and

averaging time, form, and level of revised PM10 standards

(Unit II.G. of this preamble).

2. Summary of the health effects evidence. In brief, since the last

review of the PM criteria and standards, the most significant new

evidence on the health effects of PM is the greatly expanded body of

community epidemiological studies. The Criteria Document stated that

these recent studies provide ``evidence that serious health effects

(mortality, exacerbation of chronic disease, increased hospital

admissions, etc.) are associated with exposures to ambient levels of PM

found in contemporary U.S. urban airsheds even at concentrations below

current U.S. PM standard'' (U.S. EPA, 1996a; p. 13-1). Although a

variety of

[[Page 38656]]

responses to constituents of ambient PM have been hypothesized to

contribute to the reported health effects, the relevant toxicological

and controlled human studies published to date have not identified any

accepted mechanism(s) that would explain how such relatively low

concentrations of ambient PM might cause the health effects reported in

the epidemiological literature.

Unit II.A. of the proposal further outlines key information

contained in the Criteria Document, Chapters 10-13, and the Staff

Paper, Chapter V, on the known and potential health effects associated

with airborne PM, alone and in combination with other pollutants that

are routinely present in the ambient air. The information highlighted

there summarizes:

(1) The nature of the effects that have been reported to be

associated with ambient PM, which include premature mortality,

aggravation of respiratory and cardiovascular disease (as indicated by

increased hospital admissions and emergency room visits, school

absences, work loss days, and restricted activity days), changes in

lung function and increased respiratory symptoms, changes to lung

tissues and structure, and altered respiratory defense mechanisms.

(2) Sensitive subpopulations that appear to be at greater risk to

such effects, specifically individuals with respiratory disease and

cardiovascular disease and the elderly (premature mortality and

hospitalization), children (increased respiratory symptoms and

decreased lung function), and asthmatic children and adults

(aggravation of symptoms).

(3) An integrated evaluation of the health effects evidence, with

an emphasis on the key issues raised in assessing community

epidemiological studies, including alternative interpretations of the

evidence, both for individual studies and for the evidence as a whole.

(4) The PM fractions of greatest concern to health.

The summary in the proposal will not be repeated here. EPA emphasizes

that the final decisions on these standards take into account the more

comprehensive and detailed discussions of the scientific information on

these issues contained in the Criteria Document and Staff Paper, which

were reviewed by the CASAC and the public.

3. Key insights from the risk assessment. The Staff Paper presents

the results of a quantitative assessment of health risks for two

example cities, including risk estimates for several categories of

health effects associated with: existing PM air quality levels,

projected PM air quality levels that would occur upon attainment of the

current PM10 standards, and projected PM air quality levels

that would occur upon attainment of alternative PM2.5

standards. The risk assessment is intended as an aid to the

Administrator in judging which alternative PM NAAQS would reduce risks

sufficiently to protect public health with an adequate margin of

safety, recognizing that such standards will not be risk-free. The risk

assessment is described more fully in the Staff Paper and summarized in

the proposal. Related technical reports and updates7 have

been placed in the docket (Abt Associates, 1996a,b; 1997a,b).

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7 The risk assessment results that appear in the Staff Paper and

are summarized in the proposal have been updated to include analyses

of the particular forms of standard alternatives contained in the

proposal and to correct estimates for one effects category

(mortality from long-term exposure) to reflect the actual statistics

used in the study upon which they were based (Pope et al., 1995).

The corrections, which cumulatively reduce estimates of mortality

associated with long-term exposures by 20 to 35%, have no effect on

risk estimates for mortality associated with short-term exposures or

the estimates for any other effects. Because the key sensitivity

analyses that provide additional insights regarding thresholds,

copollutants, averaging time and related issues involved the short-

term exposure studies, none of these results are affected by changes

to the long-term exposure risk estimates.

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EPA emphasizes that it places greater weight on the overall

conclusions derived from the studies--that PM air pollution is likely

causing or contributing to significant adverse effects at levels below

those permitted by the current standards--than on the specific

concentration-response functions and quantitative risk estimates

derived from them. These quantitative risk estimates include

significant uncertainty and, therefore, should not be viewed as

demonstrated health impacts. EPA believes, however, that they do

represent reasonable estimates as to the possible extent of risk for

these effects given the available information. Keeping in mind the

important uncertainties inherent in any such analyses, the key insights

from the risk assessment that are most pertinent to the current

decision include:

(1) Fairly wide ranges of estimates of the incidence of PM-related

mortality and morbidity effects and risk reductions associated with

attainment of alternative standards were calculated for the two

locations analyzed when the effects of key uncertainties and

alternative assumptions were considered. Significantly, the combined

analysis for these two cities alone found that the risk remaining after

attaining the current PM10 standards was on the order of

hundreds of premature deaths each year, hundreds to thousands of

respiratory-related hospital admissions, and tens of thousands of

additional respiratory related symptoms in children.

(2) Based on the results from the sensitivity analyses of key

uncertainties and the integrated uncertainty analyses, the single most

important factor influencing the uncertainty associated with the risk

estimates is whether or not a threshold concentration exists below

which PM-associated health risks are not likely to occur.

(3) Over the course of a year, the few peak 24-hour

PM2.5 concentrations appear to contribute a relatively small

amount to the total health risk posed by the entire air quality

distribution as compared to the aggregated risks associated with the

low to mid-range concentrations.

(4) There is greater uncertainty about both the existence and the

magnitude of estimated excess mortality and other effects associated

with PM exposures as one considers increasingly lower concentrations

approaching background levels.

B. Need for Revision of the Current Primary PM Standards

1. Introduction. The overarching issue in the present review of the

primary NAAQS is whether, in view of the advances in scientific

knowledge reflected in the Criteria Document and Staff Paper, the

existing PM standards should be revised and, if so, what revised or new

standards would be appropriate. The concluding section of the

integrative synthesis of health effects information in the Criteria

Document, which CASAC characterized as EPA's ``best ever example of a

true integrative summary of the state of knowledge about the health

effects of airborne PM,'' (Wolff, 1996b) provides the following summary

of the science with respect to this issue:

The evidence for PM-related effects from epidemiological studies

is fairly strong, with most studies showing increases in mortality,

hospital admissions, respiratory symptoms, and pulmonary function

decrements associated with several PM indices. These epidemiological

findings cannot be wholly attributed to inappropriate or incorrect

statistical methods, misspecification of concentration-effect

models, biases in study design or implementation, measurement errors

in health endpoint, pollution exposure, weather, or other variables,

nor confounding of PM effects with effects of other factors. While

the results of the epidemiological studies should be interpreted

cautiously, they nonetheless provide ample reason to be concerned

that there are detectable health effects attributable

[[Page 38657]]

to PM at levels below the current NAAQS. [U.S. EPA, 1996a, p. 13-92]

Given the nature of the health effects in question, this finding,

which is based on a large number of studies that used PM10

measurements, as well as studies using other indicators of PM, clearly

indicates that revision of the current PM NAAQS is appropriate. Quite

apart from the issue of whether PM10 should be the sole

indicator for the PM NAAQS, the extensive PM epidemiological data base

provides evidence of serious health effects (e.g., mortality,

exacerbation of chronic disease, increased hospital admissions) in

sensitive populations (e.g., the elderly, individuals with

cardiopulmonary disease), as well as significant adverse health effects

(e.g., increased respiratory symptoms, school absences, and lung

function decrements) in children. Moreover, these effects associations

are observed in areas or at times when the levels of the current

PM10 standards are met. Although the increase in relative

risk is small for the most serious outcomes, EPA believes it is

significant from an overall public health perspective, because of the

large number of individuals in sensitive populations that are exposed

to ambient PM, as well as the significance of the health effects

involved (U.S. EPA, 1996a, p. 1-21). The results of the two-city PM

risk assessment reinforce these conclusions regarding the significance

of the public health risk--even under a scenario in which the current

PM10 standards are attained.

While the lack of demonstrated mechanisms that explain the

extensive body of epidemiological findings is an important caution,

which presents difficulties in providing an integrated assessment of PM

health effects research, a number of potential mechanisms have been

hypothesized in the recent literature (U.S. EPA, 1996b; p. V-5 to V-8;

appendix D). Moreover, qualitative information from laboratory studies

of the effects of particle components at high concentrations and

dosimetry considerations suggest that the kinds of effects observed in

community studies (e.g., respiratory- and cardiovascular-related

responses) are at least plausibly related to inhalation of

PM.8 Indeed, as discussed in the Criteria Document and

section V.E of the Staff Paper, the consistency of the results of the

epidemiological studies from a large number of different locations and

the coherent nature of the observed effects9 are suggestive

of a likely causal role of ambient PM in contributing to the reported

effects.

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8 As discussed more fully below in this unit, epidemiological

studies alone cannot be used to demonstrate mechanisms of action,

but they can provide evidence useful in making inferences with

regard to causal relationships (U.S. EPA, 1996b, p. V-9).

9 As noted in the proposal, the kinds of effects observed in the

epidemiological studies are logically related. For example, the

association of PM with mortality is mainly linked to respiratory and

cardiovascular causes, which is coherent with observed PM

associations with respiratory and cardiovascular hospital admissions

and respiratory symptoms. Further, similar categories of effects are

seen in long- and short-term exposure studies.

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2. Comments on scientific basis for revision. A majority of the

public comments received on the proposal agreed that, based on the

available scientific information, the current PM10 standards

are not of themselves sufficient to protect public health and it would

be appropriate to revise them. Included in those calling for revisions

to the current standards are many public health professionals,

including numerous medical doctors and academic researchers. For

example, a group of 27 members of the scientific and medical community

recognized as having substantial expertise in conducting research on

the health effects of air pollution stated:

Health studies conducted in the U.S. and around the world have

demonstrated that levels of particulate and ozone air pollution

below the current U.S. National Air Quality Standards exacerbate

serious respiratory disease and contribute to early death. A large

body of scientific and medical evidence clearly indicates that the

current NAAQS are not sufficiently protective of public health.

[Thurston, 1997]

Similar conclusions were reached in a letter signed by more than

1,000 scientists, clinicians, researchers, and other health care

professionals (Dickey, 1997). The cosigners to this letter argued that

tens of thousands of hospital visits and premature deaths could be

prevented with the proposed air quality standard revisions. In fact,

these commenters argued that even stronger standards than those

proposed by EPA are needed to protect the health of the most vulnerable

residents of our communities.

A number of State and local government authorities also submitted

comments in support of adopting new air quality standards for fine

particulate matter. The commenters concurred with conclusions reached

through the EPA's peer review process that the PM standards should be

revised to protect public health. A number of these commenters

suggested that the standards proposed by EPA should be even stronger,

while several other State agencies recommended that EPA adopt

PM2.5 standards, but at less stringent levels. A number of

the comments from states supporting even stronger standards

acknowledged the lack of demonstrated mechanism(s) and other

uncertainties but stressed the strength of the other evidence in urging

EPA to set protective standards.

Many comments were also received from representatives of

environmental or community health organizations that supported the

adoption of air quality standards for PM2.5. These

commenters agreed with EPA's finding that a large body of compelling

evidence demonstrates that exposure to particulate matter pollution, in

general, is associated with premature death, aggravation of heart and

lung diseases, increased respiratory illness and reduced lung function.

They agreed with EPA that these studies present a consistent and

coherent relationship between exposure to PM and both mortality and

various measures of morbidity. However, the majority of these

commenters argued that EPA's proposed standards for PM2.5

were inadequate and recommended adoption of more stringent levels of

the 24-hour and/or annual air quality standards for PM2.5.

Many of these commenters also urged EPA to revise the NAAQS for

PM10 to be more protective of public health. These

commenters based their recommendations on the findings of the studies

that were reviewed in the preparation of the Criteria Document and

Staff Paper. One commenter used results from five of these studies as

the basis for recommending PM2.5 standards of 10 g/

m3 (annual) and 18 g/m3 (24-hour)

(Dockery et al., 1993; Pope et al., 1995; Schwartz et al., 1996;

Schwartz et al., 1994; Thurston et al., 1994). The commenters agreed

with EPA on the significance of these studies' results and the need to

revise the PM standards, while differing with EPA's interpretation of

the findings for purposes of developing the proposed PM standards.

Several commenters made reference to the conclusions of a number of

international scientific panels regarding the health effects of

exposure to airborne particulate matter--the British Expert Panel on

Air Quality Standards, the British Committee on the Medical Effects of

Air Pollutants, the World Health Organization, the Canadian Ministry of

Environment, Lands and Parks, and the Health Council of the Netherlands

-- and argued that all these panels found that PM concentrations

equivalent to the current U.S. standards for PM10 are not

protective of human health and made recommendations for greater

protection. One commenter noted that the findings of the British Health

Panel have resulted in a British

[[Page 38658]]

proposal to adopt a 24-hour PM10 standard of 50 g/

m3, which is one-third the level of the current U.S. NAAQS.

In these comments, some toxicological studies were cited as

providing evidence for toxicity of particulate pollution. These

commenters disagreed with arguments that PM standards cannot be adopted

due to a lack of a sufficient understanding of the biological mechanism

of injury. The commenters argued that there is sufficient evidence that

particulate pollution is associated with adverse health effects to make

it inappropriate to delay the establishment of standards while further

studies are undertaken. This group of commenters was also critical of

arguments against the establishment of additional PM standards based on

the possibility of confounding by other pollutants, and urged that more

attention be paid instead to the possible additive or synergistic

effects of multiple pollutant exposures.

In general, the EPA agrees with these commenters' arguments

regarding the need to revise the PM standards. The scientific studies

cited by these commenters were the same studies used in the development

of the Criteria Document and the Staff Paper, and the EPA agrees that

there is a sufficient body of evidence that the current NAAQS for PM

are not adequately protective of the public health. For reasons

detailed in Unit II.F. of this preamble and in the Response to

Comments, EPA disagrees with aspects of these commenters' views on the

level of protection that is appropriate and supported by the available

scientific information.

Another body of commenters, including almost all commenters

representing businesses and industry associations, many local

governmental groups and private citizens, and some States opposed

revising the standards. Many of these commenters argued that the

available scientific evidence does not provide an adequate basis for

revising the current standards. The central arguments made by these

commenters can be divided into two categories: (1) General comments on

the appropriateness of relying on the epidemiological evidence for

making regulatory decisions, and (2) more specific comments challenging

EPA's appraisal of the consistency and coherence of the available

information, EPA's conclusions regarding causality, and the use of

these studies for risk assessment and decisions on whether to revise

the standards. While EPA has included comprehensive responses to these

comments in the Response to Comments, certain key points are summarized

below in this unit.

a. General comments on the use of epidemiological studies. The

first category of comments was largely derived from ad hoc panels of

occupational and other epidemiological experts, consulting groups, and

individual consultants. Most of these individuals and groups commented

on the use of epidemiology in reaching scientific and policy

conclusions primarily from an occupational or hazard assessment

perspective, in contrast to the perspective of the review of ambient PM

criteria and standards, where the use of community air pollution

epidemiological studies are central. Citing accepted criteria used in

evaluating epidemiological studies to assess the likelihood of

causality (most notably those of Sir Austin Bradford Hill, 1965), these

commenters argued that in the absence of a demonstrated biological

mechanism, the relative risks of effects in the PM epidemiological

studies are too low (less than values variously cited as 1.5 to 2.0) to

reach any conclusions regarding causality or to form the basis for

regulations. In general, the commenters applied these criteria to a

subset of studies evaluated in the Criteria Document, including as few

as two long-term exposure studies (EOP Group) (API, 1997), a group of 9

selected studies (Greenland panel) (API, 1997), those studies cited in

the proposal (AIHC, 1997), or as many as 23 selected short-term

exposure studies examined in a recently published review paper (Gamble

and Lewis, 1996).

Based on a careful review of these comments, EPA notes a number of

limitations in these commenters' evaluations of the epidemiological

studies that they considered, as discussed in detail in the Response to

Comments. In summary, EPA notes that these commenters provided

scientific advice and conclusions that are in substantial disagreement

with the conclusions of the review reflected in the Criteria Document

and Staff Paper. EPA stands behind the scientific conclusions reached

in these documents regarding the appropriate use of the available

community epidemiological studies. These documents were the product of

an extended public process that included conducting public workshops

involving the leading researchers in the field, drafts of the Criteria

Document and Staff Paper providing opportunities for public scrutiny

and comment on, and, not least, receiving the advice of an independent

panel of air pollution experts, including epidemiologists.

EPA clearly specified the key criteria by which it evaluated the

available epidemiological studies in section 12.1.2 of the Criteria

Document, with substantial reliance on those specified by Hill (1965).

In rejecting results with relative risks less than 1.5 to 2 as

meaningful absent demonstrated biological mechanisms, the commenters

fail to note that Hill and other expert groups (U.S. DHEW, 1964) have

emphasized that no one criterion is definitive by itself, nor is it

necessary that all be met in order to support a determination of

causality (U.S. EPA, 1996a, p. 12-3).

With respect to biological plausibility, Hill noted that ``this is

a feature I am convinced we cannot demand. What is biologically

plausible depends upon the biological knowledge of the day'' (Hill,

1965). This statement is clearly pertinent to the toxicological and

mechanistic understanding of the effects of PM and associated air

pollutants, especially at lower concentrations. It is also important to

stress that while the mechanistic evidence published as of the time the

Criteria Document closed does not provide quantitative support for the

epidemiological results, neither can such limited evidence refute these

findings. It is also important to stress that our understanding of

biological mechanisms for PM pollution effects is not sufficient to

explain the effects observed at much higher concentrations in air

pollution episodes, for which causality is generally accepted.

Moreover, the toxicological literature has only recently begun to

examine animal models (or controlled human studies) that might reflect

the sensitive populations in question (the elderly, individuals with

chronic respiratory and cardiovascular disease) or that adequately

reproduce all of the physico-chemical properties of particles in the

ambient atmosphere. In short, the absence of evidence of a particular

mechanism is hardly proof that there are no mechanisms that could

explain the effects observed so consistently in the epidemiological

studies. The absence of biological mechanisms did not deter CASAC from

recommending revisions to the PM standards in 1982, 1986, and again in

1996.

While Hill appropriately emphasized the strength of the association

as important (e.g., size of the relative risk), he also pointed out

that ``We must not be too ready to dismiss a cause-and-effect

hypothesis merely on the ground that the observed association appears

to be slight. There are many occasions in medicine when this in truth

is so'' (Hill, 1965). EPA believes that the effects of air pollution

containing PM is such a

[[Page 38659]]

case. Unlike the ``textbook'' examples of unlikely significant

associations provided by some commenters (e.g., ice cream consumption

correlated with heat stroke), the abundant epidemiological literature

on combustion particles documents numerous occasions in which single

short-term episodes of high air pollution produced unequivocally

elevated relative risks. For the week of the well documented 1952

London air pollution episode, for example, the relative risk of

mortality for all causes was 2.6, while the relative risk for

bronchitis mortality was as high as 9.3 (Ministry of Health, 1954).

Hospital admissions also increased by more than a factor of two.

British epidemiologists in the 1950s concluded that increased mortality

was likely when PM (as mass calibrated British Smoke m in

aerodynamic diameter) exceeded 500 g/m3 (Martin and

Bradley, 1960). This is only about a factor of 3 higher than that

allowed by the current PM standard. Unlike the ``textbook'' and other

unlikely statistical associations noted by some commenters, where the

only evidence is for low relative risk, clear and convincing links

between high-level PM concentrations and mortality and morbidity

buttress the findings of similar associations at much lower PM

concentrations as suggested in the more recent epidemiological

literature.

These commenters also appear to ignore several epidemiological

studies conducted at low PM concentrations in U.S. and European cities,

including both short- and long-term exposures to PM air pollution, that

find statistically significant relative risks of respiratory symptom

categories in children in the range of 1.5 to 5 (Schwartz et al., 1994;

Pope and Dockery, 1992; Braun-Fahrlander et al., 1992; Dockery et al.,

1989; Dockery et al., 1996). Concentrations in these studies extend

from moderately above to well below those permitted by the current

PM10 standards. While, as noted in the proposal, most of the

recent epidemiological studies of mortality and hospital admissions

report comparatively small relative risks, the findings of relative

risks well in excess of the 1.5 to 2 criterion noted by commenters for

earlier studies of high PM episodes, as well as the relative risks of

1.5 to 5 reported in more recent studies of less serious, but still

important effects categories, lend credibility to EPA's interpretation

of the results.

In addition to basing their conclusions primarily on their own

assessment of a limited set of studies, this group of commenters

reached different conclusions about the consistency of the observed

associations because of their assumptions that all model building

strategies by all authors are equally valid. Even the most thorough of

these treatments (Gamble and Lewis, 1996) shared this flaw,

particularly in the discussion of the series of Philadelphia mortality

studies and in the discussion of modeling approaches. The authors'

treatment of modeling and confounding issues was further limited

because they did not include the most recent Philadelphia results

(Samet et al., 1996a,b) sponsored by the Health Effects Institute (HEI,

1997). One of the important functions of the Criteria Document is to

evaluate the strengths and limitations of various studies. As discussed

more fully below in this unit, the Criteria Document found that some of

the studies cited by commenters as suggesting a lack of consistency had

important limitations. In general, these commenters' analyses suffered

by ignoring the much more thorough critical review of these studies and

issues contained in the Criteria Document, notably that in section 12.6

on alternative modeling approaches.

EPA also rejects the notion advanced by these commenters that

epidemiological studies must use personal exposure monitoring to be

considered for regulatory purposes. In particular, commenters ignore

the significant strengths of the time-series studies and prospective

cohort studies relied on by EPA as compared to cross-sectional

epidemiological studies. Time-series studies, such as the daily

mortality studies, look at changes in response rate in relation to

changes in weather and air pollution over time intervals of a few days.

This controls for other factors such as smoking and socioeconomic

status, which are little changed during such short intervals.

Prospective cohort studies (e.g., Pope et al., 1995; Raizenne et al.,

1996), on the other hand, look at changes in health status in a

selected cohort of individuals, which allows direct adjustment for

smoking status, socioeconomic status, and other subject-specific

factors. The commenters also ignore the Criteria Document conclusions

on how properly conducted monitoring can provide an adequate index of

population exposure to ambient air pollution in such studies that, as

detailed below, is more relevant to establishing ambient air quality

standards (U.S. EPA 1996a, chapter 7). Although personal monitoring may

be practical for some occupational and epidemiological studies, and has

been employed in some past studies of air pollution, it is not

realistic to require personal monitors in air pollution studies of

daily mortality, which require urban scale population data over a

period of years. Furthermore, the use of community monitoring-based

epidemiological studies as a basis for establishing standards and

guidelines has a long history in air pollution, including the British

authorities' response to the London episodes and the establishment of

the original U.S. NAAQS in 1971. Rejecting the use of the vast array of

such studies on this basis alone would also go against the advice of

the independent scientific experts on every CASAC panel that has

addressed the subject of PM pollution through the years, each of which

has recommended general PM standards based primarily on the results of

community epidemiological studies (Friedlander, 1982; Lippmann, 1986;

Wolff, 1996b). As noted above in this unit, EPA has included a more

detailed discussion of its responses to these comments in the Response

to Comments.

b. Specific comments on epidemiologic studies. The second group of

commenters noted above made more specific challenges to EPA's

assessment of the epidemiological studies. These comments, although

overlapping some of those made by the first group, were generally made

by commenters who have taken a more active role in the review of the

Criteria Document and Staff Paper. These commenters asserted that the

epidemiological evidence on PM is not as consistent and coherent as EPA

has claimed, and, in particular, charged that EPA ignored or downplayed

a number of studies that the commenters argue contradict the evidence

the Agency cited as supporting the consistency and coherence of PM

effects. The studies, all of which commenters contend do a better job

of addressing one or more key issues, such as confounding pollutants,

weather, exposure misclassification, and model specification, than

earlier studies, include several that were available during preparation

of the Criteria Document, and a number that appeared after the Criteria

Document and Staff Paper were completed. Because the status of the

later studies differ from that of the earlier ones for purposes of

decisions under section 109 of the Act, the two categories are

discussed separately below in this unit. Additional responses to

comments relating to both sets of studies have been included in the

Response to Comments. In addition to the inclusion of specific studies,

commenters also raised other issues regarding the limitations of the

[[Page 38660]]

epidemiological information and the use of these studies in EPA's two-

city risk assessment. Both of these topics are also discussed below in

this unit.

(i) Studies available for inclusion in the criteria review. With

some exceptions, most of the above commenters cited somewhat similar

lists of ``negative'' studies that they argue EPA ignored or downplayed

in arriving at conclusions on consistency and coherence. Of the most

commonly cited studies, the following were available for inclusion in

the Criteria Document: daily mortality studies by Styer et al. (1995),

Lyon et al. (1995), Li and Roth (1995), Moolgavkar (1995a,b), Wyzga and

Lipfert (1995), Lipfert and Wyzga (1995), and Samet et al. (1995,

1996a,b); the long-term exposure mortality study by Abbey et al.

(1991); and the re-examination of the Six-City mortality results

(Dockery et al., 1993) by Lipfert (1995).

The written record of EPA's evaluations of these studies

effectively refutes the claim that the Agency ignored any of these

studies and supports the treatment the Agency accorded to each of them.

All of the studies available to EPA at the time of CASAC closure on the

PM Criteria Document (March 1996) were examined for inclusion in the

Criteria Document and Staff Paper, which form the basis for the PM

proposal. ``Negative''10 studies were evaluated in detail

along with ``positive'' studies when they were found to have no

critical methodological deficiencies, or to point out strengths and

limitations. Studies that had more serious problems were generally

discussed in less detail, whether positive or negative, than studies

with fewer or small deficiencies. The EPA assessments were evaluated by

peer reviewers, by CASAC, and by the public.

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10 The term ``negative'' studies, as used in these comments,

should not be construed to mean those in which there is a negative

effects estimate (either significant or non-significant) for the

nominal cause. As used by these commenters, the term also includes

statistically non-significant positive effect estimates. In other

words, the commenters define ``positive'' studies as including only

those in which the effect estimate is both positive and

statistically significant.

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Most of the short-term exposure studies cited above in this unit

are reanalyses and extensions of PM/mortality studies that had been

published by other investigators. In general, the Criteria Document

concluded that the most comprehensive and thorough reanalyses were

those in the series conducted for the HEI, which reanalyzed data sets

used in studies from six urban areas in Phase I.A (Samet et al.,

1995)11, with extended analyses for Philadelphia in Phase

I.B (Samet et al., 1996a,b). The most important finding in the HEI

Phase I.A reanalyses of the six areas is ``the confirmation of the

numerical results of the earlier analyses of all six data sets'' (HEI,

1995)12. After replicating the original investigators'

analyses, Samet et al. (1995) also found similar results analyzing the

data using an improved statistical model. The HEI Oversight Committee

found

[I]t is reasonable to conclude that, in these six data sets,

daily mortality from all causes combined, and from cardiovascular

and respiratory causes in particular, increases as levels of

particulate air pollution indexes increase. [HEI, 1995]

It is important to note that these reanalyses by respected

independent scientists confirm the reliability and reproducibility of

the work of the original investigators, particularly in view of the

concerns some commenters have expressed about EPA's reliance on a

number of PM studies published by these authors.

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11 Data sets were those used in the original studies by Dockery

et al. (1992) for St. Louis and Eastern Tennessee; Pope et al.

(1992) for Utah Valley; Schwartz and Dockery (1992a) for

Philadelphia; Schwartz (1993) for Birmingham; and a portion of the

Santa Clara data from Fairley (1990). The data set from the

Moolgavkar et al. (1995a) Philadelphia reanalysis was also included

(Samet et al., 1995).

12 The HEI Board of Directors appointed an eight member

Oversight Committee consisting of leading scientists in several

disciplines relevant to air pollution epidemiology to oversee key

aspects of the project and to prepare HEI's assessment of the

results.

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The Phase I.A HEI results for Philadelphia also found that it was

difficult to separate the effects of PM from those of co-occurring

SO2, in agreement with the Moolgavkar et al.(1995a)

analysis. Subsequent HEI work, and several of the other so-called

``negative'' studies cited above in this unit, further examined this

issue in terms of confounding or effects modification by one or more

co-occurring gaseous pollutants or weather. Contrary to commenters'

claims, this issue and these studies received considerable attention in

the Criteria Document and Staff Paper, and the overall implications and

conclusions from these assessments were summarized in the proposal. In

particular, the so-called ``negative'' and other findings of

Moolgalvkar et al. (1995a,b) in their Philadelphia and Steubenville

studies were discussed in great detail in section 12.6 of the PM

Critera Document and compared to those of the original investigators

(Schwartz and Dockery, 1992a,b) and other investigators (Li and Roth,

1995; Wyzga and Lipfert, 1995). Further analytical studies of the

Philadelphia data set were carried out by HEI (Samet et al., 1996a,b)

and have largely resolved many of the uncertainties in the earlier

analyses; in EPA's opinion, these studies supersede the results of the

original investigators (Schwartz and Dockery, 1992a) and the several

earlier reanalyses, including Moolgavkar (1995a), Moolgavkar and

Luebeck (1996), Li and Roth (1995), Wyzga and Lipfert (1995), and Samet

et al. (1995). Even though TSP is not the best PM indicator for health

effects, since it includes a substantial fraction of non-thoracic

particles, the extended Criteria Document assessment (U.S. EPA, 1996a,

pp. 12-291 to -299; 12-327) of the Phase I.B HEI analyses in

Philadelphia (Samet et al., 1996a,b) serves to support the following

findings:

(1) The mortality effects estimates for TSP do not depend heavily

on statistical methods when appropriate models are used.

(2) Estimated PM effects are not highly sensitive to appropriate

methods for adjusting for time trends and for weather.

(3) Air pollution has significant health effects above and beyond

those of weather.

(4) Copollutants such as ozone, CO, and NO2 may be

important predictors of mortality, but their effects can be

substantially separated from those of TSP and SO2.

(5) The health effects of TSP in Philadelphia cannot be completely

separated from SO2, which is itself a precursor of fine

particles, based solely on the epidemiological analyses in this single

city.

The most recent HEI Oversight Committee comments on these studies

(HEI, 1997), which were submitted to the docket by HEI, state that:

Although individual air pollutants (TSP, SO2, and

ozone) are associated with increased daily mortality in these data,

the limitations of the Philadelphia data make it impossible to

establish that particulate air pollution alone is responsible for

the widely observed associations between increased mortality and air

pollution in that city. All we can conclude is that it appears to

play a role. [HEI, 1997; p.38.]

While recognizing the limitations in the conclusions that can be made

based on studies in a single city, the Oversight Committee endorses the

approach taken by EPA in evaluating a broader set of epidemiological

studies:

Consistent and repeated observations in locales with different

air pollution profiles can provide the most convincing

epidemiological evidence to support

[[Page 38661]]

generalizing the findings from these models. This has been the

approach reported by the EPA in its recent Criteria Document and

Staff Paper. [HEI, 1997; p. 38.]

As noted in the proposal, based on this approach, EPA's assessment

of numerous mortality studies concludes that when studies are evaluated

on an individual basis, the PM-effects associations are valid and, in a

number of studies, not seriously confounded by co-pollutants (U.S. EPA,

1996a; p. 13-57); and when a collection of studies from multiple areas

with differing concentrations of PM and co-pollutants are examined

together, the association with PM10 remains reasonably

consistent across a wide range of concentrations of these potentially

influential pollutants (U.S. EPA, 1996a; p. 12-33; U.S. EPA, 1996b; p.

V-55).

In addition to relying on the most comprehensive and best analyses

in evaluating the reanalysis in Philadelphia and other areas, the

Criteria Document gave less weight to both so-called ``negative'' and

``positive'' studies with methodogical limitations. In particular, EPA

agreed with the epidemiological experts on CASAC (Lippmann et al.,

1996; Samet, 1995) that the Li and Roth (1995) study approach of using

a ``panoply'' of different modeling strategies to produce seemingly

conflicting findings provides little useful insight and is superseded

by the HEI report. The attempt by Lipfert and Wyzga (1995) to address

relative effects of different pollutants was considered inconclusive

(Lippmann et al., 1996) and flawed by the use of a metric (elasticity)

that ignores the absolute concentrations of the pollutants being

compared (see Response to Comments).

Further, the Steubenville studies and reanalyses (Schwartz and

Dockery, 1992b; Moolgavkar, 1995b) were discussed in detail to examine

methodologies, and the differences in relative risks between the two

were regarded as small (U.S. EPA, 1996a, p. 12-280 to 283). Both

studies used TSP as the PM indicator variable, and they are augmented

by the more recent findings of Schwartz et al. (1996) that examine

PM10 and its components. The mixed results by Lyon et al.

(1995) in Utah Valley are compromised by loss of information related to

the methodology (U.S. EPA, 1996a, p. 12-58). As noted above, subsequent

reanalyses of the Utah Valley study by HEI (Samet et al., 1995) as well

as by Pope and Kalkstein (1996) confirmed the original findings of Pope

et al. (1992) using different model specifications. The Salt Lake City

study by Styer et al. (1995) was mentioned in the PM Criteria Document,

but received little discussion because aspects of the methodological

approach limited its statistical power to detect effects. The analysis

of Chicago mortality data in the same paper shared these problems,

particularly for seasonal analyses; in this larger city, they

nonetheless found significant associations on an annual basis between

PM10 and mortality that are consistent with other studies.

In short, the record shows that EPA did not ignore these short-term

exposure studies cited by commenters; moreover, EPA's assessment of

these studies is consistent with the views of four researchers on the

CASAC panel who have extensive involvement in conducting population

studies of air pollution (Lippmann et al., 1996).13

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13 Their March 20, 1996 letter to the Administrator concludes

that the HEI analysis of Philadelphia supersedes earlier analyses,

specifically Moolgavkar et al. (1995a), Lipfert and Wyzga (1995),

and Li and Roth (1995), and points out the limitations of Styer et

al. (1995).

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Similarly, EPA believes that appropriate treatment and weight were

given to studies of long-term exposure and mortality. EPA concluded

that the lack of associations in the Abbey et al. (1991) prospective

cohort study were not inconsistent with two other such studies because

the use of days of peak TSP levels as the PM indicator (instead of

PM10 or PM2.5) is inappropriate for California

cohorts exposed to both urban smog and fugitive dust episodes, and the

overall sample size may have been too small to detect significant

effects (U.S. EPA, 1996b; pp. V-17 to -18). The inadequacy of Lipfert's

(1995) application of state-wide average sedentary lifestyle data to

adjust mortality for the six cities studied by Dockery et al. (1993),

in which superior subject-specific body mass index data had already

been considered, was also noted and addressed in the Staff Paper (U.S.

EPA, 1996b; p. V-16). Again, EPA did not ignore these studies; the

rationale for giving them less weight was clearly articulated in the

documents reviewed by CASAC and judged appropriate for use in standard

setting.

While the proposal presents only a summary discussion of key

Criteria Document and Staff Paper findings, EPA believes that

discussion is fully consistent with the state of the science.

Furthermore, the proposal highlights the nature of alternative

viewpoints on the epidemiology in a quotation from the Criteria

Document (61 FR 65644, December 13, 1996) and cites explicitly the

views of most of the authors noted above in this unit (Moolgavkar et

al., 1995b; Moolgavkar and Luebeck, 1996; Li and Roth, 1995; Samet et

al., 1996; Wyzga and Lipfert, 1995). The proposal also summarizes EPA

conclusions based on all of the literature as assessed in the Criteria

Document and Staff Paper with respect to issues raised in these and

other studies, including potential confounding by independent risk

factors such as weather and other pollutants, choice of statistical

models, use of outdoor monitors, and exposure misclassification.

More specifically, in the proposal EPA has not ignored the view

advanced by some that the results of individual studies of multiple

pollutants, such as the HEI Philadelphia studies, are more suggestive

of an ``air pollution'' effect than an effect of PM alone. Indeed, the

proposal notes that it is reasonable to expect that other pollutants

may play a role in modifying the magnitude of the estimated effects of

PM on mortality, either through pollutant interactions or independent

effects (61 FR 65645, December 13, 1996). Based on the large body of

evidence at hand, however, EPA cannot accept the suggestion that such

multi-pollutant studies are in any way ``negative'' with respect to

EPA's conclusions that PM, alone or in combination with other

pollutants, is associated with adverse effects at levels below those

allowed by the current standards. This conclusion is based not only on

the consistency of PM effects across areas with widely varying

concentrations of potentially confounding copollutants, but also on the

extended analyses of the Philadelphia studies in the Criteria Document

and Staff Paper.

Because commenters have tended to ignore the latter analyses, it is

appropriate to summarize them here briefly. As noted above in this

unit, the Criteria Document assessment of the Philadelphia studies

finds that PM can reasonably be distinguished from potential effects of

all pollutants except SO2. The Staff Paper builds on this

analysis through an integrated assessment that draws on information

from atmospheric chemistry, human exposure studies, and respiratory

tract penetration results to provide insight as to which of these two

pollutants is more likely to be responsible for mortality in the

elderly and individuals with cardiopulmonary disease (U.S. EPA 1996b;

pp. V-46 to -50). That assessment notes that the inhalable

(PM10), including the fine (PM2.5), components of

TSP are more likely than SO2 to penetrate and remain indoors

where the sensitive population resides most of the time.14

In addition, these PM

[[Page 38662]]

components, especially PM2.5, penetrate far more effectively

to the airways and gas exchange regions of the lung than does

SO2. Furthermore, in Philadelphia, it is possible that

SO2 is a surrogate for fine particulate acid sulfates. For

these reasons, even though statistical analyses of the Philadelphia

data set cannot fully distinguish between these two highly correlated

pollutants, EPA believes that the weight of the available evidence from

an integrated assessment more strongly supports the notion that PM is

playing an important direct role in the observed mortality effects

associations in Philadelphia. Moreover, as noted above in this unit, in

some other locations with significant PM-mortality associations,

ambient SO2 levels are too low to confound PM.

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14 In response to comments on this rulemaking, some papers

submitted by industry commenters make statements that are in

substantial agreement with these staff conclusions with respect to

the likelihood of SO2 penetrating to indoor environments

and the lesser likelihood of affecting sensitive populations indoors

(Lipfert and Wyzga, 1997; Lipfert and Urch, 1997).

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(ii) Recent studies available after completion of criteria review.

As noted above in this unit, other studies cited by some commenters as

so-called ``negative'' evidence ignored by EPA were published or

otherwise made available only after completion of the PM Criteria

Document. EPA agrees that it did not rely on these studies, based on

its long-standing practice of basing NAAQS decisions on studies and

related information included in the pertinent air quality criteria and

available for CASAC review.15 Although EPA has not relied on

such studies in this review and decision process, the Agency

nevertheless has conducted a provisional examination of these and other

recent studies to assess their general consistency with the much larger

body of literature evaluated in the Criteria Document.16 EPA

has placed its examination of recent studies in the rulemaking docket.

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15 Since the 1970 amendments, the EPA has taken the view that

NAAQS decisions are to be based on scientific studies that have been

assessed in air quality criteria [see e.g., 36 FR 8186 (April 30,

1971) (EPA based original NAAQS for six pollutants on scientific

studies discussed in the air quality criteria and limited

consideration of comments to those concerning validity of scientific

basis); 38 FR 25678, 25679-25680 (September 14, 1973) (EPA revised

air quality criteria for sulfur oxides to provide basis for

reevaluation of secondary NAAQS)]. This longstanding interpretation

was strengthened by new legislative requirements enacted in 1977

(section 109(d)(2) of the Act; section 8(c) of the Environmental

Research, Development, and Demonstration Authorization Act of 1978)

for CASAC review of air quality criteria and reaffirmed in EPA's

decision not to revise the ozone standards in 1993. 58 FR 13008,

13013-13014 (March 9, 1993). Some of the commenters now criticizing

EPA for not considering the most recent PM studies strongly

supported the Agency's interpretation in the 1993 decision (UARG,

1992).

16 As discussed in EPA's 1993 decision not to revise the NAAQS

for ozone, new studies may sometimes be of such significance that it

is appropriate to delay a decision on revision of NAAQS and to

supplement the pertinent air quality criteria so the new studies can

be taken into account. 58 FR at 13014, March 9, 1993. In the present

case, EPA's provisional examination of recent studies suggests that

reopening the air quality criteria review would not be warranted

even if there were time to do so under the court order governing the

schedule for this rulemaking. Accordingly, EPA believes that the

appropriate course of action is to consider the newly published

studies during the next periodic review cycle.

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Among the most frequently cited new studies relied on by commenters

were Davis et al. (1996), Moolgavkar et al. (1997), and Roth and Li

(1997). Davis et al. (1996) conducted a reanalysis of the Birmingham

mortality data set originally investigated in Schwartz (1993). At the

time of the close of the public comment period, the paper based on this

manuscript had not been accepted for publication in a peer reviewed

journal (Sacks, 1997). Commenters nevertheless highlight the authors'

claim that ``when humidity is included among the meteorological

variables (it is excluded in the analysis by Schwartz [1993]), we find

that the PM10 effect is not statistically significant.''

EPA's review found important factual errors in this study. Contrary to

Davis et al., Schwartz did include humidity in his 1993 study, and his

finding of a hot-and-humid-day effect was reported there. In addition,

the PM-related variables used by Davis et al. in their manuscript were

not, as the authors claimed, the same as those in Schwartz (1993).

Davis et al. also used a different humidity indicator, specific

humidity. Reanalysis by one of the co-authors (R. Smith, personal

communication, February 8, 1997) showed that when Schwartz's PM metric

was used, the estimated PM10 effect was of about the same

magnitude, and statistically significant at the 0.05 level, even using

the characterization of humidity effect proposed by Davis et al. It

therefore appears that the Davis et al. PM10 result was, in

fact, consistent with that of Schwartz, and robust against a very

different weather model specification.

Based on its examination of both the content and the publication

status of this study, EPA believes the heavy reliance and attention

given to it are misguided. In contrast to commenters' assertions, this

study does not contradict EPA's conclusions with respect to consistency

of the epidemiological evidence and confounding by weather variables;

indeed, the consideration of the corrected results would actually

support EPA's conclusions. EPA believes this example reinforces the

importance of relying on peer reviewed studies and also conducting the

kind of critical examination of such studies that takes place in the

criteria and standards review process.

Several commenters note that Roth and Li (1997) also reexamined the

Birmingham mortality data, as well as hospital admissions data from

Schwartz (1994), and produced a number of negative and inconsistent

results that depend on temperature effects and choice of statistical

model. Preliminary findings from this study were presented by Roth at

the May 1996 CASAC meeting. CASAC epidemiologists and statisticians at

the meeting pointed out a number of shortcomings, both in the

analytical strategy and in details of the models being

evaluated.17 As discussed in more detail in the Response to

Comments, the materials from Roth and Li (1997) recently provided to

EPA as attachments to public comments show that the deficiencies

pointed out at the May 1996 CASAC meeting have not been adequately

addressed. EPA concludes that this study does not support commenters'

claims.

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17 For example, commenting on the Roth examination of

alternative model specifications, Dr. Stolwijk noted ``If you select

out of his [Roth's] matrix the things that other people have done,

he comes to a different conclusion than when he takes his whole

matrix * * *. [Y]ou are going to get a random effect that shows that

there is no effect. He [Roth] did this, I think, on purpose in this

case. Most epidemiologists, I think, have been trained to limit

their observations to something that they can state or would have

stated before they started and observe that and base their

conclusions on it'' [U.S. EPA 1996(c); May 17, 1996 Transcript,

pages 45-46].

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The paper recently accepted for publication by Moolgavkar et al.

(1997) examines hospital admissions and air pollution in Minneapolis

and Birmingham and comes to different conclusions than earlier

investigators with respect to the role of PM10. While the

paper is a useful addition to the literature, the authors clearly do

not attempt to replicate the original studies, making the kind of

direct comparisons suggested by commenters difficult. The paper finds

an air pollution effect in one city that implicates ozone but is unable

to separate effects of PM from a group of other pollutants. EPA's

provisional examination of this study raises some questions about the

methodology, which might usefully be supplemented to further separate

pollutants as was done by Samet et al. (1996a,b) in Philadelphia, and

about the authors' interpretation of the results in both cities. In any

event, EPA does not believe this study negates the PM associations with

hospital admissions

[[Page 38663]]

reported in a number of other studies cited in the Criteria Document.

Another recent paper by Lipfert and Wyzga (1997) provides analyses

suggesting that differential measurement error might account for some

or all of the observation by Schwartz et al. (1996) that daily

mortality is more strongly associated with fine (PM2.5) than

with coarse (PM10-2.5) PM. EPA staff and CASAC accounted for

this possibility, however, and it was factored into both the Staff

Paper and CASAC recommendations.18

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18 CASAC panelists recommended a discussion of this issue in the

Staff Paper. The Staff Paper notes: ``While greater measurement

error for the coarse fraction could depress a potential coarse

particle effect, this would not explain the results in Topeka

relative to other cities. Even considering relative measurement

error, these results provide no clear evidence implicating coarse

particles in the reported effects.'' (U.S. EPA, 1996b p. V-64).

EPA's provisional examination of the Lipfert and Wyzga (1997) paper

in the Response to Comments, finds that it is implausible that most

of the effect attributed to PM2.5 could in fact be due to

PM10-2.5, since differential measurement error cannot

make a weaker effect appear stronger than a stronger one, except

under extremely unusual circumstances.

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Some commenters have highlighted selected individual papers or

summaries from the APHEA19 project conducted in Europe, and

from Roth (1996), calling attention particularly to negative results

found in heavily polluted regions of Eastern Europe. EPA notes that a

number of the recent APHEA and other studies in Western Europe have

shown significant associations between mortality and air pollution

including PM, and that a meta-analysis of 12 Western and Central-

eastern European studies ``is supportive of a causal association

between PM and SO2 exposure and all-cause mortality''

(Katsouyanni et al., 1997). The Eastern and Western European studies

used differing measurement methods for PM, including PM10,

gravimetric ``suspended particles,'' and the British Smoke

method.20 The differences in aerometry and the substantial

differences in location and strength of primary PM emissions sources in

central and eastern Europe as compared to western Europe or the U.S.

might well explain the different results in these unique areas.

Consequently, integration of these results would involve comprehensive

examination of the various PM instruments used, monitor siting in

relation to sources, mass calibration procedures and other aspects of

these studies.21 EPA notes that a number of European

authorities, who are familiar with this recent literature, have

proceeded with recommendations to strengthen their health guidelines,

risk assessments, or regulations for PM.22

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19 The APHEA (Air Pollution and Health: a European Approach)

project was supported by the European Union Environment 1991-1994

Programme to investigate the possible short-term health effects of

exposure to low or moderate levels of ambient air pollutants. Eleven

European research groups carried out studies in 15 cities

(Amsterdam, Athens, Barcelona, Bratislava, Cracow, Helsinki, Koln,

Lodz, London, Lyon, Milan, Paris, Poznan, Rotterdam and Wroclaw) in

which air pollutant concentration data had been collected for at

least 5 years. Initial findings of studies on mortality and hospital

admissions were published in a series of papers in Supplement 1 to

the Journal of Epidemiology and Community Health in 1996 and a meta-

analysis of the mortality data from 12 cities is currently in press

(Katsouyanni et al., 1997).

20 The Roth et al. (1997) study in Prague used a measurement

termed ``suspended particles'' that appears to be close to TSP. The

relation of this indicator to PM10 or PM2.5 in

this city is not reported. Moreover, this study uses a variant of

the problematic methodology in the Roth analyses cited above.

21 These concerns are consistent with EPA's treatment of a

number of European and South American studies that are included in

the Criteria Document and contributed to the evaluation of the

epidemiology in Chapter 12. Because of differences in aerometry

methods and characteristic source classes between North America and

other regions of the world, however, the integrative assessment

chapter reported results only from studies conducted in the U.S. and

Canada (cf. Tables 13-3 to 13-5) in reaching quantitative

conclusions for effects estimates.

22 See, for example, the United Kingdom Air Quality Strategy,

1997; Swiss Federal Commission of Air Hygiene, 1996; World Health

Organization Revised Air Quality Guidelines for Europe, In Press).

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Aside from the recent literature cited by these commenters, there

are a number of other recent epidemiological studies that, if

considered in today's decision, would tend to support EPA's conclusions

about the effects of PM at lower concentrations, assuming their results

were accepted following a full review in the criteria and CASAC

process. For example, in addition to the APHEA studies, several other

recent epidemiologic studies have reported significant positive

associations between PM and health effects (Lipsett et al., 1997;

Peters et al., 1997; Borja-Aburto et al., 1997; Delfino et al., 1997;

Scarlett et al., 1996; Woodruff et al., 1997; Wordley et al., 1977). In

addition, a number of recent toxicologic papers have been accepted or

appear in proceedings (Costa and Dreher, 1997; Killingsworth et al.,

1997; Godleski et al., 1997) that involve exposure to concentrated

ambient fine particles or PM constituents and appear to provide

supportive evidence as to the plausibility of the effects that have

been reported epidemiologically. If considered in this decision, these

studies would also provide biological support for the epidemiological

observation that certain susceptible groups (notably those with

cardiopulmonary disease) are most likely to be affected by PM, again

assuming the results were sustained in the full criteria and CASAC

review process.

In summary, EPA has conducted a provisional assessment of the more

recent scientific literature. Based on this provisional assessment, EPA

disagrees with commenters' assertion that full consideration of

selected new studies in this decision would materially change the

Criteria Document and Staff Paper conclusions on the consistency and

coherence of the PM data, or on the need to revise the current

standards.

(iii) Other specific comments on the epidemiological studies. Aside

from their assertion that EPA ignored or downplayed particular studies,

this second group of commenters raise additional objections, based on

the statistical modeling strategies used and the potential importance

of personal exposure misclassification, to EPA's conclusions regarding

the consistency of the epidemiological evidence. EPA conclusions on

these topics were summarized in the proposal and supported by extensive

treatments in the Criteria Document and Staff Paper. With respect to

the first issue, commenters argued that sufficient flexibility exists

in the analyses of large data sets that it may be possible to obtain

almost any result desired through choice of statistical method.

Analytical choices include the specific statistical model; methods used

to adjust for seasonal variation and the trends in the data; treatment

of other variables (e.g., other pollutants, weather, and day of week);

``lag'' structure; and study population.

A more detailed discussion of this issue, which expands on the

assessment summarized in the Criteria Document, is included in the

Response to Comments. In summary, EPA must reject commenters'

contention that legitimate alternative analyses can obtain ``almost any

result.'' As outlined above in this unit, EPA's detailed reviews of

individual studies have shown that not all methods are equally valid or

legitimate. Moreover, strong arguments can be made that the methods and

analytical strategies in the studies EPA relied upon are more

appropriate approaches than those cited by commenters (e.g., Li and

Roth, 1995; Lipfert and Wyzga, 1995; Davis et al., 1996; Roth and Li,

1997). While not all studies have addressed each of the above issues in

this unit equally well, the most comprehensive analyses of these issues

(e.g., Samet et al., 1995, 1996a,b; Pope and Kalkstein, 1996), as well

as the EPA analyses comparing study results for each issue (U.S. EPA,

1996a, pp. 12-261 to 12-305) found that the authors of studies on which

EPA

[[Page 38664]]

chiefly relied made appropriate modeling choices. The Criteria Document

concludes that: ``[T]he largely consistent specific results, indicative

of significant positive associations of ambient PM exposures and human

mortality/morbidity effects, are not model specific, nor are they

artifactualy derived due to misspecification of any specific model. The

robustness of the results of different modeling strategies and

approaches increases our confidence in their validity [U.S. EPA, 1996a,

p. 13-54].'' While it is true, as evidenced in Li and Roth (1995), that

PM-effects data can be randomly manipulated to produce apparently

conflicting results, commenters have provided no evidence that

different plausible model specifications could lead to markedly

different conclusions.

Some commenters have expressed concerns about the reliability of

the epidemiological results because some studies showed a lack of

correlation in cross-sectional comparisons between outdoor PM measured

at central locations and indoor or personal exposures to PM (which

includes PM from the outdoor, indoor and personal

environments).23 EPA acknowledged and responded to this

issue in chapter 7 of the Criteria Document and the proposal (61 FR

65645, December 13, 1996). The major premise underlying commenters'

arguments on this issue is incorrect.24 The question is not

whether central monitoring site measurements contain a signal

reflecting actual exposures to total PM from both outdoor and indoor

sources at the individual level; the relevant question is whether

central monitoring site measurements contain a signal reflecting actual

exposures to ambient PM for the subject population, including both

ambient PM, while individuals are outdoors, and ambient PM that has

infiltrated indoors, while individuals are indoors. The PM standards

are intended to protect the public from exposure to ambient PM, not PM

generated by indoor or personal sources. There is ample evidence, as

discussed in chapter 7 of the Criteria Document, that personal exposure

to ambient PM, while outdoors and while in indoor micro-environments,

does correlate on a day-to-day basis with concentrations measured at

properly sited central monitors (U.S. EPA, 1996a, p. 1-10). EPA has,

therefore, concluded that it is reasonable to presume that a reduction

in ambient PM concentrations will reduce personal exposure to ambient

PM, and that this will protect the public from adverse health outcomes

associated with personal exposure to ambient PM.

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23 Paradoxically, some commenters have argued (e.g., Valdberg,

1997) that the PM results are confounded because the weather and

other factors that cause daily variations in outdoor pollution will

cause similar daily variations in indoor generated air pollution.

For this to be true, outdoor ambient pollution concentrations would

have to be correlated with personal exposure to indoor generated air

pollution such as that from smoking, cleaning, and cooking. This

argument is logically inconsistent with the other comments on the

lack of any such correlation with personal exposure, and these

commenters have offered no scientific evidence to support their

claim. In response, EPA has performed and included in the Response

to Comments a numerical analysis of the relevant information from

the PTEAM exposure study that finds no evidence for such a

correspondence in the actual data.

24 As documented in Chapter 7 of the Criteria Document, time-

series community studies observe the effects of varying levels of

ambient air pollution; therefore the effects of indoor-generated air

pollution would be independent of and in addition to the effects

found in these epidemiological studies. Commenters apparently

believe EPA is claiming such studies are detecting the effects of

daily variations in total PM personal exposure from indoor and

outdoor sources. This misunderstanding is evidenced, for example, by

Wyzga and Lipfert's (1995) treatment of the difference between

ambient monitors and actual personal exposures as ``exposure

errors'' and Brown's comment for API that ``if (ambient) PM is

causally related to mortality/morbidity, then it is personal PM

exposure that must be reduced to have an effect.'' On the contrary,

it is personal exposure to ambient PM that must be reduced to

address the risk identified in community air pollution studies. Any

lack of significant correlation between outdoor PM concentrations

and personal exposure to total PM from all sources is irrelevant,

except to the extent it may decrease the power of time-series

studies to detect the effects of ambient pollution.

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Commenters have also restated theoretically based concerns on a

related issue, namely errors in the measurement of the concentrations

of air pollutants, that was summarized in the proposal. In multiple

pollutant analyses, measurement error or, more generally, exposure

misclassification, could theoretically bias effects estimates of PM or

co-pollutants in either direction, introducing further uncertainties in

the estimated concentration-response relationships for all pollutants

(U.S. EPA, 1996b, pp. V-39 to V-43). Relevant insights on this issue in

material appended to public comments (Ozkaynak and Spengler, 1996) have

prompted an expanded statistical analysis of the conditions under which

such errors could inflate the magnitude of the effects estimates or the

significance of PM relative to gaseous pollutants, as has been

suggested by Lipfert and Wyzga (1995). This analysis, which is

summarized in the Response to Comments, finds that the conditions under

which measurement error could inflate the effects estimates or

significance of PM relative to other pollutants are restricted to a

limited set of statistical relationships. Commenters have not provided

evidence that suggest such conditions are likely to occur with respect

to the measurement of ambient PM in relation to those for gaseous co-

pollutants commonly used in epidemiological studies.25

Therefore, it appears unlikely that measurement and exposure errors for

PM and other pollutants have inflated the estimated effects of PM, even

in multivariate analyses. More importantly, the available evidence on

the consistency of the PM-effects relationships in multiple urban

locations, with widely varying indoor/outdoor conditions and a variety

of monitoring approaches, makes it less likely that the observed

associations of PM with serious health effects at levels allowed under

the current NAAQS are an artifact of errors in measurement of pollution

or of exposure (U.S. EPA 1996b, pp. V-39 to V-43).

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25 The EPA analysis finds that in order for measurement errors

in one pollutant variable to significantly bias the estimated effect

of another pollutant, three conditions are necessary: (1) The

measurement error in the poorly measured pollutant must be very

large, roughly at least the same size as the population variability

in that pollutant; (2) the poorly measured pollutant must be highly

correlated with the other pollutant, either positively or

negatively; and (3) the measurement errors for the two pollutants

must be highly negatively correlated (Response to Comments, Appendix

D). This important factor was not considered in Lipfert and Wyzga

(1995) or by commenters.

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(iv) Comments on the PM risk assessment. As noted in the proposal,

uncertainties about measurement errors, exposure misclassification, and

the relative effects of copollutants are more important to the

quantitative estimates of risk associated with PM than to the existence

of valid PM-effects associations at levels found in recent studies. A

number of commenters argued that EPA's risk assessment is flawed and

incomplete. Chief among the reasons they advanced is that the

assessment is based on the same epidemiological studies these

commenters argued are inadequate for the reasons summarized and

responded to above. Specific comments also addressed the extent to

which the risk assessment might overstate risk estimates because it

assumes a linear no-threshold relationship and the use of studies that

might inflate PM risk due to inadequate consideration of co-pollutants

and other potential confounders. The full risk assessment acknowledges

these issues and uncertainties, however, and it illustrates the

potential influence of such uncertainties in sensitivity analyses (U.S.

EPA 1996b; chapter 6, appendix F; Abt Associates, 1996a,b; 1997a,b).

For example, Figure 2c in the proposal (61 FR 65653, December 13, 1996)

[[Page 38665]]

illustrates the potential influence of what appears to be the most

significant uncertainty in current information, whether a population

threshold exists below which the effects of PM no longer occur (61 FR

65653, December 13, 1996). EPA notes that a full consideration of the

uncertainties, including the analysis summarized above on measurement

error, suggests that the epidemiological studies might well have

understated the total effects of air pollution; thus, both the

direction and the extent of any bias in the risk estimates are less

clear than commenters suggest.

EPA believes that, even recognizing the large uncertainties, the

key qualitative insights derived from the risk assessment and

summarized in Unit II.A.3. of this preamble remain appropriate. While

not placing great weight on the specific numerical estimates, EPA

believes that the risk analysis confirms the general conclusions drawn

primarily from the epidemiological results themselves, that there is

ample reason to be concerned that exposure to ambient PM at levels

allowed under the current air quality standards presents a serious

public health problem.

3. Key considerations informing the decision. Having carefully

considered the public comments on the above matters, EPA believes the

fundamental scientific conclusions on the effects of PM reached in the

Criteria Document and Staff Paper, and restated in the introduction to

this unit, remain valid. That is, the epidemiological evidence for

ambient PM, alone or in combination with other pollutants, shows

associations with premature mortality, hospital admissions, respiratory

symptoms, and lung function decrements. Despite extensive critical

examination in the criteria and standards review, these findings cannot

be otherwise explained by analytical, data, or other problems inherent

in the conduct of such studies. Although the evidence from

toxicological studies available during the criteria review has not

revealed demonstrated mechanisms that explain the range of effects

reported in epidemiological studies, it does not and cannot refute the

observation of such effects in exposed populations. Moreover, the

effects observed in the recent epidemiological studies at lower PM

concentrations are both coherent with each other and plausible based on

the categories of effects observed at much higher concentrations in

historic air pollution episodes, laboratory studies of PM effects at

high doses, and particle dosimetry studies. The consistency of the

results from a large number of locations and the coherent nature of the

observed results suggest a likely causal role of ambient PM in

contributing to the reported effects (U.S. EPA, 1996a; p. 13-1). Many

of the studies showing PM effects were conducted in areas where the

current PM10 standards are largely met, and both the studies

and EPA's risk assessment suggest that the collective magnitude of the

effects reflects a significant public health problem.

For these reasons, and having considered public comments on this

issue, the Administrator concludes that the review of the criteria and

standards provides strong evidence that the current PM10

standards do not adequately protect public health, and that revision of

the standards is not only appropriate, but necessary.

Aside from that conclusion, the appropriateness of continuing to

rely on the use of PM10 as the sole indicator for revised PM

standards is also relevant here. While the basis for decisions on

specific indicators is discussed more fully in Unit II.C. of this

preamble, this issue is related to the Administrator's decision on the

need to revise the standards. Based on both the staff review (U.S. EPA,

1996b, p. VII-3) and the recommendations of some commenters (e.g.,

California EPA), there are two alternative approaches for providing

additional health protection in revising the standards: Adopt tighter

PM10 standards and/or recognize the fundamental differences

between fine and coarse particles and develop separate standards for

the major components of PM10, including fine particles.

Conceptually, the first approach would give weight to comments that

standards should be based on pollutant indicators for which the most

data have been collected, with less consideration of the evidence that

suggests that the current standards provide adequate protection against

the effects of coarse particles, and that tightening the current

PM10 standards in an attempt to control fine particles would

place unnecessary requirements on coarse particles. Because the

PM10 network is in place, a more stringent PM10

standard would also respond to commenters who have expressed a desire

for more immediate implementation of revised standards. The second

approach is based on the view that, in the long run, more effective and

efficient protection can be provided by separately targeting

appropriate levels of controls to fine and coarse PM.

The Staff Paper examined this issue in detail (U.S. EPA 1996b, pp.

VII-3 to VII-11), and concluded that the available information was

sufficient to develop separate indicators for fine and coarse fractions

of PM10, based on the recent health evidence, the

fundamental differences between fine- and coarse-fraction particles,

and implementation experience with PM10. Further, the staff

concluded that:

[C]onsideration of comparisons between fine and coarse fractions

suggests that fine fraction particles are a better surrogate for

those particle components linked to mortality and morbidity effects

at levels below the current standards. In contrast, coarse fraction

particles are more likely linked with certain effects at levels

above those allowed by the current PM10 standards. In

examining alternative approaches to increasing the protection

afforded by PM10 standards, the staff concludes that

reducing the levels of the current PM10 standards would

not provide the most effective and efficient protection from these

health effects. [U.S. EPA 1996b; p. 7-45]

As discussed in Unit II.C. of this preamble, the Administrator

believes that it is more appropriate to provide additional protection

against the risk posed by PM by adding new standards for the fine

fraction of PM10, as opposed to tightening the current

PM10 standards. Although fewer epidemiological studies have

used PM2.5 and other fine particle indicators (e.g.,

sulfates, acids), there are nonetheless significant indications from

the scientific evidence - drawn from the physicochemical studies of PM,

air quality and exposure information, toxicological studies, and

respiratory tract deposition data - that this approach will provide the

most effective and efficient protection of public health.

Several commenters have argued that the decision on whether to

revise the PM standards should be deferred, particularly with regard to

fine particle standards, pending establishment and operation of a

national monitoring network to characterize fine PM and a research

program to reduce uncertainties in the effects information. These

commenters expressed concerns that establishing fine PM standards now

might result in needless regulation of PM components that may be

unrelated to observed health effects. As discussed more fully in Unit

II.F. of this preamble, such commenters recommended, at most, that if

fine PM standards were established, they should be set at a level

``equivalent'' to the current PM standards.

EPA strongly disagrees that the decision on revising the standards

should be delayed to await the results of new PM monitoring and

research programs. Under section 109(d) of the Act, EPA's obligation

after reviewing the

[[Page 38666]]

existing criteria and standards for PM is to make such revisions in the

standards and to promulgate such new standards as are appropriate under

section 109(b) of the Act. Based on her review of the criteria and

standards for PM, the Administrator has concluded that the current

standards are not adequate to protect public health and that revisions

are appropriate. In the face of the available evidence, a delay in

revising the standards would not only be inconsistent with the statute

but -- even under the optimistic assumption that the same extensive

monitoring and strategy assessment as now contemplated would occur in

the absence of a revised standard -- would add approximately 2 years to

the time when significant health benefits can be realized, resulting in

potentially significant numbers of additional premature deaths and even

larger numbers of children and individuals with air pollution-related

illness and symptoms. On the other hand, establishing standards now

will set into motion the development of implementation programs and

monitoring that can be conducted in parallel with additional scientific

research, without undue delays inherent in waiting for the research.

The question of which pollutant components to regulate has been an

issue since the inception of the first PM standards. Other ambient

pollutants (e.g., NO2 or CO) are uniquely defined as

individual chemicals, whether or not they serve as proxies for a larger

class of substances (e.g., ozone as an index of photochemical

oxidants). Regulating general PM, as opposed to multiple chemical

components of PM, raises the spectre of a host of particulate materials

of varying composition, size, and other physicochemical properties, not

all of which are likely to produce identical effects.

Both EPA's past and present regulatory experience with PM control

programs and its successive reviews of the standards have reaffirmed

the wisdom of retaining standards that control particles as a group,

rather than eliminating such standards and waiting for scientific

research to develop information needed to identify more precise limits

for the literally thousands of particle components. Each such decision

recognized the possibility that potentially less harmful particles

might be included in the mix that was regulated, but concluded that the

need to provide protection against serious health effects nonetheless

required action under section 109 of the Act. The success of this

approach is evident in early U.S. control programs that dramatically

reduced ``smoke'' and ``TSP'' in major cities in the 1960's and 1970's

and in the continued improvement in air quality through the current PM

standards. The major refinements that have been recommended through the

course of reviews of PM standards have been to improve the focus of

control efforts by defining scientifically based size classes (i.e.,

moving from TSP to PM10 and now, PM2.5) that will

permit more effective and efficient regulation of those fractions most

likely to present significant risks to health and the environment.

As discussed in Unit II.C. of this preamble, the current review has

examined the available evidence to determine whether it would tend to

support inclusion or exclusion of any physical or chemical classes of

PM, for example sulfates, nitrates, or ultra-fine particles. That

examination concludes that, while both fine and coarse particles can

produce health effects, the fine fraction appears to contain more of

the reactive substances potentially linked to the kinds of effects

observed in the recent epidemiological studies (U.S. EPA 1996b, section

V.F.). However, the available scientific information does not rule out

any one of these components as contributing to fine particle effects.

Indeed, it is reasonable to anticipate that no single component will

prove to be responsible for all of the effects of PM.

EPA recognizes that whether the standards are set for

PM10 only or also for fine particles, there are

uncertainties with respect to the relative risk presented by various

components of PM. In this regard, the Administrator places greater

weight on the concern that by failing to act now, the PM NAAQS would

not control adequately those components of air pollution that are most

responsible for serious effects, than on the possibility they might

also control some component that is not. EPA believes that moving

simultaneously to establish standards based on the best available

scientific evidence and to conduct an aggressive monitoring and

scientific research program designed to help resolve current

uncertainties is a prudent and responsible approach for addressing both

the risks and the uncertainties inherent in this important public

health issue.

In summary, given the evidence that PM-related health effects

appear likely to occur at levels below the current standards, the

serious nature and potential magnitude of the public health risks

involved, and the need to consider the fine and coarse fractions as

distinct classes of particles, the Staff Paper and the CASAC (Wolff,

1996b) concluded that revision of the current standards is clearly

appropriate. Moreover, at their May 1996 public meeting (U.S. EPA,

1996c), and in separate written comments (including Lippmann et al.,

1996), a majority of CASAC panel members recommended revisions that

would strengthen the health protection provided by the current PM

standards. Based on the rationale and recommendations contained in the

Staff Paper and the advice of CASAC, and taking into account public

comments, the Administrator concludes that it is appropriate at this

time to revise the current PM standards to increase the public health

protection provided against the known and potential effects of PM

identified in the air quality criteria.

C. Indicators of PM

In establishing adequately protective, effective, and efficient PM

standards, it is necessary to specify the fraction of particles found

in the ambient air that should be used as the indicator(s) for the

standards. In this regard, EPA concludes that the most recent

assessment of scientific information in the Criteria Document,

summarized in chapters IV and V of the Staff Paper, continues to

support past staff and CASAC recommendations regarding the selection of

size-specific indicators for PM standards. More specifically, EPA

continues to find that the following conclusions reached in the Staff

Paper and in the 1987 review remain valid:

(1) Health risks posed by inhaled particles are influenced both by

the penetration and deposition of particles in the various regions of

the respiratory tract and by the biological responses to these

deposited materials.

(2) The risks of adverse health effects associated with deposition

of ambient fine and coarse fraction particles in the thoracic

(tracheobronchial and alveolar) regions of the respiratory tract are

markedly greater than for deposition in the extrathoracic (head)

region. Maximum particle penetration to the thoracic region occurs

during oronasal or mouth breathing.

(3) The risks of adverse health effects from extrathoracic

deposition of general ambient PM are sufficiently low that particles

which deposit only in that region can safely be excluded from the

standard indicator.

(4) The size-specific indicator(s) should represent those particles

capable of penetrating to the thoracic region, including both the

tracheobronchial and alveolar regions.

[[Page 38667]]

These conclusions, together with information on the dosimetry of

particles in humans, were the basis for the promulgation in 1987 of a

new size-specific indicator for the PM NAAQS, PM10, that

includes particles with an aerodynamic diameter smaller than or equal

to a nominal 10 m. The recent information on human particle

dosimetry contained in the Criteria Document provides no basis for

changing 10 m as the appropriate cut point for particles

capable of penetrating to the thoracic regions.

As noted in Unit II.B. of this preamble, however, the Staff Paper

concludes that continued use of PM10 as the sole indicator

for the PM standards would not provide the most effective and efficient

protection from the health effects of PM (U.S. EPA, 1996b, pp. VII-4 to

VII-11). Based on the recent health effects evidence and the

fundamental physical and chemical differences between fine and coarse

fraction particles, the Criteria Document and Staff Paper conclude that

fine and coarse fractions of PM10 should be considered

separately (U.S. EPA, 1996a, p. 13-93; 1996b, p. VII-18). Taking into

account such information, CASAC found sufficient scientific and

technical bases to support establishment of separate standards relating

to these two fractions of PM10. Specifically, CASAC advised

the Administrator that ``there is a consensus that retaining an annual

PM10 NAAQS * * * is reasonable at this time'' and that there

is ``also a consensus that a new PM2.5 NAAQS be

established'' (Wolff, 1996b).

Some commenters have noted that it is often difficult to

distinguish the effects of either fine or coarse fraction particles

from those of PM10; this is to be expected because both

fractions are themselves components of PM10, and hence not

fully independent. EPA believes that it is more meaningful to examine

comparisons between the fine and coarse fraction components. Such

comparisons presented in the Staff Paper suggest that fine particles

are a better surrogate for those components of PM that are linked to

mortality and morbidity effects at levels below the current standards

(U.S. EPA, 1996b, p. VII-18). Moreover, a regulatory focus on fine

particles would likely also result in controls on gaseous precursors of

fine particles (e.g., SOx, NOx, VOC), which are

all components of the complex mixture of air pollution that has most

generally been associated with mortality and morbidity effects. The

Staff Paper concludes that, in contrast to fine particles, coarse

fraction particles are more clearly linked with certain morbidity

effects at levels above those allowed by the current 24-hour standard.

Public comments received on the proposed indicators were

overwhelmingly in favor of EPA's proposal to maintain PM10

as an indicator for PM, whether as an indicator of coarse particles in

conjunction with a fine PM standard, or as the sole PM indicator. This

near unanimity shows strong support for retaining general PM standards.

While a substantial number of commenters supported EPA's proposal to

add an indicator for fine PM, a number of other commenters objected to

any standard revisions, including addition of a fine PM indicator.

Beyond the general points about the basis for any revisions discussed

in Unit II.B. of this preamble, these commenters argued either that the

available epidemiological data did not provide a basis for separating

fine and coarse fraction particles, or that there were not enough fine

particle studies to support selecting standard levels. Most of these

commenters also expressed concerns that there were insufficient ambient

fine particle data by which to evaluate the relative protection

afforded by new standards.

EPA notes that issues relating to the basis for separating

PM10 fractions were addressed in the Criteria Document and/

or Staff Paper assessments, and these perspectives were also available

for CASAC consideration in developing its recommendations. The proposal

states that the main basis for separating the fine and coarse fractions

of PM10 is that, because they are fundamentally different PM

components with significantly different physico-chemical properties and

origins (U.S. EPA 1996b, section V.D), separate standards would permit

more effective and efficient regulation of PM. While the difficulty in

separating these classes in the epidemiological studies is noted above,

the preponderance of the available evidence suggests that strategies to

control fine particles will more effectively reduce population exposure

to substances associated with health effects in the recent

epidemiological studies. Although the number of studies using fine PM

indicators is more limited than for PM10, there are more

than 20 community studies showing significant associations for a

consistent set of mortality and morbidity effects. A substantial subset

of these studies (Tables V-12 to V-13; U.S. EPA, 1996b) provides a

sufficient quantitative basis for selecting standard levels, without

the need to rely on estimates based on PM2.5/PM10

ratios.

Having considered the public comments on this issue, the

Administrator concurs with staff and CASAC recommendations to control

particles of health concern (i.e., PM10) through separate

standards for fine and coarse fraction particles. The following units

outline the basis for the Administrator's decision on specific

indicators for fine and coarse fraction particle standards.

1. Indicators for the fine fraction of PM10. The

Administrator continues to conclude that it is appropriate to control

fine particles as a group, as opposed to singling out particular

components or classes of fine particles. The more qualitative

scientific literature, evaluated in Chapter 11 of the Criteria Document

and summarized in section V.C of the Staff Paper, has reported various

health effects associated with high concentrations of a number of fine

particle components (e.g., sulfates, nitrates, organics, transition

metals), alone or in some cases in combination with gases. Community

epidemiolgical studies have found significant associations between fine

particles or PM10 and health effects in various areas across

the U.S. where such fine particle components correlate significantly

with particle mass. As noted above in this unit, it is not possible to

rule out any one of these components as contributing to fine particle

effects.26 Thus, the Administrator finds that the present

data more readily support a standard based on the total mass of fine

particles. EPA will conduct additional research, in cooperation with

other Federal agencies and in partnership with State and local agencies

and the private sector, to better identify which species are of concern

for human health, and the sources and relative magnitude of such

species.

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26 As discussed above, a number of commenters expressed concerns

that various portions of fine particles might not be responsible for

any observed effects. One group (PG&E, 1997) recommended that

nitrates should be excluded from fine PM mass collected on the basis

of their assessment of available effects literature on particulate

and gas phase inorganic nitrates. Based on an examination of this

information as well as the earlier staff assessment, EPA maintains

its conclusion that the available evidence is not sufficient to

exclude nitrates or any other class of fine particles that are

collected by PM monitors comparable to those used in the recent

epidemiological studies.

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In specifying a precise size range for a fine particle standard,

both the staff and CASAC recommended PM2.5 as the indicator

of fine particles (Wolff, 1996b). The particle diameter reflecting the

mass minimum between the fine and coarse modes typically lies between 1

and 3 m, and the scientific data support a sampling ``cut

point'' to delineate fine particles somewhere in this range. Because of

the potential

[[Page 38668]]

overlap of fine and coarse particle mass in this intermodal region, EPA

recognizes that any specific sampling cut point would result in only an

approximation of the actual fine-mode particle mass. Thus, the choice

of a specific diameter within this size range is largely a policy

judgment. The staff and CASAC recommendations for a 2.5 m

sampling cut point were based on considerations of consistency with the

community health studies, the limited potential for intrusion of coarse

fraction particles into the fine fraction, and availability of

monitoring technology.27 PM2.5 encompasses all of

the potential agents of concern in the fine fraction, including most

sulfates, acids, fine particle transition metals, organics, and

ultrafine particles, and includes most of the aggregate surface area

and particle number in the entire distribution of atmospheric

particles.

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27 The National Mining Association (NMA) and related companies

submitted comments favoring ultimate selection of a smaller cutpoint

of 1 m (PM1) to further reduce coarse particle

intrusion. EPA considered this approach in developing the Staff

Paper and proposal. PM1 has not been used in health

studies, although in most cases collected mass should be similar to

those for cutpoints of 2.1 or 2.5 m. While a PM1

indicator could reduce intrusion of coarse particles, it might also

omit portions of hygroscopic PM components such as acid sulfates,

nitrates, and some organic compounds in higher humidity environments

picked up by PM2.5 measurements. PM1 sampling

technologies have been developed, but have not been widely used in

the field to date; there are some concerns about loss of certain

organic materials in available models relative to an instrument with

a larger size cut. NMA has also recommended consideration of a

methodology that could subtract coarse mass from PM2.5

measurements where undue coarse particle intrusion resulted in fine

standard violations. EPA will evaluate this recommendation in the

context of implementation policies.

---------------------------------------------------------------------------

The Administrator concurs with the staff and CASAC recommendations

and concludes that PM2.5 is the appropriate indicator for

fine particle standards. As discussed in Unit VI.B. of this preamble,

technical details of how PM2.5 is to be measured in the

ambient air are specified in the Federal Reference Method (40 CFR part

50, Appendix L).

2. Indicators for the coarse fraction of PM10.

The Criteria Document and Staff Paper conclude that

epidemiological information, together with dosimetry and toxicological

information, support the need for a particle indicator that addresses

the health effects associated with coarse fraction particles within

PM10 (i.e., PM10-2.5). As noted above, coarse

fraction particles can deposit in those sensitive regions of the lung

of most concern. Although the role of coarse fraction particles in much

of the recent epidemiological results is unclear, limited evidence from

studies where coarse fraction particles are the dominant fraction of

PM10 suggest that significant short-term effects related to

coarse fraction particles include aggravation of asthma and increased

upper respiratory illness. In addition, qualitative evidence suggests

that potential chronic effects may be associated with long-term

exposure to high concentrations of coarse fraction particles.

In selecting an indicator for coarse fraction particles, the

Administrator took into account the views of several CASAC panel

members who suggested using the coarse fraction directly (i.e.,

PM10-2.5) as the indicator. However, the Administrator notes

that the existing ambient data base for coarse fraction particles is

smaller than that for fine particles, and that the only studies of

clear quantitative relevance to effects most likely associated with

coarse fraction particles have used undifferentiated PM10.

In fact, it was the consensus of CASAC that it is reasonable to

consider PM10 itself as a surrogate for coarse fraction

particles, when used together with PM2.5 standards. The

monitoring network already in place for PM10 is large.

Therefore, in conjunction with the decision to have separate standards

for PM2.5, the Administrator concludes, consistent with

CASAC recommendations and public comments, that it is appropriate to

retain PM10 as the indicator for PM standards intended to

protect against the effects most likely associated with coarse fraction

particles.

D. Averaging Time of PM2.5 Standards

As discussed above in this unit, the Administrator has concluded

that PM2.5 is an appropriate indicator for standards

intended to provide protection from effects associated primarily with

fine particles. The recent health effects information includes reported

associations with both short-term (from less than 1 day to up to 5

days) and long-term (from a year to several years) measures of PM.

On the basis of this information, summarized in chapter V of the

Staff Paper and in the rationale presented in the proposal, the

Administrator has considered both short- and long-term PM2.5

standards.

1. Short-term PM2.5 standard. The current 24-hour

averaging time is consistent with the majority of community

epidemiological studies, which have reported associations of health

effects with 24-hour concentrations of various PM indicators such as

PM10, fine particles, and TSP. Such health effects,

including premature mortality and increased hospital admissions, have

generally been reported with same-day, previous day, or longer lagged

single-day concentrations, although some studies have reported stronger

associations with multiple-day average concentrations. In any case, the

Administrator recognizes that a 24-hour PM2.5 standard can

effectively protect against episodes lasting several days, since

attainment of such a standard would provide protection on each day of a

multi-day episode, while also protecting sensitive individuals who may

experience effects after even a single day of exposure.

Although most reported effects have been associated with daily or

longer measures of PM, evidence also suggests that some effects may be

associated with PM exposures of shorter durations. For example,

controlled human and animal exposures to specific components of fine

particles, such as acid aerosols, suggest that bronchoconstriction can

occur after exposures of minutes to hours. Some epidemiological studies

of exposures to acid aerosols have also found changes in respiratory

symptoms in children using averaging times less than 24 hours. However,

such reported results do not provide a satisfactory quantitative basis

for setting a fine particle standard with an averaging time of less

than 24 hours, nor do current gravimetric mass monitoring devices make

such shorter durations generally practical at present. Further, the

Administrator recognizes that a 24-hour average PM2.5

standard which leads to reductions in 24-hour average concentrations is

likely to lead as well to reductions in shorter-term average

concentrations in most urban atmospheres, thus providing some degree of

protection from potential effects associated with shorter duration

exposures.

2. Long-term PM2.5 standard. Community epidemiological

studies have reported associations of annual and multi-year average

concentrations of PM10, PM2.5, sulfates, and TSP

with an array of health effects, notably premature mortality, increased

respiratory symptoms and illness (e.g., bronchitis and cough in

children), and reduced lung function. The relative risks associated

with such measures of long-term exposures, although highly uncertain,

appear to be larger than those associated with short-term exposures.

Based on the available epidemiology, and consistent with the limited

relevant toxicological and dosimetric information, the Administrator

concludes that significant, and potentially independent, health

consequences are likely associated with long-term PM exposures.

[[Page 38669]]

The Administrator has considered this evidence, which suggests that

some health endpoints reflect the cumulative effects of PM exposures

over a number of years. In such cases, an annual standard would provide

effective protection against persistent long-term (several years)

exposures to PM. Requiring a much longer averaging time would also

complicate and unnecessarily delay control strategies and attainment

decisions.

The Administrator has also considered the seasonality of emissions

of fine particles and their precursors in some areas (e.g., wintertime

smoke from residential wood combustion, summertime regional acid

sulfate and ozone formation), which suggests that some effects

associated with annual average concentrations might be the result of

repeated seasonally high exposures. However, different seasons are

likely of concern in different parts of the country, and the current

evidence does not provide a satisfactory quantitative basis for setting

a national fine particle standard in terms of a seasonal averaging

time.

In addition, the Administrator recognizes that an annual standard

would have the effect of improving air quality broadly across the

entire annual distribution of 24-hour PM2.5 concentrations,

although such a standard would not as effectively limit peak 24-hour

concentrations as would a 24-hour standard. The risk assessment

summarized above found that because such 24-hour peaks contribute much

less to the total health risk over a year than the more numerous low-

to mid-range PM2.5 levels, an annual standard could also

provide effective protection from health effects associated with short-

term exposures to PM2.5 as well as those associated with

long-term exposures (see figure 2; 61 FR 65652-65653, December 13,

1996).

3. Combined effect of annual and 24-hour standards. For the reasons

outlined in Units II.C.1. and 2. of this preamble, the Administrator

concluded in the proposal that a short-term PM2.5 standard

with a 24-hour averaging time can serve to control short-term ambient

PM2.5 concentrations, thus providing protection from health

effects associated with short-term (from less than 1-day to up to 5-

day) exposures to PM2.5. Further, a long-term

PM2.5 standard with an annual averaging time can serve to

control both long- and short-term ambient PM2.5

concentrations, thus providing protection from health effects

associated with long-term (seasonal to several years) and, to some

degree, short-term exposures to PM2.5.

EPA received comparatively few public comments on these proposed

averaging times. Those supporting PM2.5 standards also

strongly supported adopting both annual and 24-hour averaging times.

Many of those opposing PM2.5 standards, for the reasons

discussed in Unit II.B. of this preamble, provided contingent comments

that variously supported both averaging times for PM2.5

standards in the event the Administrator disagreed with their overall

recommendations. Other opponents of PM2.5 standards

disagreed with having two standards on administrative grounds, or

because some CASAC members did not support both averaging times.

The relationship between standards for the two averaging times is

discussed below in this unit. In essence, based on its examination of

the effects data and air quality relationships, EPA believes that a

single PM2.5 standard (24-hour or annual) either would not

provide adequate protection against effects of concern for all

averaging times, or would be inefficient in the sense that it was more

stringent than necessary for at least one averaging time. Contrary to

commenters who focused on minority CASAC opinions, EPA notes that a

clear majority of CASAC supported both 24-hour and annual

standards28. After considering public comments on averaging

time and the rationale outlined above, the Administrator has concluded

that both 24-hour and annual PM2.5 standards are

appropriate.

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28 Of the 19 panel members who joined in the consensus for

PM2.5 standards, 17 (90 percent) recommended a 24-hour

standard and 13 (70 percent) recommended an annual standard (Wolff,

1996b).

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The Administrator next considered the potential combined effects of

such standards on PM concentration levels and distributions. The

existing health effects evidence could, of course, be used to assess

the form and level of each standard independently, with short-term

exposure health effects evidence being used as the basis for a 24-hour

standard and the long-term exposure health effects evidence as the

entire basis for an annual standard. Some CASAC panel members

apparently used this approach as a basis for their views on appropriate

averaging times and standard levels. In particular, a few members

focused only on a 24-hour PM2.5 standard in light of the

relative strength of the short-term exposure studies. On the other

hand, two members focused only on an annual standard, recognizing that

strategies to meet an annual standard would provide protection against

effects of both short- and long-term exposures.

As noted above in this unit, attempting to provide protection for

all of the effects identified in long- and short-term PM exposure

studies with a single averaging time would result in either inadequate

protection for some effects, or unnecessarily stringent control for

others. The Administrator has, instead, emphasized a policy approach

that considers the consistency and coherence, as well as the

limitations, of the body of evidence as a whole, and recognizes that

there are various ways to combine two standards to achieve an

appropriate degree of public health protection. Such an approach to

standard setting, which integrates the body of health effects evidence

and air quality analyses, and considers the combined effect of the

standards, has the potential to result in a more effective and

efficient suite of standards than an approach that only considers

short- and long-term exposure evidence, analyses, and standards

independently.

In considering the combined effect of such standards, the

Administrator notes that while an annual standard would focus control

programs on annual average PM2.5 concentrations, it would

also result in fewer and lower 24-hour peak concentrations.

Alternatively, a 24-hour standard that focuses controls on peak

concentrations could also result in lower annual average

concentrations. Thus, either standard could be viewed as providing both

short- and long-term protection, with the other standard serving to

address situations where the daily peaks and annual averages are not

consistently correlated.

The Administrator proposed that the suite of PM2.5

standards could most effectively and efficiently be defined by treating

the annual standard as the generally controlling standard for lowering

both short- and long-term PM2.5 concentrations. In

conjunction with the annual standard, the 24-hour standard would serve

to provide protection against days with high peak PM2.5

concentrations, localized ``hot spots,'' and risks arising from

seasonal emissions that would not be well controlled by a national

annual standard.

Relatively few public comments were addressed specifically to the

proposal that the annual standard be directed toward controlling both

24-hour and annual levels (thereby basing the annual standard on an

evaluation of both the short- and long-term health effects

information), with the 24-hour standard being used to address more

localized short-term peaks. A number of commenters, notably some among

the groups opposing any revised PM

[[Page 38670]]

standards, appeared to have ignored this fundamental aspect of the

proposal, judging by their assertions that the sole basis for EPA's

proposed annual standards was two long-term exposure studies (Dockery

et al., 1993; Pope et al. 1995). This is incorrect; as the proposal

states, EPA based the proposed annual standard level on a wider range

of short- and long-term exposure studies. Other commenters, including

some environmental groups, reserved comment on this specific issue, but

expressed concerns that the specific levels for both standards were not

stringent enough, regardless of which standard is intended to be

controlling. Issues regarding specific levels are discussed below in

Unit II.F. of this preamble.

Some commenters, however, disagreed with the proposition that EPA's

proposed approach would necessarily provide the most effective and

efficient standards. In the view of some who opposed PM2.5

standards, the likelihood that there are thresholds below which no

effects occur means that a 24-hour standard would be more efficient

than an annual standard. In this view, the reductions made on days that

were below the threshold would provide no protection.29 Some

commenters also noted that while a majority of CASAC members favored

both annual and 24-hour standards, more recommended 24-hour standards.

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29 A related comment criticized the risk assessment conclusion

that peak 24-hour concentrations contribute much less to the total

risk over a year as inconsistent with the experience in historic air

pollution episodes. EPA disagrees. While the historic London

episodes were quantitatively different from those assumed in the

risk assessment, the record over 14 London winters indicates a

continuum of effects down to the lowest levels. It is therefore

likely that the cumulative increase in mortality calculated for all

the days in the whole 14-year period would not be dominated by the

more limited number of episode days.

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While the available epidemiological studies provide strong evidence

suggesting that PM causes or contributes to health effects at levels

below the current standards, EPA agrees, as stated previously, that

uncertainties increase markedly at lower concentrations. Nevertheless,

the level or even existence of population thresholds below which no

effects occur cannot be reliably determined by an examination of the

results from the available studies. Analyses have placed some limits,

however, and EPA has considered hypothetical thresholds in its risk

assessment. As noted in Unit II.A. of this preamble, even assuming an

example threshold of 18 g/m3, the risk assessment

(see Figure 2c; 61 FR 65653, December 13, 1996) finds that most of the

annual aggregate risk associated with short-term exposures still

results from the large number of days at lower to mid-range values

above the mean. Given that neither the Criteria Document nor commenters

have provided quantitative evidence regarding the likelihood of a

threshold at levels much higher than the above example, EPA believes

that the evidence provided in the risk assessment does not support the

commenters' position. As noted above, EPA believes that most CASAC

opinions on averaging time reflect panelists' judgments on the relative

strength of the short-term exposure epidemiological studies, a judgment

that EPA shares. Although most CASAC panel members did not offer an

opinion on the use of short-term exposure studies in specifying annual

standards, two panelists did support this notion. EPA therefore

believes this approach is neither inconsistent with the underlying

science nor discordant with the advice of CASAC.

Another concern was raised by some air pollution control officials

who otherwise supported revised PM standards. These commenters state

that, from an implementation perspective, it is often easier to design

control strategies for single short-term events than for annual

averages. Aside from whether this is a proper consideration in

establishing NAAQS, the point in fact highlights one of the important

strengths of an annual standard in addressing short-term risks

associated with PM2.5. As noted by the commenters, risk

management for a short-term standard focuses on a characteristic

``design value'' episode responsible for peak concentrations. For PM,

such peak values can be associated with single source contributions.

Meteorology, relative source contributions, and resulting particle

composition for that day may or may not be typical for the area or for

the year. Yet the short-term exposure epidemiological results are

largely drawn from studies that associated variations in area-wide

effects with monitor(s) that gauged the variation in daily levels over

the course of up to 8 years. The strength of the associations in these

data is demonstrably in the numerous ``typical'' days in the upper to

middle portion of the annual distribution, not on the peak

days.30 For these reasons, strategies that focus only on

reducing peak days are less likely to achieve reduction of the mix and

sources of urban and regional-scale PM pollution most strongly

associated with health effects. Although designing control strategies

to reduce annual levels may be more difficult than for 24-hour

standards, the available short- and long-term epidemiological data

suggest it is also likely to result in a greater reduction in area-wide

population exposure and risk.

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30 This point is buttressed by studies that have taken out a

limited number of higher PM concentration days with little effect on

the effects estimates or significance of the association (e.g.,

Schwartz et al., 1996; Pope and Dockery, 1992).

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The Administrator concludes that the most effective and efficient

approach to establishing PM2.5 standards is to treat the

annual standard as the generally controlling standard for lowering both

short- and long-term PM2.5 concentrations, while the 24-hour

standard would serve to provide protection against days with high peak

PM2.5 concentrations, localized ``hot spots,'' and risks

arising from seasonal emissions that would not be well controlled by a

national annual standard. In reaching this view, the Administrator took

into account the public comments and the factors discussed below in

this unit.

(1) Based on one of the key observations from the quantitative risk

assessment summarized above (see Figures 2a,b,c; 61 FR 65652-65653,

December 13, 1996), the Administrator notes that much if not most of

the aggregate annual risk associated with short-term exposures results

from the large number of days during which the 24-hour average

concentrations are in the low- to mid-range, below the peak 24-hour

concentrations. As a result, lowering a wide range of ambient 24-hour

PM2.5 concentrations, as opposed to focusing on control of

peak 24-hour concentrations, is the most effective and efficient way to

reduce total population risk. Further, there is no evidence suggesting

that risks associated with long-term exposures are likely to be

disproportionately driven by peak 24-hour concentrations. Thus, an

annual standard that controls an area's attainment status is likely to

reduce aggregate risks associated with both short- and long-term

exposures with more certainty than a 24-hour standard.

(2) The consistency and coherence of the health effects data base

are, therefore, more directly related to the more frequently occurring

PM exposures reflected in study period mean measures of air quality

(e.g., the annual distributions of 24-hour PM concentrations), than to

the potentially site-specific and/or otherwise infrequent PM exposures

reflected in a limited number of peak 24-hour concentrations. More

specifically, judgments about the quantitative consistency of the large

number of short-term exposure studies

[[Page 38671]]

reporting associations with 24-hour concentrations arise from comparing

the relative risk results per PM increment as derived from analyzing

the associations across the entire duration of the studies. These

studies typically spanned at least an annual time frame and the

reported associations are most strongly influenced by the large number

of days toward the middle of the distribution.

(3) An annual average measure of air quality is more stable over

time than are 24-hour measures. Thus, a controlling annual standard is

likely to result in the development of more consistent risk reduction

strategies over time, since an area's attainment status will be less

likely to change due solely to year-to-year variations in

meteorological conditions that affect the formation of fine particles,

than under a controlling 24-hour standard.

Under this policy approach, the annual PM2.5 standard

would serve in most areas as the target for control programs designed

to be effective in lowering the broad distribution of PM2.5

concentrations, thus protecting not only against long-term effects but

also short-term effects as well. In combination with such an annual

standard, the 24-hour PM2.5 standard would be set so as to

protect against the occurrence of peak 24-hour concentrations,

particularly peak concentrations that present localized or seasonal

exposures of concern in areas where the highest 24-hour-to-annual mean

PM2.5 ratios are appreciably above the national average.

E. Form of PM2.5 Standards

1. Annual standard. As discussed in some detail during the last

review of the PM NAAQS (see 49 FR 10408, March 20, 1984; 52 FR 24634,

July 1, 1987) and in the December 13, 1996 proposal, the annual

arithmetic mean form of the current annual PM10 standard

(i.e., the annual arithmetic mean averaged over 3 years) is a

relatively stable measure of air quality that reflects the total

cumulative dose of PM to which an individual or population is exposed.

Short-term peaks have an influence on the arithmetic mean that is

proportional to their frequency, magnitude, and duration, and, thus,

their contribution to cumulative exposure and risk. As a result, the

annual arithmetic mean form of an annual standard provides protection

across a wide range of the air quality distribution contributing to

exposure and risk, in contrast to other forms, such as the geometric

mean, that de-emphasize the effects of short-term peak concentrations.

While almost no commenters took specific issue with use of an

annual arithmetic mean, a number of commenters disagreed with averaging

over 3 years for both the annual and 24-hour standards because of their

desire for quick action in the initial implementation of

PM2.5 controls. The Administrator recognizes the importance

of promptly implementing appropriate control programs, but she does not

believe that implementation start-up concerns are an adequate basis for

adopting a form (e.g., a single year annual average) that would provide

less stable risk reduction in the long-run. Therefore, the

Administrator continues to concur with the Staff Paper recommendation,

supported by CASAC, to use the annual arithmetic mean, averaged over 3

years, as the form for an annual PM2.5 standard consistent

with the current form of the annual PM10 standard.

Nevertheless, EPA intends to address the concerns of those who

commented that the 3-year form might prevent the public from being

informed about the air quality status of their communities. As outlined

in Unit II.H. of this preamble, EPA plans to issue revised Pollutant

Standard Index criteria for PM2.5, to ensure the public is

informed promptly about air quality status.

The Staff Paper and some CASAC panel members also recommended that

consideration be given to calculating the PM2.5 annual

arithmetic mean for an area by averaging the annual arithmetic means

derived from multiple monitoring sites within a monitoring planning

area. In proposing a calculation method for annual arithmetic averages

that involves spatial averaging of monitoring data, the Administrator

reasoned as follows:

(1) Many of the community-based epidemiological studies examined in

this review used spatial averages, when multiple monitoring sites were

available, to characterize area-wide PM exposure levels and the

associated population health risk. In those studies that used only one

monitoring location, the selected site was chosen to represent

community-wide exposures, not the highest value likely to be

experienced within the community. Thus, spatial averages are most

directly related to the epidemiological studies used as the basis for

the proposed revisions to the PM NAAQS.

(2) As a part of the overall policy approach discussed in Unit

II.D. of this preamble, the annual PM2.5 standard would be

intended to reduce aggregate population risk from both long- and short-

term exposures by lowering the broad distribution of PM2.5

concentrations across the community. An annual standard based on

spatially averaged concentrations would better reflect area-wide PM

exposure levels than would a standard based on concentrations from a

single monitor with the highest measured values.

(3) Under this policy approach, the 24-hour PM2.5

standard would be intended to work in conjunction with a spatially

averaged annual PM2.5 standard by providing protection

against peak 24-hour concentrations, localized ``hot spots,'' and

higher PM2.5 concentrations arising from seasonal emissions

and meteorology that would not be as well controlled by an annual

standard. Accordingly, the 24-hour PM2.5 standard should be

based on the single population-oriented monitoring site within the

monitoring planning area with the highest measured values.

Based on these considerations, the Administrator proposed that the

form of an annual PM2.5 standard be expressed as the annual

arithmetic mean, temporally averaged over 3 years and spatially

averaged over all designated monitoring sites,31 which, in

conjunction with a 24-hour PM2.5 standard, was intended to

provide the most appropriate target for reducing area-wide population

exposure to fine particle pollution. Recognizing the complexities that

spatial averaging might introduce into risk management programs, in the

proposal the Administrator also requested comment on the alternative of

basing the annual standard for PM2.5 solely on the single

population-oriented monitor site within the monitoring planning area

with the highest 3-year average annual mean.

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31 The notice of proposed revisions to 40 CFR part 58 recognized

that a single appropriately sited monitor could suffice for an area

in place of an average of multiple monitors.

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The proposed approach to designating sites that are appropriate for

spatial averaging was based on criteria and constraints contained in

the proposed revision to the monitoring siting and network planning

requirements in 40 CFR part 58. In proposing this approach, the

Administrator noted concerns regarding the development and

implementation of appropriate and effective criteria for the selection

of sites and designations of areas for spatial averaging.

A number of commenters who otherwise favored setting

PM2.5 standards objected to the concept of population-

oriented monitors and expressed the view that any monitor regardless of

where it was sited should be eligible for comparison to the annual

PM2.5 standard. They further maintained that the proposed

provisions for spatial averaging would fail to provide adequate health

protection because

[[Page 38672]]

``clean areas'' and ``dirty areas'' would be averaged together. Some

commenters expressed concern that the proposed constraints on spatial

average would not be sufficient to prevent use of such averaging to

avoid pollution abatement. Others may not have fully understood the

implications of the specific constraints and siting requirements

discussed in the proposed revisions to 40 CFR part 58, which were

intended to ensure that the population-oriented monitors used for the

annual standard were actually reflective of community-wide exposures

and that the spatial averages did not include non-representative

monitored values from either ``clean areas'' or ``dirty

areas.''32 In order to clarify the intent that the spatially

averaged annual standard protect those in smaller communities, as well

as those in larger population centers, the final revisions to 40 CFR

part 58 adopt the term ``community-oriented'' monitors.

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32 The 40 CFR part 58 proposed rule identified the proposed

criteria for monitors to be averaged; namely, monitors must be

properly sited to reflect population-orientation, primarily

influenced by similar sources, and within +/-20 percent of the

average levels and a specific degree of correlation (or meet a

``

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