# Lowering Miners' Exposure to Respirable Crystalline Silica and Improving Respiratory Protection

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URL: https://www.frixlaw.com/law-library/documents/fr%3A2024-06920

## Record

- **Collection:** Federal Register
- **Document type:** Rule
- **Published:** April 18, 2024
- **Citation:** 89 FR 28218

## Text

DEPARTMENT OF LABOR
Mine Safety and Health Administration
30 CFR Parts 56, 57, 60, 70, 71, 72, 75, and 90
[Docket No. MSHA-2023-0001]
RIN 1219-AB36
Lowering Miners' Exposure to Respirable Crystalline Silica and Improving Respiratory Protection

AGENCY:

Mine Safety and Health Administration (MSHA), Department of Labor.

ACTION:

Final rule.

SUMMARY:

The Mine Safety and Health Administration (MSHA) is amending its existing standards to better protect miners against occupational exposure to respirable crystalline silica, a significant health hazard, and to improve respiratory protection for miners from exposure to airborne contaminants. MSHA's final rule also includes other requirements to protect miner health, such as exposure sampling, corrective actions to be taken when a miner's exposure exceeds the permissible exposure limit, and medical surveillance for metal and nonmetal mines.

DATES:

Effective date:
The final rule is effective June 17, 2024, except for amendments 21, 22, 25, 26, 27, 30, 31, 34, 35, 36, 38, 39, 42, 43, 46, 47, 50, 51, 54, 55, 59, 60, 63, 64, 68, 69, 73, 74, 77, 78, 81, 82, 83, 86, 87, 90, 91, 94, 95, 98, 99, 102, 103, 106, 107, 110, and 111, which are effective April 14, 2025, and amendments 4, 5, 8, 9, 13, 14, 17, and 18, which are effective April 8, 2026.

Incorporation by reference date:
The incorporation by reference of certain materials listed in the rule is approved by the Director of the Federal Register beginning June 17, 2024, except for the material in amendment 60, which is approved beginning April 14, 2025, and the material in amendments 9 and 18, which is approved beginning April 8, 2026. The incorporation by reference of certain other material listed in the rule was approved by the Director of the Federal Register as of July 10, 1995.

Compliance dates:
Compliance with this final rule is required April 14, 2025 for coal mine operators and April 8, 2026 for metal and nonmetal mine operators.

FOR FURTHER INFORMATION CONTACT:

S. Aromie Noe, Director, Office of Standards, Regulations, and Variances, MSHA, at:
silicaquestions@dol.gov
(email); 202-693-9440 (voice); or 202-693-9441 (facsimile). These are not toll-free numbers.

SUPPLEMENTARY INFORMATION:

The preamble to the final standard follows this outline:

I. Executive Summary

II. Pertinent Legal Authority

III. Regulatory History

IV. Background

V. Health Effects Summary

VI. Final Risk Analysis Summary

VII. Feasibility

VIII. Summary and Explanation of the Final Rule

IX. Summary of Final Regulatory Impact Analysis and Regulatory Alternatives

X. Final Regulatory Flexibility Analysis

XI. Paperwork Reduction Act

XII. Other Regulatory Considerations

XIII. References

XIV. Appendix

Acronyms and Abbreviations

COPD chronic obstructive pulmonary disease

ESRD end-stage renal disease

FEV forced expiratory volume

FRA final risk analysis

FRIA final regulatory impact analysis

FVC forced vital capacity

L/min liters per minute

mg milligram

mg/m
3
milligrams per cubic meter

mL milliliter

µg/m
3
micrograms per cubic meter

MNM metal and nonmetal

MRE Mining Research Establishment

NMRD nonmalignant respiratory disease

PEL permissible exposure limit

PMF progressive massive fibrosis

PRA preliminary risk analysis

RCMD respirable coal mine dust

REL recommended exposure limit

SiO
2
silica

TB tuberculosis

TLV® Threshold Limit Value

TWA time-weighted average

I. Executive Summary

A. Purpose of the Regulatory Action

The purpose of this final rule is to reduce occupational disease in miners and to improve respiratory protection against airborne contaminants. The rule sets the permissible exposure limit (PEL) of respirable crystalline silica at 50 micrograms per cubic meter of air (µg/m
3
) for a full-shift exposure, calculated as an 8-hour time weighted average (TWA) for all mines. This rule also establishes an action level for respirable crystalline silica of 25 µg/m
3
for a full-shift exposure, calculated as an 8-hour TWA for all mines. In addition to the PEL and action level, the rule includes provisions for methods of compliance, exposure monitoring, corrective actions, respiratory protection, medical surveillance for metal and nonmetal (MNM) mines, and recordkeeping.

The statutory authority for this rule is provided by the Mine Act under sections 101(a), 103(h), and 508. 30 U.S.C. 811(a), 813(h), and 957. A full discussion of Mine Act legal requirements can be found in
Section II. Pertinent Legal Authority.
MSHA implements and administers the provisions of the Mine Act to prevent death, illness, and injury from mining and promote safe and healthful workplaces for miners.

Respirable crystalline silica is classified by the International Agency for Research on Cancer (IARC) as a human carcinogen. Occupational exposure to respirable crystalline silica results in adverse health effects and increases risk of death. The adverse health effects include silicosis (
i.e.,
acute silicosis, accelerated silicosis, chronic silicosis, and progressive massive fibrosis), nonmalignant respiratory diseases (
e.g.,
emphysema and chronic bronchitis), lung cancer, and kidney disease. Each of these effects is chronic, irreversible, and potentially disabling or fatal. Occupational exposure to respirable crystalline silica at mines occurs most commonly from respirable dust generated during mining activities, such as cutting, sanding, drilling, crushing, grinding, sawing, scraping, jackhammering, excavating, and hauling of materials that contain silica.

Existing standards pertaining to respirable crystalline silica for both MNM and coal mines have been in place since the early 1970s. For MNM mines, the existing standards, established by the Department of Interior, Bureau of Mines, in 1974, helped protect miners from the most dangerous levels of exposure to respirable crystalline silica. The existing MNM PELs for the three polymorphs of respirable crystalline silica are: 0.1 mg/m
3
or 100 micrograms per cubic meter of air (µg/m
3
) for quartz; 0.05 mg/m
3
or 50 µg/m
3
for cristobalite; and 0.05 mg/m
3
or 50 µg/m
3
for tridymite. Existing standards for coal mines, first established by the Federal Coal Mine Health and Safety Act of 1969 as interim standards in 1970, control miners' exposures to respirable crystalline silica indirectly by reducing the respirable coal mine dust standard when quartz is present. The exposure limit for respirable crystalline silica during a coal miner's shift is 100 µg/m
3
, reported as an equivalent concentration as measured by the Mining Research Establishment (MRE) instrument.

However, since the promulgation of these existing standards, the National Institute for Occupational Safety and Health (NIOSH) has recommended a

lower respirable crystalline silica exposure level of 50 µg/m
3
for all workers, including miners. In 2016, the Occupational Safety and Health Administration (OSHA) established a PEL of 50 and an action level of 25 µg/m
3
as an 8-hour TWA in the general and construction industries and maritime sector that it regulates. In the mining industry, however, the higher PELs have remained in place for miners in both the MNM sector and the coal sector.

To better protect miners' health, therefore, with this final rule MSHA is lowering its existing exposure limits for quartz or respirable crystalline silica to 50 µg/m
3
and setting an action level of 25 μg/m
3
for all miners. As discussed in
Section V. Health Effects Summary
and
Section VI. Final Risk Analysis Summary,
lowering the PEL will substantially reduce health risks to miners. This final rule also provides a uniform, streamlined regulatory framework to ensure consistent protection across mining sectors and make compliance more straightforward. As discussed in
Section VII. Feasibility
and
Section IX. Summary of Final Regulatory Impact Analysis and Regulatory Alternatives,
compliance with the final rule is technologically and economically feasible, and the final rule has quantified benefits in terms of avoided deaths and illnesses that greatly outweigh the costs, as well as other important unquantified benefits.

B. Summary of Major Provisions

MSHA amends its existing standards on respirable crystalline silica or quartz, after considering all the testimonies and written comments the Agency received from a variety of stakeholders, including miners, mine operators, labor unions, industry trade associations, government officials, and public health professionals, in response to its notice of proposed rulemaking. Below is a summary of major provisions in the final rule.
Section VIII. Summary and Explanation of the Final Rule
discusses each provision in the final rule.

This final rule:

1.
Establishes a uniform permissible exposure limit (PEL) and action level for all mines.
The rule sets a PEL for respirable crystalline silica at 50 micrograms per cubic meter of air (µg/m
3
) over a full shift, calculated as an 8-hour TWA and an action level at 25 µg/m
3
over a full shift, calculated as an 8-hour TWA for all mines.

2.
Requires exposure monitoring for respirable crystalline silica.
Mine operators are required to conduct sampling to assess miners' exposures to respirable crystalline silica. Mine operators are also required to evaluate the impact of mining production, processes, equipment, engineering controls, and geological condition changes on respirable crystalline silica exposures.

3.
Updates the standard for respirable crystalline silica sampling.
ISO 7708:1995(E),
Air quality—Particle size fraction definitions for health-related sampling,
First Edition, 1995-04-01 (ISO 7708:1995), is incorporated by reference. The final rule requires mine operators to conduct sampling for respirable crystalline silica using respirable particle size-selective samplers that conform to ISO 7708:1995, which is the international consensus standard that defines sampling conventions for particle size fractions used in assessing possible health effects of airborne particles in the workplace and ambient environment.

4.
Requires immediate reporting and corrective action to remedy overexposures.
Whenever an overexposure is identified, mine operators must immediately report to MSHA and take corrective action to lower the concentration of respirable crystalline silica to at or below the PEL, resample to determine the efficacy of the corrective action taken, and make a record of all sampling and corrective actions that were taken.

5.
Specifies methods of controlling respirable crystalline silica.
All mines are required to install, use, and maintain feasible engineering controls as the primary means of controlling respirable crystalline silica; administrative controls may be used, when necessary, as a supplementary control.

6.
Requires temporary use of respirators at metal and nonmetal mines when miners must work in concentrations above the PEL.
When MNM miners must work in concentrations of respirable crystalline silica above the PEL while engineering controls are being developed and implemented or it is necessary by nature of the work involved, the mine operator shall use respiratory protection as a temporary measure.

7.
Updates the respiratory protection standard.
ASTM F3387-19,
Standard Practice for Respiratory Protection,
approved August 1, 2019 (ASTM F3387-19), is incorporated by reference. When approved respirators are used, the mine operator must have a written respiratory protection program to protect miners from airborne contaminants, including respirable crystalline silica, in accordance with ASTM requirements.

8.
Requires medical surveillance at MNM mines.
Metal and nonmetal mine operators are required to provide to all miners, including those who are new to the mining industry, periodic medical examinations performed by a physician or other licensed health care professional (PLHCP) or specialist, at no cost to the miner. Like coal miners, MNM miners will be able to monitor their health and detect early signs of respiratory illness.

The requirements in the new part 60 will take effect on June 17, 2024. For coal mine operators, compliance with part 60 is required by 12 months after the publication date; for MNM operators, compliance is required by 24 months after the publication date. The delayed compliance is to strike a balance between meeting the urgent need to protect miners from this health hazard and giving mining operators adequate preparation time to allow them to comply effectively with the new requirements.

In addition, conforming amendments to parts 56, 57, 70, 71, 72, 75, and 90 will take effect on June 17, 2024. Compliance with conforming amendments to parts 56 and 57 is required by 24 months after the publication date; and compliance with conforming amendments to parts 70, 71, 72, 75, and 90 is required by 12 months after the publication date.

C. Summary of Final Regulatory Impact Analysis

MSHA's economic analysis estimates that the final rule would cost approximately an average of $89 million per year in 2022 dollars at an undiscounted rate, $90 million at a 3 percent discount rate, and $92 million at a 7 percent discount rate. Based on the results of the Final Regulatory Impact Analysis (FRIA), MSHA estimates that this final rule's monetized benefits would exceed its costs, with or without discount rates. Monetized benefits are estimated from avoidance of 531 deaths related to NMRD, silicosis, ESRD, and lung cancer and 1,836 cases of silicosis associated with silica exposure over the first 60-year period after the promulgation of the final rule. The estimated annualized net benefit is approximately $294 million at an undiscounted rate, $157 million at a 3 percent discount rate, and $40 million at a 7 percent discount rate.

A rule is significant under Executive Order 12866 Section 3(f)(1), as amended by E.O. 14094, if it is likely to result in “an annual effect on the economy of $200 million or more.” The Office of Management and Budget has determined that the final rule is significant under E.O. 12866 Section 3(f)(1).

In summary, this final rule will strengthen MSHA's existing regulatory framework and improve health protections for the nation's miners. It establishes a uniform PEL that aligns respirable crystalline silica exposure limits for MNM and coal miners with workers in other industries. Moreover, the final rule updates the existing respiratory protection standard to require mine operators to provide miners with NIOSH-approved respiratory equipment that has been fitted, selected, maintained, and used in accordance with recent consensus standards. It also requires all MNM operators to provide medical surveillance in the form of a medical examination regime similar to the one that already covers coal miners. Cumulatively, the final rule will lower miners' risks of developing chronic, irreversible, disabling, and potentially fatal health conditions, consistent with MSHA's mission and statutory mandate to prevent occupational diseases and protect U.S. miners from suffering material health impairments.

II. Pertinent Legal Authority

The statutory authority for this final rule is provided by the Mine Act under sections 101(a), 103(h), and 508. 30 U.S.C. 811(a), 813(h), and 957. MSHA implements the provisions of the Mine Act to prevent death, illness, and injury from mining and promote safe and healthful workplaces for miners. The Mine Act requires the Secretary of Labor (Secretary) to develop and promulgate improved mandatory health or safety standards to prevent hazardous and unhealthy conditions and protect the health and safety of the nation's miners. 30 U.S.C. 811(a).

Congress passed the Mine Act to address these dangers, finding “an urgent need to provide more effective means and measures for improving the working conditions and practices in the Nation's coal or other mines in order to prevent death and serious physical harm, and in order to prevent occupational diseases originating in such mines.” 30 U.S.C. 801(c). Congress concluded that “the existence of unsafe and unhealthful conditions and practices in the Nation's coal or other mines is a serious impediment to the future growth of the coal or other mining industry and cannot be tolerated.” 30 U.S.C. 801(d). Accordingly, “the Mine Act evinces a clear bias in favor of miner health and safety.”
Nat'l Mining Ass'n
v.
Sec'y, U.S. Dep't of Lab.,
812 F.3d 843, 866 (11th Cir. 2016).

Section 101(a) of the Mine Act gives the Secretary the authority to develop, promulgate, and revise mandatory health standards to address toxic materials or harmful physical agents. Under Section 101(a), a standard must protect lives and prevent injuries in mines and be “improved” over any standard that it replaces or revises.

The Secretary must set standards to assure, based on the best available evidence, that no miner will suffer material impairment of health or functional capacity from exposure to toxic materials or harmful physical agents over their working lives. 30 U.S.C. 811(a)(6)(A). In developing standards that attain the “highest degree of health and safety protection for the miner,” the Mine Act requires that the Secretary consider the latest available scientific data in the field, the feasibility of the standards, and experience gained under the Mine Act and other health and safety laws.
Id.
As a result, courts have found it “appropriate to `give an extreme degree of deference' ” to MSHA “ `when it is evaluating scientific data within its technical expertise.' ”
Nat'l Mining Ass'n,
812 F.3d at 866 (quoting
Kennecott Greens Creek Mining Co.
v.
MSHA,
476 F.3d 946, 954 (D.C. Cir. 2007)). Consequently, MSHA's “duty to use the best evidence and to consider feasibility . . . cannot be wielded as counterweight to MSHA's overarching role to protect the life and health of workers in the mining industry.”
Nat'l Mining Ass'n,
812 F.3d at 866. Thus, “when MSHA itself weighs the evidence before it, it does so in light of its congressional mandate” in favor of protecting miners' health.
Id.
Moreover, “the Mine Act does not contain the `significant risk' threshold requirement” from the OSH Act.
Nat'l Mining Ass'n
v.
United Steel Workers,
985 F.3d 1309, 1319 (11th Cir. 2021);
see also Nat'l Min. Ass'n
v.
Mine Safety & Health Admin.,
116 F.3d 520, 527-28 (D.C. Cir. 1997) (contrasting the Mine Act at 30 U.S.C. 811(a) with the OSH Act at 29 U.S.C. 652 and noting that “[a]rguably,
this
language does not mandate the same risk-finding requirement as OSHA” and holding that “[a]t most, . . . [MSHA] was required to identify a significant risk associated with having no oxygen standard at all”).

Section 103(h) of the Mine Act gives the Secretary the authority to promulgate standards involving recordkeeping and reporting. 30 U.S.C. 813(h). Additionally, section 103(h) requires that every mine operator establish and maintain records, make reports, and provide this information as required by the Secretary.
Id.
Section 508 of the Mine Act gives the Secretary the authority to issue regulations to carry out any provision of the Mine Act. 30 U.S.C. 957.

MSHA's final rule to lower the exposure limits for respirable crystalline silica adopts an integrated monitoring approach across all mining sectors and updates the existing respiratory protection requirements. The final rule fulfills Congress' direction to protect miners from material impairments of health or functional capacity caused by exposure to respirable crystalline silica and other airborne contaminants.

III. Regulatory History

On August 29, 2019, MSHA published a Request for Information (RFI) in the
Federal Register
to solicit information and data on a variety of topics concerning silica (quartz) in respirable dust (84 FR 45452). In the RFI, MSHA requested data and information on technologically and economically feasible best practices to protect MNM and coal miners' health from exposure to quartz, including a lowered permissible exposure limit (PEL), new or developing protective technologies, and/or effective technical and educational assistance (84 FR 45456).

Specifically, MSHA requested input from industry, labor, and other interested parties on the following four topics: (1) new or developing technologies and best practices that can be used to protect miners from exposure to quartz dust; (2) how engineering controls, administrative controls, and personal protective equipment can be used, either alone or concurrently, to protect miners from exposure to quartz dust; (3) additional feasible dust-control methods that could be used by mining operations to reduce miners' exposures to respirable quartz during high-silica cutting situations, such as on development sections, shaft and slope work, and cutting overcasts; and (4) any other experience, data, or information that may be useful to MSHA in evaluating miners' exposures to quartz (84 FR 45456).

The Agency received 57 comments from citizens, labor, industry, and public health stakeholders in response to the RFI. Stakeholders expressed various and differing opinions on how and to what extent MSHA should address the protection of miners' health from exposure to silica. Many of these stakeholders also commented on MSHA's proposed rulemaking, summarized below.

On June 30, 2023, MSHA made an informal copy of the proposed rule available on the Agency's website, prior to publication in the
Federal Register
, so the public and stakeholders could

review it in advance of the comment period.

On July 13, 2023, MSHA published the proposed rule,
Lowering Miners' Exposure to Respirable Crystalline Silica and Improving Respiratory Protection,
in the
Federal Register
(88 FR 44852). The standalone documents “
Health Effects of Respirable Crystalline Silica,
” “
Preliminary Risk Analysis,
” and “
Preliminary Regulatory Impact Analysis
” were also made publicly available at that time. MSHA proposed to set the PEL of respirable crystalline silica at 50 micrograms
1

per cubic meter of air (µg/m
3
) for a full-shift exposure, calculated as an 8-hour time-weighted average. MSHA's proposal included other requirements for sampling, qualitative evaluations, corrective actions, and medical surveillance for MNM mines. Finally, the proposal included requirements for respiratory protection, including the incorporation by reference of
ASTM F3387-19 Standard Practice for Respiratory Protection.

1
One microgram is equal to one-thousandth of a milligram (1 milligram = 1000 micrograms).

On July 26, 2023, MSHA published a notice in the
Federal Register
scheduling three public hearings on the proposed rule (88 FR 48146). Hearings were held on: (1) August 3, 2023, in Arlington, Virginia; (2) August 10, 2023, in Beckley, West Virginia; and (3) August 21, 2023, in Denver, Colorado. Speakers and attendees could participate in-person or online. There were 14 speakers and over 150 attendees at the Arlington hearing; 24 speakers and over 200 attendees at the Beckley hearing; and 10 speakers and over 175 attendees at the Denver hearing. Speakers included active and retired miners and representatives from the mining industry, unions, the health care profession, advocacy groups, industry groups, trade associations, and law firms. Transcripts from the public hearings are available at
www.regulations.gov
and on the MSHA website.

On August 14, 2023, in response to requests from the public, MSHA published a notice in the
Federal Register
extending the comment period by changing the closing date from August 28, 2023, to September 11, 2023 (88 FR 54961).

During the comment period, MSHA received 157 written comments on the proposed rule from miners, mine operators, individuals, government officials, labor organizations, advocacy groups, industry groups, trade associations, and health organizations. Some commenters supported various aspects of the proposal. Other commenters opposed aspects of the proposal and offered recommendations for suggested changes to the proposed rule. All public comments and supporting documentation are available at
www.regulations.gov
and on the MSHA website. MSHA carefully reviewed and considered the written comments on the proposed rule and the speakers' testimonies from the hearings and addresses them in the relevant sections below.

IV. Background

A. Respirable Crystalline Silica Hazard and Mining

Silica is a common component of rock composed of silicon and oxygen (chemical formula SiO
2
), existing in amorphous and crystalline states. Silica in the crystalline state is the focus of this rulemaking. Respirable crystalline silica consists of small particles of crystalline silica that can be inhaled and reach the alveolar region of the lungs, where they can accumulate and cause disease. In crystalline silica, the silicon and oxygen atoms are arranged in a three-dimensional repeating pattern. The crystallization pattern varies depending on the circumstances of crystallization, resulting in a polymorphic state, meaning several different structures with the same chemical composition. The most common form of crystalline silica found in nature is quartz, but cristobalite and tridymite also occur in limited circumstances. Quartz accounts for the overwhelming majority of naturally occurring crystalline silica. In fact, quartz accounts for almost 12 percent of the earth's crust by volume. All soils contain at least trace amounts of quartz, and it is present in varying amounts in almost every type of mineral. Quartz is also abundant in most rock types, including granites, sandstones, and shale. Moreover, quartz bands and veins are commonly found in limestone formations, although limestone itself does not contain quartz. Because of its abundance, crystalline silica in the form of quartz is present in nearly all mining operations.

Cristobalite and tridymite are formed at very high temperatures and are associated with volcanic activity. Naturally occurring cristobalite and tridymite are rare, but they can be found in volcanic ash and in a relatively small number of rock types limited to specific geographic regions. Although rare, exposure to cristobalite can occur when volcanic deposits are mined. In addition, when other materials are mined, miners can potentially be exposed to cristobalite during certain processing steps (
e.g.,
heating silica-containing materials) and contact with refractory materials (
e.g.,
replacing fire bricks in mine processing facility furnaces). Tridymite is rarely found in nature and miner exposure to tridymite is much more infrequent.

Most mining activities generate silica dust because silica is often contained in the ore being mined or in the overburden (
i.e.,
the soil and surface material surrounding the commodity being mined). Such activities include, but are not limited to, cutting, sanding, drilling, crushing, grinding, sawing, scraping, jackhammering, excavating, and hauling materials that contain silica. These activities can generate respirable crystalline silica and therefore may lead to miner exposure.

Inhaled small particles of silica dust can be deposited throughout the lungs. Because of their small size, many of these particles can reach and remain in the deep lung (
i.e.,
alveolar region), although some can be cleared from the lungs. Because respirable crystalline silica particles are not water-soluble and do not undergo metabolism into less toxic compounds, those particles remaining in the lungs result in a variety of cellular responses that may lead to pulmonary diseases, such as silicosis and lung cancer. The respirable crystalline silica particles that are cleared from the lungs can be distributed to lymph nodes, blood, liver, spleen, and kidneys, potentially accumulating in those other organ systems and causing renal disease and other adverse health effects.

In the U.S. in 2021, a total of 12,162 mines produced a variety of commodities. As shown in Table IV-1, of those 12,162 total mines, 11,231 mines were MNM mines and 931 mines were coal mines. MNM mines can be broadly divided into five commodity groups: metal, nonmetal, stone, crushed limestone, and sand and gravel. These broad categories encompass approximately 98 different commodities.
2

Table IV-1 shows that a majority of MNM mines produce sand and gravel, while the largest number of MNM miners work at metal mines, not including MNM contract workers (
i.e.,

independent contractors and employees of independent contractors who are engaged in mining operations).

2
Commodities such as sand, gravel, silica, and/or stone are used in road building, concrete construction, the manufacture of glass and ceramics, molds for metal castings in foundries, abrasive blasting operations, plastics, rubber, paint, soaps, scouring cleansers, filters, hydraulic fracturing, and various architectural applications. Some commodities naturally contain high levels of crystalline silica, such as high-quartz industrial and construction sands and granite dimension stone and gravel (both produced for the construction industry).

ER18AP24.131

The 931 coal mines—underground and surface—produce bituminous, subbituminous, anthracite, and lignite coal. Coal mining activities generate mixed coal mine dust that contains respirable silicates such as kaolinite, oxides such as quartz, and other components (IARC, 1997). These activities include the general mining activities previously mentioned (
e.g.,
cutting, sanding, drilling, crushing, hauling, etc.), as well as roof bolter operations, continuous mining machine operations, longwall mining, and other activities. Table IV-1 shows that there are more surface coal mines than underground coal mines, but more miners are working in underground coal mines than surface coal mines (not including coal contract workers).

B. Existing Standards

Since the early 1970s, MSHA has maintained health standards to protect MNM and coal miners from excessive exposure to airborne contaminants, including respirable crystalline silica. These standards require mine operators to use engineering controls as the primary means of suppressing, diluting, or diverting dust generated by mining activities. They also require mine operators to provide miners with respiratory protection in limited situations for a short period. The existing standards for MNM and coal mines differ in some respects, including exposure limits and monitoring requirements. This section describes MSHA's existing standards for respirable crystalline silica and presents respirable crystalline silica sampling data to show how MNM and coal mine operators have complied with the standards in recent years.

1. Existing Standards—Metal and Nonmetal Mines

MSHA's existing standards for exposure to airborne contaminants in MNM mines, including respirable crystalline silica, are found in 30 CFR 56 subpart D (Air Quality and Physical Agents) and 30 CFR 57 subpart D (Air Quality, Radiation, Physical Agents, and Diesel Particulate Matter). These standards include PELs for airborne contaminants (§§ 56.5001 and 57.5001), exposure monitoring (§§ 56.5002 and 57.5002), and control of exposure to airborne contaminants (§§ 56.5005 and 57.5005).

Permissible Exposure Limits.
The existing PELs for the three polymorphs of respirable crystalline silica are based on the
TLVs® Threshold Limit Values for Chemical Substances in Workroom Air Adopted by the American Conference of Governmental Industrial Hygienists (ACGIH) for 1973,
incorporated by reference in 30 CFR 56.5001 and 57.5001 (ACGIH, 1974). The 1973 TLV® establishes limits for respirable dust containing 1 percent quartz or greater and is calculated in milligrams per cubic meter of air (mg/m
3
) for each respirable dust sample. The resulting TLVs® for respirable dust containing 1 percent respirable crystalline silica or greater are designed to limit exposures to less than 0.1 mg/m
3
or 100 micrograms per cubic meter of air (µg/m
3
) for quartz, to less than 0.05 mg/m
3
or 50 µg/m
3
for cristobalite, and to less than 0.05 mg/m
3
or 50 µg/m
3
for tridymite. Throughout the remainder of this preamble, the concentrations of respirable dust and respirable crystalline silica are expressed in µg/m
3
.

Exposure Monitoring.
Under 30 CFR 56.5002 and 57.5002, MNM mine operators must conduct respirable dust “surveys . . . as frequently as necessary to determine the adequacy of control measures.” Mine operators can satisfy the survey requirement through various activities, such as respirable dust sampling and analysis, walk-through inspections, wipe sampling, examination of dust control system and ventilation system maintenance, and

review of information obtained from injury, illness, and accident reports.

MSHA encourages MNM mine operators to conduct sampling for airborne contaminants to ensure a healthy and safe work environment for miners, because sampling provides more accurate information about miners' exposures and the effectiveness of existing controls in reducing exposures. When a mine operator's respirable dust survey indicates that miners have been overexposed to any airborne contaminant, including respirable crystalline silica, the operator is expected to adjust its control measures (
e.g.,
exhaust ventilation) to reduce or eliminate the identified hazard. After doing so, the mine operator is expected to conduct additional surveys to determine whether its adjustments to control measures were successful. Re-surveying should be done as frequently as necessary to ensure that the sampling results comply with the PEL and the implemented control measures remain adequate.

Exposure Controls.
MSHA's existing standards for controlling a miner's exposure to harmful airborne contaminants in §§ 56.5005 and 57.5005 require, if feasible, prevention of contamination, removal by exhaust ventilation, or dilution with uncontaminated air. These requirements to use feasible engineering controls, supplemented by administrative controls, are consistent with widely accepted industrial hygiene principles and NIOSH's recommendations (NIOSH, 1974). Engineering controls designed to remove or reduce the hazard at the source are the most effective. Although administrative controls are considered a supplementary or secondary measure to engineering controls, mine operators may use administrative controls to further reduce miners' exposures to respirable crystalline silica and other airborne contaminants.

The use of respiratory protective equipment is also allowed under specified circumstances, such as where engineering controls are not yet developed or when it is necessary due to the nature of the work—for example, while establishing controls or during occasional entry into hazardous atmospheres to perform maintenance or investigation. Respirators approved by NIOSH and suitable for their intended purpose must be provided by mine operators at no cost to the miner and must be used by miners to protect themselves against the health and safety hazards of respirable crystalline silica and other airborne contaminants. When respiratory protective equipment is used, MNM mine operators must implement a respiratory protection program consistent with the requirements of
American National Standards Practices for Respiratory Protection ANSI Z88.2-1969
(ANSI Z88.2-1969).

2. Existing Standards—Coal Mines

Under the existing coal mine standards, there is no separate standard for respirable crystalline silica. MSHA's existing standards for exposure to respirable quartz in coal mines, found in 30 CFR 70.101 and 71.101, establish a respirable dust standard when quartz is present for underground and surface coal mines, respectively. Under 30 CFR part 90 (Mandatory Health Standards—Coal Miners Who Have Evidence of the Development of Pneumoconiosis), § 90.101 also sets the respirable dust standard when quartz is present for Part 90 miners.
3

Coal miners' exposures to respirable quartz are indirectly regulated through reductions in the overall respirable dust standards.

3
A “Part 90 miner” is defined in 30 CFR 90.3 as a miner employed at a coal mine who shows evidence of having contracted pneumoconiosis based on a chest X-ray or based on other medical examinations, and who is afforded the option to work in an area of a mine where the average concentration of respirable dust in the mine atmosphere during each shift to which that miner is exposed is continuously maintained at or below the applicable standard.

Under its existing respirable coal mine dust standards, MSHA defines quartz as crystalline silicon dioxide (SiO
2
), which includes not only quartz but also two other polymorphs, cristobalite and tridymite.
4

Therefore, the terms quartz and respirable crystalline silica are used interchangeably in the discussions of MSHA's existing standards for controlling exposures to respirable crystalline silica in coal mines.

4
Quartz is defined in 30 CFR 70.2, 71.2, and 90.2 as crystalline silicon dioxide (SiO
2
) not chemically combined with other substances and having a distinctive physical structure. Crystalline silicon dioxide is most commonly found in nature as quartz but sometimes occurs as cristobalite or, rarely, as tridymite. Quartz accounts for the overwhelming majority of naturally occurring crystalline silica and is present in varying amounts in almost every type of mineral.

Exposure Limits.
The exposure limit for respirable crystalline silica during a coal miner's shift is 100 µg/m
3
, reported as an equivalent concentration as measured by the Mining Research Establishment (MRE) instrument.
5

The equivalent concentration of respirable crystalline silica must not be exceeded during the miner's entire shift, regardless of duration. When the equivalent concentration of respirable quartz exceeds 100 µg/m
3
, under §§ 70.101, 71.101, and 90.101, MSHA imposes a reduced respirable dust standard designed to ensure that respirable quartz will not exceed 100 µg/m
3
. Various sections within a mine may have different reduced respirable coal mine dust (RCMD) exposure limits. Therefore, when a respirable dust sample collected by MSHA indicates that the average concentration of respirable quartz dust exceeds the exposure limit, the mine operator is required to comply with the applicable dust standard. Because respirable crystalline silica is a percentage of RCMD, by reducing the amount of respirable dust to which miners are exposed during their shifts, the miners' exposures to respirable crystalline silica are reduced to a level at or below the exposure limit of 100 µg/m
3
.

5
As defined in 30 CFR 70.2, an MRE instrument is a gravimetric dust sampler with a four channel horizontal elutriator developed by the Mining Research Establishment of the National Coal Board, London, England. MSHA inspectors use Dorr-Oliver 10-mm nylon cyclones operated at a 2.0 L/min flow rate (reported as MRE-equivalent concentrations) for coal mine sampling.

Exposure Monitoring.
Under §§ 70.208, 70.209, 71.206, and 90.207, coal mine operators are required to sample for respirable dust on a quarterly basis for specified occupations and work areas. The occupations and work areas specified in the existing coal dust standards are the occupations and work areas at a coal mine that are expected to have the highest concentrations of respirable dust—typically in locations where respirable dust is generated. Respirable dust sampling must be representative of respirable dust exposures during a normal production shift and must occur while miners are performing routine, day-to-day activities. Part 90 miners must be sampled for the air they breathe while performing their normal work duties, in their normal work locations, from the start of their work day to the end of their work day.

Exposure Controls.
Under §§ 70.208, 70.209, 71.206, and 90.207, coal mine operators are required to use engineering or environmental controls as the primary means of complying with the respirable dust standards. For many underground coal mines, providing adequate ventilation is the primary engineering control for respirable dust, ensuring that dust concentrations are continuously diluted with fresh air and exhausted away from miners.

When a respirable dust sample exceeds the exposure limit of 100 µg/m
3
for respirable quartz, the operator must reduce the average concentration of RCMD to a level designed to maintain the quartz level at or below 100 µg/m
3
. If operators exceed the RCMD standard, they are required to take corrective

action to reduce exposure and comply with the reduced standard. Corrective actions that lower respirable coal mine dust, thus lowering respirable quartz exposures, are selected after evaluating the cause or causes of the overexposure.

When taking corrective actions to reduce the exposure to respirable dust, coal mine operators must make approved respiratory equipment available to miners under §§ 70.208, 70.209, and 71.206. Whenever respiratory protection is used, § 72.700 requires coal mine operators to comply with requirements specified in ANSI Z88.2-1969.

C. MSHA Inspection and Respirable Dust Sampling

Under the existing standards, MSHA collects respirable dust samples at mines and analyzes them for respirable crystalline silica to determine whether the respirable crystalline silica exposure limits are exceeded and whether exposure controls are adequate. MSHA's inspection and respirable dust sampling were discussed in detail in the proposal (88 FR 44862). This section, for ease of reference, briefly summarizes the process for MSHA's inspection and respirable dust sampling.

1. Respirable Dust Sample Collection

Under the existing standards, MSHA inspectors arrive at mines, determine which miners and which areas of the mine to select for respirable dust sampling, and place gravimetric samplers on the selected miners and at the selected locations. The gravimetric samplers capture air from the breathing zone of each selected miner and from each selected work area for the entire duration of the work shift. Full-shift sampling is used to minimize errors associated with fluctuations in airborne contaminant concentrations during the miners' work shifts and to avoid any speculation about the miners' exposures during unsampled periods of the work shift. Once sampling is completed, MSHA inspectors send cassettes containing the full-shift respirable dust samples to the MSHA Laboratory for analysis.

2. Respirable Dust Sample Analysis

The MSHA Laboratory analyzes respirable dust samples following the standard operating procedures summarized below.
6

Any samples that are broken, torn, or visibly wet are voided and removed before analysis. Samples are weighed and then examined for validity based on mass gain. All valid samples that meet the minimum mass gain criteria per the associated MSHA analytical method are then analyzed for respirable crystalline silica and for the compliance determination.
7

6
The MSHA Laboratory has fulfilled the requirements of the AIHA Laboratory Accreditation Programs (AIHA-LAP), LLC accreditation to the ISO/IEC 17025:2017 international standard for industrial hygiene.

7
The minimum mass gain criteria used by the MSHA Laboratory for the different samples are:

• MNM mine respirable dust samples: greater than or equal to 0.100 mg;

• Underground coal mine respirable dust samples: greater than or equal to 0.100 mg; and

• Surface coal mine respirable dust samples: greater than or equal to 0.200 mg.

Exception:
For six surface occupations that have been deemed “high risk,” the laboratory uses a minimum mass gain criterion of greater than or equal to 0.100 mg.

If cristobalite analysis is requested for MNM mine respirable dust samples, filters having a mass gain of 0.05 mg or more are analyzed. In the rare instance when tridymite analysis is requested, a qualitative analysis for the presence of the polymorph is conducted concurrently with the cristobalite analysis.

The MSHA Laboratory uses two analytical methods to determine the concentration of quartz (and cristobalite and tridymite, if requested) in respirable dust samples: X-ray diffraction (XRD) for samples from MNM mines and Fourier transform infrared spectroscopy (FTIR) for samples from coal mines.
8

The percentage of silica in the MNM mine dust sample is calculated using the mass of quartz or cristobalite determined from the XRD analysis and the measured mass of respirable dust. Similarly, in the respirable coal mine dust sample, the percentage of quartz is calculated using the quartz mass determined from the FTIR analysis and the sample's mass of dust. Current FTIR methods, however, cannot quantify quartz and cristobalite, and/or tridymite, in the same sample.

8
Details on MSHA's analytical procedures for respirable crystalline silica analysis can be found in “MSHA P-2: X-Ray Diffraction Determination of Quartz and Cristobalite in Respirable Metal/Nonmetal Mine Dust” and “MSHA P-7: Determination of Quartz in Respirable Coal Mine Dust by Fourier Transform Infrared Spectroscopy.”

Department of Labor, Mine Safety and Health Administration, Pittsburgh Safety and Health Technology Center, X-Ray Diffraction Determination of Quartz and Cristobalite in Respirable Metal/Nonmetal Mine Dust.
https://arlweb.msha.gov/Techsupp/pshtcweb/MSHA%20P2.pdf
(last accessed Jan. 10, 2024). Department of Labor, Mine Safety and Health Administration, Pittsburgh Safety and Health Technology Center, MSHA P-7: Determination of Quartz in Respirable Coal Mine Dust By Fourier Transform Infrared Spectroscopy.
https://arlweb.msha.gov/Techsupp/pshtcweb/MSHA%20P7.pdf
(last accessed Jan. 10, 2024).

MSHA calculates full-shift exposures to respirable crystalline silica (and other airborne contaminants) in the same way for MNM and coal miners when the miner works an 8-hour shift, but the calculated exposures differ for longer shifts. For work shifts that last longer than 8 hours, a coal miner's full-shift exposure is calculated using the entire duration of the coal miner's shift. For the MNM miner, by contrast, MSHA calculates extended full-shift exposure for respirable dust samples using 480 minutes (8 hours) as the sampling time, meaning that contaminants collected over extended shifts (
e.g.,
600-720 minutes) are calculated as if they had been collected over 480 minutes.

D. Respirable Crystalline Silica Sampling Results—Metal and Nonmetal Mines

MSHA's respirable crystalline silica sampling results for MNM mines were discussed in detail in the proposal (88 FR 44863). This section, for ease of reference, summarizes the results of respirable dust samples that were collected by MSHA inspectors at MNM mines from 2005 to 2019. From January 1, 2005, to December 31, 2019, a total of 104,354 valid samples were collected. Of this total, 57,769 samples met the minimum mass gain criteria and were analyzed for respirable crystalline silica. The vast majority of the 46,585 valid samples that were excluded from the analysis did not meet the mass gain criteria. Further information on the valid respirable dust samples that were excluded from the analysis can be found in Appendix A of the preamble.

1. Annual Results of MNM Respirable Crystalline Silica Samples

Table IV-2 below shows the variation between 2005 and 2019 in: (1) the number of MNM respirable dust samples analyzed for respirable crystalline silica; and (2) the number and percentage of samples that had concentrations of respirable crystalline silica greater than 100 µg/m
3
. Of the 57,769 MNM respirable dust samples analyzed for respirable crystalline silica over the 15-year period, about 6 percent (3,539 samples) had respirable crystalline silica concentrations exceeding the existing PEL of 100 µg/m
3
. The average annual rates of overexposure ranged from a maximum of approximately 10 percent in 2006 (the second year) to a minimum of approximately 4 percent in 2019 (the last year of the time series). Compared with the rates in 2005-2008, overexposure rates were substantially lower in 2009-2017, with a further drop in 2018-19.

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ER18AP24.132

BILLING CODE 4520-43-C
2. Analysis of MNM Respirable Crystalline Silica Samples by Commodity

Because the MNM mining industry produces commodities that contain varying degrees of respirable crystalline silica, it is important to examine each commodity separately. MNM mines can be grouped by five commodities: metal, sand and gravel, stone, crushed limestone, and nonmetal (where nonmetal includes all other materials that are not metals, besides sand, gravel, stone, and limestone). This grouping is based on the mine operator-reported mining products and the North American Industry Classification System (NAICS) codes. (Appendix B of the preamble provides a list of the NAICS codes relevant for MNM mining and how each code is assigned to one of the five commodities.)

Table IV-3 shows the distribution of the respirable dust samples analyzed for respirable crystalline silica by mine commodity. The percentage of samples with respirable crystalline silica concentrations greater than the existing exposure limit of 100 µg/m
3
varies across the different commodities. It is highest for the metal, sand and gravel, and stone commodities (at approximately 11, 7, and 7 percent, respectively), and lowest for the nonmetal and crushed limestone commodities (at approximately 4 and 3 percent, respectively).

ER18AP24.133

3. Analysis of MNM Respirable Crystalline Silica Samples by Occupation

To examine how miners who perform different tasks differ in occupational exposure to respirable crystalline silica, MSHA grouped MNM mining jobs into 11 occupational categories. These categories include jobs that are similar in terms of tasks performed, equipment used, and engineering or administrative controls used to control miners' exposure. For example, backhoe operators, bulldozer operators, and tractor operators were grouped into “operators of large powered haulage equipment,” whereas belt crew, belt cleaners, and belt vulcanizers were grouped into “conveyer operators.” The 121 MNM job codes used by MSHA inspectors were grouped into the following occupational categories:
9

9
For a full crosswalk of job codes included in each of these 11 Occupational Categories, please see Appendix C of the preamble. Also, note that the order of the presentation of the 11 Occupational Categories here follows the general sequence of mining activities: first development and production, then ore/mineral processing, then loading, hauling, and dumping, and finally all others.

(1) Drillers (
e.g.,
Diamond Drill Operator, Wagon Drill Operator, and Drill Helper),

(2) Stone Cutting Operators (
e.g.,
Jackhammer Operator, Cutting Machine Operator, and Cutting Machine Helper),

(3) Kiln, Mill, and Concentrator Workers (
e.g.,
Ball Mill Operator, Leaching Operator, and Pelletizer Operator),

(4) Crushing Equipment and Plant Operators (
e.g.,
Crusher Operator/Worker, Scalper Screen Operator, and Dry Screen Plant Operator),

(5) Packaging Equipment Operators (
e.g.,
Bagging Operator and Packaging Operations Worker),

(6) Conveyor Operators (
e.g.,
Belt Cleaner, Belt Crew, and Belt Vulcanizer),

(7) Truck Loading Station Tenders (
e.g.,
Dump Operator and Truck Loader),

(8) Operators of Large Powered Haulage Equipment (
e.g.,
Tractor Operators, Bulldozer Operator, and Backhoe Operators),

(9) Operators of Small Powered Haulage Equipment (
e.g.,
Bobcat Operator, Scoop-Tram Operator, and Forklift Operator),

(10) Mobile Workers (
e.g.,
Laborers, Electricians, Mechanics, and Supervisors), and

(11) Miners in Other Occupations (
e.g.,
Welder, Dragline Operator, Ventilation Crew and Dredge/Barge Operator).

Table IV-4 shows sample numbers and overexposure rates by MNM occupation. Operators of large powered haulage equipment accounted for the largest number of samples analyzed for silica (17,016 samples), whereas conveyor operators accounted for the fewest (215 samples). Table IV-4 also shows the number and percentage of the samples exceeding the existing respirable crystalline silica PEL of 100 µg/m
3
. In every occupational category, some MNM miners were exposed to respirable crystalline silica levels above the existing PEL. In 9 out of the 11 occupational categories, the percentage of samples exceeding the existing PEL is less than 10 percent, although two have higher rates, ranging up to more than 19 percent (in the case of stone cutting operators).

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BILLING CODE 4520-43-C
4. Conclusion

This analysis of MSHA inspector sampling data shows that MNM operators have generally met the existing standard. Of the 57,769 respirable dust samples from MNM mines, approximately 6 percent exceeded the existing respirable crystalline silica PEL of 100 µg/m
3
, although there are several outliers with much higher overexposures. For 9 of the 11 occupational categories, less than 10 percent of the respirable dust samples had concentrations over the existing PEL of 100 µg/m
3
for respirable crystalline silica. While stone-cutting operators have historically had high exposures to respirable dust and respirable crystalline silica
10

and continue to experience the highest overexposures of any MNM occupation, about 80 percent of samples taken from stone cutting operators did not exceed the existing PEL. For the categories of drillers, miners in other occupations, and operators of large powered haulage equipment, approximately 5 percent or less of the respirable dust samples showed concentrations over the existing exposure limit.

10
Analysis of MSHA respirable dust samples from 2005 to 2010 showed that stone and rock saw operators had approximately 20 percent of the sampled exposures exceeding the PEL. Watts
et al.
(2012).

In summary, the analysis of MSHA inspector sampling data indicates that the controls that MNM mine operators are using, together with MSHA's enforcement, have generally been effective in keeping miners' exposures at or below the existing limit of 100 µg/m
3
.

E. Respirable Crystalline Silica Sampling Results—Coal Mines

MSHA's respirable crystalline silica sampling results for coal mines were discussed in detail in the proposal (88 FR 44866). This section, for ease of reference, summarizes the results of RCMD samples collected by MSHA inspectors from 2016 to 2021. (The data analyses for this rulemaking do not include any respirable dust samples collected by coal mine operators.) The analysis below is based on the samples collected by MSHA inspectors starting on August 1, 2016, when Phase III of MSHA's 2014
Lowering Miners' Exposure to Respirable Coal Mine Dust, Including Continuous Personal Dust Monitors
(referred to throughout the preamble as the 2014 RCMD Standard) (79 FR 24813) went into effect. At that time, the exposure limits for RCMD were lowered from 2.0 mg/m
3
to 1.5 mg/m
3
(MRE equivalent) at underground and surface coal mines, and from 1.0 mg/m
3
to 0.5 mg/m
3
(MRE equivalent) for intake air at underground coal mines and for Part 90 miners. From August 1, 2016, to July 31, 2021, MSHA inspectors collected a total of 113,607 valid RCMD samples. Of the valid samples, only those collected from the breathing zones of miners were used in the analysis for this rulemaking; no environmental dust

samples were included.
11

Of the valid breathing zone samples, there were 63,127 samples that met the minimum mass gain criteria and were analyzed for respirable quartz. The majority of the non-environmental valid samples excluded from this rulemaking analysis were excluded due to insufficient mass. Further information on the valid respirable dust samples that are not included in the rulemaking analysis can be found in Appendix A of the preamble.

11
Environmental samples were not included in the analysis to be consistent with the proposed sampling requirements to determine individual miner exposure.

Of the 63,127 valid samples analyzed for respirable crystalline silica and used for this analysis, about 1 percent (777 samples) were over the existing quartz exposure limit of 100 µg/m
3
(MRE equivalent) for a full shift, calculated as a TWA.
12

Overexposure rates decreased by nearly a quarter between the first half and the second half of the 2016-2021 period. As in MNM mines, different miner occupations had different overexposure rates. Using broader groupings, surface mines experienced higher rates of overexposure than underground mines (2.4 percent versus 1.0 percent, respectively).

12
The conversion between ISO values and MRE values uses the NIOSH conversion factor of 0.857. In the 1995b Criteria Document, NIOSH presented an empirically derived conversion factor of 0.857 for comparing current (MRE) and recommended (ISO) respirable dust sampling criteria using the 10 mm Dorr-Oliver nylon cyclone operated at 2.0 and 1.7 L/min, respectively (
i.e.,
1.5 mg/m
3
BMRC-MRE = 1.29 mg/m
3
ISO).

1. Annual Results of Coal Respirable Crystalline Silica Samples

In examining trends from one year to the next, the discussion below focuses on the samples collected in the 6 calendar years from 2016 to 2021. The number of samples per year was stable from 2017 to 2019 before decreasing in 2020.
13

The overexposure rate decreased across the entire 2016 to 2021 period, from 1.41 percent in 2016 to 0.95 percent in 2021. As shown in Table IV-5, a review of the 6 calendar years reveals that the overexposure rate decreased by nearly a quarter from 2016-2018 (1.38 percent) to 2019-2021 (1.07 percent).

13
The coal samples for 2016 begin in August of that year and the coal samples for 2021 end in July of that year.

ER18AP24.135

2. Analysis of Coal Respirable Crystalline Silica Samples by Location

Coal mining activities differ depending on the characteristics and locations of coal seams. When coal seams are several hundred feet below the surface, miners tunnel into the earth and use underground mining equipment to extract coal, whereas miners at surface coal mines remove topsoil and layers of rock to expose coal seams. Due to these differences, it is important to examine the respirable crystalline silica data by location to determine how underground and surface coal miners differ in occupational exposure to respirable crystalline silica.

Table IV-6, which presents the overexposure rate by type of mine where respirable coal mine dust samples were collected, shows that samples from surface coal mines reflected higher rates of overexposure than samples from underground mines. Out of the 53,095 respirable coal mine dust samples from underground mines, 1 percent (537 samples) were over the existing exposure limit. By contrast, there were 10,032 samples from surface coal mines, and approximately 2.4 percent (240 samples) of those samples were over the existing exposure limit.

ER18AP24.136

3. Analysis of Coal Respirable Crystalline Silica Samples by Occupation

To assess the exposure to respirable crystalline silica of miners in different occupations, MSHA has consolidated the 220 job codes for coal mines into 9 occupational categories (using a similar process to the one it used for the MNM mines, but with different job codes and categories). For the coal mine occupational categories,
14

a distinction is made between occupations based on whether the job tasks are being performed at the surface of a mine or underground. For example, bulldozer operators are assigned to the job category of operators of large powered haulage equipment grouping and then sorted into separate occupational categories based on whether they are working at the surface of a mine or underground.

14
For a full crosswalk of which job codes were included in each of these nine Occupational Categories, please see Appendix C of the preamble.

Of the nine occupational categories used for coal miners, the five underground categories are:

(1) Continuous Mining Machine Operators (
e.g.,
Coal Drill Helper and Coal Drill Operator),

(2) Longwall Workers (
e.g.,
Headgate Operator and Jack Setter (Longwall)),

(3) Roof Bolters (
e.g.,
Roof Bolter and Roof Bolter Helper),

(4) Operators of Large Powered Haulage Equipment (
e.g.,
Shuttle Car Operator, Tractor Operator/Motorman, Scoop Car Operator), and

(5) All Other Underground Miners (
e.g.,
Electrician, Mechanic, Belt Cleaner and Laborer, etc.).

The four surface occupational categories are:

(1) Drillers (
e.g.,
Coal Drill Operator, Coal Drill Helper, and Auger Operator),

(2) Crusher Operators (
e.g.,
Crusher Attendant, Washer Operator, and Scalper-Screen Operator),

(3) Operators of Large Powered Haulage Equipment (
e.g.,
Backhoe Operator, Forklift Operator, and Bulldozer Operator), and

(4) Mobile Workers (
e.g.,
Electrician, Mechanic, Blaster, Laborer, etc.).

The most sampled occupational category was operators of large powered haulage equipment (underground), representing approximately 34 percent of the samples taken. The least sampled occupational category was crusher operators (surface), consisting of 1 percent of the samples taken. Table IV-7 displays the number and percent of respirable coal mine dust samples with quartz greater than the existing exposure limit for each occupational category.

ER18AP24.137

Looking at trends, every occupational category shows a decrease in overexposure rates over time. See Figure IV-1. Most of the nine categories had lower rates of overexposure in the 2019-2021 period than in the 2016-2018 period.

Figure IV-1: Percent of RCMD Samples With Respirable Crystalline Silica Concentration Greater Than 100 MRE µg/m
3
(MRE) by Occupational Category *

ER18AP24.076

* For Crusher Operators (Surface), only one sample with a quartz concentration greater than 100 µg/m
3
MRE occurred (in 2018); and for Mobile Workers (Surface), only nine samples with a quartz concentration greater than 100 µg/m
3
MRE occurred (three in 2017, five in 2018 and one in 2021). Source: MSHA MSIS respirable crystalline silica data for the Coal Industry, August 1, 2016, through July 31, 2021 (version 20220617).

In all occupational categories, coal miners were sometimes exposed to respirable crystalline silica levels above the existing exposure limit. But the sampling data showed that coal mine operators can generally comply with the existing exposure limit. For example, although mining tasks performed by the occupational category of roof bolters (underground) historically resulted in high levels of overexposure to quartz, the low levels of overexposure for that occupation in 2016-2021 (
i.e.,
1 percent) suggest that roof bolters now benefit from the improved respirable dust standard, improved technology, and better training.
15

Over the 2016-2021 period, coal miners in the occupational category drillers (surface) were the most frequently overexposed, with approximately 6 percent of samples over the existing quartz limit; they were followed by longwall workers (underground) (about 4 percent), operators of large powered haulage equipment (surface) (about 3 percent), and continuous mining machine operators (underground) (about 2 percent). For all other occupational categories, the overexposure rate was less than 1 percent.

15
The drilling operation in the roof bolting process, especially in hard rock, generates excessive respirable coal and quartz dusts, which could expose the roof bolting operator to continued health risks (Jiang and Luo, 2021).

4. Conclusion

This analysis of MSHA inspector sampling data shows that coal mine operators generally comply with the existing standards related to quartz. Of the 63,127 valid respirable dust samples from coal mines over the most recent 5-year period, 1.2 percent had respirable quartz over the existing exposure limit of 100 µg/m
3
(MRE equivalent) for a full-shift exposure, calculated as a TWA. Seven of the nine occupational categories had overexposure rates of 2.5 percent or less. Roof bolters (underground), which historically have had high exposures to respirable dust and respirable crystalline silica, had overexposure rates of 1 percent over this recent period. The data demonstrates that the controls that coal mine operators are using, together with MSHA's enforcement, have generally been effective in keeping miners' exposure to respirable crystalline silica at or below the existing exposure limit.

V. Health Effects Summary

This section summarizes the health effects from occupational exposure to respirable crystalline silica. MSHA's full analysis of the health effects literature is contained in the standalone document, entitled “
Effects of Occupational Exposure to Respirable Crystalline Silica on the Health of Miners
” (referred to as the standalone Health Effects document throughout the preamble), which is placed in the rulemaking docket for the MSHA silica rulemaking (RIN 1219-AB36, Docket No. MSHA-2023-0001). MSHA reviewed a wide range of health effects literature that included more than 600 studies exploring the relationship between respirable crystalline silica exposure and resultant health effects in miners and other workers across various industries. The purpose of this summary is to briefly present MSHA's findings on the nature of the hazards of exposure to respirable crystalline silica. Based on its review of the health effects literature and the weight-of-evidence approach, MSHA makes the following conclusions:

1. Miners in MNM and coal mines exposed to respirable crystalline silica at MSHA's existing exposure limits are subject to material impairment of health or functional capacity. The illnesses associated with exposure to respirable crystalline silica develop independent of other exposures.

2. Occupational exposure to respirable crystalline silica (as quartz and/or cristobalite) causes silicosis,

nonmalignant respiratory disease (NMRD) (
e.g.,
emphysema and chronic bronchitis), lung cancer, and renal disease. Each of these health effects outcomes is exposure-dependent, potentially chronic, irreversible, potentially disabling, and can be fatal.

3. Exposure to respirable crystalline silica contributes to the development of autoimmune disorders through inflammatory pathways.

4. The development of silicosis, NMRD, lung cancer, renal disease, and autoimmune disorders is largely dependent upon cumulative respirable crystalline silica exposure.

These conclusions are the basis of MSHA's Final Risk Analysis (FRA) on miners' exposure to respirable crystalline silica. In the FRA, MSHA quantifies risks associated with the five specific health outcomes mentioned above. The FRA summary is presented in
Section VI. Final Risk Analysis Summary
and a standalone document, entitled “
Final Risk Analysis
” (referred to as the standalone FRA document throughout the preamble), has been placed in the rulemaking docket for the MSHA silica rulemaking (RIN 1219-AB36, Docket No. MSHA-2023-0001).

From its health effects literature review and FRA, MSHA determines that miners exposed to respirable crystalline silica continue to face a risk of material impairment of health or functional capacity under MSHA's existing exposure limits. Thus, MSHA also makes the following conclusions:

(1) The rate of silicosis and other diseases caused by respirable crystalline silica exposure would decrease with reduction in occupational exposures, which is the most effective way to prevent these types of diseases.

(2) Regulatory action is necessary to reduce these occupational exposures and protect miners' health. Section 101(a)(6)(A) of the Federal Mine Safety and Health Act of 1977, as amended (Mine Act), requires MSHA to “set standards which most adequately assure on the basis of the best available evidence that no miner will suffer material impairment of health or functional capacity even if such miner has regular exposure to the hazards dealt with by such standard for the period of his working life.” 30 U.S.C. 811(a)(6)(A).

Regulatory action to protect miners' health is required by section 101(a)(6)(A) of the Mine Act, and MSHA's statutory authority and mission has been recognized and upheld by reviewing courts. “[T]he Mine Act evinces a clear bias in favor of miner health and safety.”
Nat'l Min. Ass'n
v.
Sec'y, U.S. Dep't of Lab.,
812 F.3d 843, 866 (11th Cir. 2016). Courts interpret MSHA's obligation to promulgate standards to protect the health of the nation's miners to include “ `prevent[ing],' not merely reduc[ing] the incidence of, `occupational diseases originating in . . . mines.' ”
Id.
at 883 (quoting 30 U.S.C. 801(c)). Where occupational disease “incidence has not been reduced
to zero
. . . MSHA has not completely fulfilled its mission to `protect the health . . . of the Nation's coal or other miners.' ”
Id.
(quoting 30 U.S.C. 801(g)). Case law instructs that MSHA must demonstrate risk before regulating: “[B]efore promulgating a health or safety standard under the Mine Act, MSHA must show that the substance being regulated presents a risk of `material impairment of health or functional capacity' for miners who are regularly exposed to the substance.”
Kennecott Greens Creek Min. Co.
v.
Mine Safety & Health Admin.,
476 F.3d 946, 952 (D.C. Cir. 2007) (quoting 30 U.S.C. 811(a)(6)(A)). Although the Mine Act requires MSHA to consider the best available evidence, the “duty to use the best available evidence . . . cannot be wielded as a counterweight to MSHA's overarching role to protect the life and health of workers in the mining industry.”
Nat'l Min. Ass'n,
812 F.3d at 866. With this regulatory action, MSHA is addressing this urgent need. See 30 U.S.C. 801(c).

On July 13, 2023, MSHA published a notice of proposed rulemaking, entitled “
Lowering Miners' Exposure to Respirable Crystalline Silica and Improving Respiratory Protection
”, along with supplemental documents. The Agency specifically sought comments on its preliminary determination from the literature review that miners' exposure to respirable crystalline silica presents a risk of material health impairment or functional capacity. MSHA also requested input on any additional adverse health effects that should be included or more recent literature that offers a different perspective. MSHA received numerous comments in response to this request and considered them in preparing the final standalone Health Effects document and the final rule.

This section will describe how MSHA conducted its review of the health effects literature on respirable crystalline silica and what the Agency has found about the toxicity of respirable crystalline silica. This section will also present the findings on the following health effects: (1) Silicosis; (2) Non-malignant respiratory disease (NMRD), excluding silicosis; (3) Lung cancer and cancer at other sites; (4) Renal disease; and (5) Autoimmune diseases. Public comments received are reflected throughout this section.

A. General Approach to Health Effects Literature Review

MSHA reviewed a wide range of health effects literature totaling over 600 studies that explore the relationship between respirable crystalline silica exposure and resultant adverse health effects in miners and other workers across various industries. The health effects literature reviewed by MSHA included both studies reviewed by OSHA for its 2016 respirable crystalline silica standard and many other newer studies and studies that focused specifically on the mining industry.

OSHA's “Health Effects Analysis and Preliminary Quantitative Risk Assessment” (2013b) included studies that were identified from previously published scientific reviews, such as the IARC (1997) and NIOSH (2002), and from newer evaluations of scientific literature, literature searches, and contact with experts and stakeholders. That document underwent extensive peer review by a panel of nationally recognized experts in occupational epidemiology, biostatistics and risk assessment, animal and cellular toxicology, and occupational medicine who had no conflict of interest (COI) or apparent bias in performing the review. These experts were asked to consider the strengths, weaknesses, interpretations, and inclusion of studies used to support the findings, and OSHA revised the document based on their feedback.

To ensure that its literature review was thorough and up to date, MSHA reviewed a large body of additional evidence beyond the studies considered by OSHA. It added many studies focused on miners' exposures to respirable crystalline silica, as well as newer studies published over the past decade. MSHA drew upon numerous studies conducted by NIOSH, the International Agency for Research on Cancer (IARC), the National Toxicology Program (NTP), and other researchers. These studies provided epidemiological data, analyses of morbidity (having a disease or a symptom of disease) and mortality (disease resulting in death), progression and pathology evaluations, death certificate and autopsy reviews, medical surveillance data, health hazard assessments, in vivo (animal) and in vitro (cell-based) toxicity data, and other toxicological reviews. These studies are cited throughout this summary and are listed in the References section of MSHA's standalone Health Effects

document. Additionally, these studies appear in the rulemaking docket.

MSHA received some comments from industry stakeholders who disagreed with MSHA's selection of studies for its literature review and therefore with its findings. The Nevada Mining Association (NVMA) and the Sorptive Minerals Institute (SMI) stated that not all relevant studies were discussed in the Health Effects literature review (Document ID 1441; 1446). NVMA also stated that the studies referenced are outdated. The National Stone, Sand, & Gravel Association (NSSGA) stated that MSHA's review is overly reliant on OSHA's review (2013b) (Document ID 1448, Attachment 3). The state mining association stated that the studies MSHA considered do not recognize that the likelihood of prolonged exposure to respirable crystalline silica has been dramatically reduced over the years, noting improvements to respirators, equipment, and engineering controls (Document ID 1441).

However, commenters from health and labor organizations stated that MSHA's review was thorough, was consistent with the scientific consensus, and addressed the primary health effects of concern. These commenters agreed with MSHA's findings and conclusions related to health risks from exposure to respirable crystalline silica (Document ID 1398; 1405; 1410; 1416). The American Public Health Association (APHA) also noted the inclusion of several recent peer-reviewed publications included in MSHA's review (Document ID 1416). The American College of Occupational and Environmental Medicine (ACOEM) commented that there has been an explosion of new information about the molecular basis for silica's adverse effects since OSHA's comprehensive summary of the medical literature in its preamble to the 2016 revisions to the silica standard (Document ID 1405). This commenter stressed that this new information only adds to the urgency of establishing and enforcing MSHA's proposed standard and applauded the Agency's review of the medical and epidemiologic literature on the health effects of silica exposure.

MSHA has taken several steps to ensure that its review of health effects literature represents the current understanding of health risks related to exposures to respirable crystalline silica. In its initial standalone Health Effects document, which was published alongside the proposed rule, MSHA included several recent publications (published as late as 2022), and since then, it has added more recent publications (through 2023) in its final standalone Health Effects document. Examples of recent literature included in the standalone Health Effects document are: Carrington and Hershberger (2022), Cohen
et al.
(2022), Descatha
et al.
(2022), Hall
et al.
(2022), and Keles
et al.
(2022). Furthermore, many of the more recent studies included miners regulated under the existing MSHA PEL of 100 µg/m
3
(
e.g.,
Almberg
et al.,
2017, 2018a; Graber
et al.,
2017; Blackley
et al.,
2018a; Cohen
et al.,
2022). In response to the comment that the initial standalone Health Effects document did not take into account improved mining conditions or contemporary engineering controls, the Agency notes that it considered several studies featuring miners in a larger range of exposure groups, including some that had lower exposure levels (
e.g.,
Mannetje
et al.,
2002b; Park
et al.,
2002; Buchanan
et al.,
2003; Attfield and Costello, 2004; Chen
et al.,
2012).

Two commenters (an industry trade association and a training consulting company) stated that MSHA presented a significant amount of data showing the consequences of the various chronic health effects that silica can and does have on the human body but no viable data on mortality and morbidity among MNM miners (Document ID 1442; 1392).

As discussed elsewhere, MSHA is not required to prove a risk of death due to silica exposure to justify regulating to reduce a silica health risk. But the evidence shows that respirable silica exposure causes death as well as chronic disease. MSHA reviewed and discussed multiple studies that reported an increase in mortality rates throughout the standalone Health Effects document (
e.g.,
Bang
et al.,
2005; Mazurek and Wood, 2008a; Liu
et al.,
2017a; Wang
et al.,
2020a). Examples of MNM morbidity studies included are Mamuya
et al.
(2007), Tse
et al.
(2007a), Rego
et al.
(2008), Reynolds
et al.
(2016), and Wang
et al.
(2020b); while MNM specific mortality studies include Attfield and Costello (2004), Chen
et al.
(2005, 2012), Schubauer-Berigan
et al.
(2009), and Vacek
et al.
(2011), among others. MSHA considered the best available evidence for MNM and concludes that MNM miners have an increased mortality and morbidity due to exposure to respirable crystalline silica.

Commenters from health and labor organizations suggested additional studies for MSHA to include in the final standalone Health Effects document (Document ID 1405; 1373; 1449). These studies included topics such as new information regarding the molecular basis for silica's adverse health effects or related to engineered stone workers. One commenter stated that MSHA should include studies from outside of the mining industry (Document ID 1448, Attachment 3).

MSHA thoroughly reviewed these studies and did not find sufficient evidence to alter MSHA's overall conclusions of health risk, as discussed in detail in the sections that follow. However, MSHA did add many of the recommended studies to its final standalone Health Effects document (
e.g.,
Chilosi
et al.,
2003; Chen
et al.,
2018; Cao
et al.,
2020). MSHA also reviewed other suggested literature, including promising animal studies exploring novel drug treatments for diseases caused by exposure to respirable crystalline silica; however, it determined that these studies are not sufficiently developed for inclusion at this time (
e.g.,
Guo
et al.,
2019; Huang
et al.,
2019; Jia
et al.,
2022). MSHA has already included several studies related to non-mining occupations throughout its standalone Health Effects document. Examples of other occupational studies include studies of health effects on granite workers (
e.g.,
Davis
et al.,
1983; Attfield and Costello, 2004), brick workers (
e.g.,
Merlo
et al.,
1991), agate stone grinders (Rastogi
et al.,
1991), pottery workers (
e.g.,
McDonald
et al.,
1995; Cherry
et al.,
1998), industrial sand workers (
e.g.,
McDonald
et al.,
2001; Rando
et al.,
2001), concrete workers (
e.g.,
Meijers
et al.,
2001), ceramic workers (
e.g.,
Forastiere
et al.,
2002), and foundry workers (
e.g.,
Hertzberg
et al.,
2002; Vihlborg
et al.,
2017), among others. Occupations such as granite, industrial sand, or concrete workers, represent similar job tasks and exposures which may overlap with mining occupations. Others such as brick, pottery, and ceramic workers involve processing of mined materials into a commercial product.

To analyze the extensive literature that it considered, MSHA used the widely accepted weight-of-evidence (WoE) approach. Under this approach, studies with varied methodologies and conclusions are evaluated for their overall quality. Causal inferences are drawn based on a determination of whether there is substantial evidence that exposure increases the risk of a particular adverse health effect. This approach is a well-accepted method of conducting health hazard assessments (NRC, 2009; NIOSH, 2019a). Additionally, it was used by OSHA in its review of health effects literature (2013b) for its 2016 respirable crystalline silica standard. Factors that MSHA considered in its WoE analysis include: (1) size of the cohort studied and power of the study to detect a

sufficiently low level of disease risk; (2) duration of follow-up of the study population; (3) potential for study bias, such as selection bias or healthy worker effects, and (4) adequacy of underlying exposure information for examining exposure-response relationships. Of the studies examined in the standalone Health Effects document, studies were deemed suitable for inclusion in the FRA if they provided adequate quantitative information on exposure and disease risks and were judged to be of sufficiently high quality according to the above criteria. MSHA's literature review expanded upon OSHA's (2013b) review of the health effects literature to support its final respirable crystalline silica rule (81 FR 16286), reviewing pertinent new research. MSHA's assessment of the literature is consistent with OSHA's conclusion from its silica literature review.

MSHA received one comment from the NSSGA challenging the validity of MSHA's literature review methodology (Document ID 1448, Attachment 3). This commenter submitted a report analyzing MSHA's health effects literature review, arguing that MSHA's review cannot be replicated or fully evaluated for its scientific validity and claiming that it is unclear whether MSHA's interpretations are sufficiently reliable as a basis for decision-making. The commenter asserted the need for literature reviews to be done pursuant to Lynch
et al.
's (2022) framework of a “systematic review,” a review method that seeks to eliminate bias by adhering to a transparent,
a priori
protocol. The commenter also expressed concerns that MSHA's methodology is inadequately explained and possibly dated. The commenter suggested further studies to be included in MSHA's review and provided specific responses to some of MSHA's statements in its literature review.

On the other hand, the APHA provided a different perspective on the methodology (Document ID 1416). This commenter stated that MSHA thoroughly describes the health risks, which include developing chronic silicosis, accelerated silicosis, progressive massive fibrosis, chronic obstructive pulmonary disease, lung cancer and kidney disease. Further, the commenter noted that MSHA's review of the health effects literature included more than three dozen peer-reviewed papers published in just the last few years. This commenter concurred with MSHA's determination that miners' exposure to respirable crystalline silica presents a risk of material impairment of health or functional capacity.

MSHA disagrees with the comment challenging MSHA's methodology. Although the “systematic review” framework outlined in Lynch
et al.
(2022) is increasingly used in review publications, it is not the only valid method of conducting a literature review of the current science. As explained in the standalone Health Effects document, MSHA's review of the scientific literature on respirable crystalline silica used a widely accepted WoE approach.

The term, “weight-of-evidence” was coined as early as 40 years ago by the NRC (1983) in their seminal publication “
Risk Assessment in the Federal Government: Managing the Process
”. It has become a fundamental element of the risk assessment process (NRC, 2009; EPA, 1986; Martin
et al.,
2018; Lee
et al.,
2023). MSHA selected this approach for use in its respirable crystalline silica risk analysis for a variety of reasons. First, it has withstood the scrutiny of scientists throughout the world (Suter
et al.,
2020). Second, it has been used successfully throughout the world for conducting a wide variety of risk assessments and analyses involving a wide range of exposures in both occupational and environmental settings (
e.g.,
drugs, pesticides, industrial chemicals) (EPA, 1986, 2016; National Research Council (NRC), 2009; Suter
et al.,
2020; Government of Canada, 2022). Third, it continues to be a solid and accepted approach that is still used today (EPA, 1986, 2016; National Research Council (NRC), 2009; Martin
et al.,
2018; Suter
et al.,
2020; Government of Canada, 2022; Lee
et al.,
2023). Current searches of the scientific literature (
e.g.,
using search engines such as PubMed or Google Scholar) continue to identify studies in which the WoE approach has been employed. Finally, numerous courts have approved of federal agencies relying on this methodology in rulemaking for over 40 years.
See Mississippi
v.
E.P.A.,
744 F.3d 1334, 1344-45 (D.C. Cir. 2013) (upholding the “weight of evidence approach” because “one type of study might be useful for interpreting ambivalent results from another type . . . and though a new study does little besides confirm or quantify a previous finding, such incremental (and arguably duplicative) studies are valuable precisely because they confirm or quantify previous findings or otherwise decrease uncertainty”) (citing
Ethyl Corp.
v.
EPA,
541 F.2d 1, 26 (D.C. Cir. 1976) (en banc));
N. Am.'s Bldg. Trades Unions
v.
OSHA,
878 F.3d 271, 284 (D.C. Cir. 2017) (rejecting challenges to OSHA's “weight of evidence” approach supporting its silica rulemaking). Thus, MSHA finds that the WoE approach is appropriate for use in its respirable crystalline silica rulemaking.

In summary, MSHA's weight-of-evidence analysis is based on OSHA's extensive literature review and peer review process; includes a substantial number of studies and data published after the OSHA rulemaking; and has received support from NIOSH experts.
16

16
MSHA's review benefitted from feedback and review from experts at NIOSH, both informally and through the interagency review process organized by OMB, during the literature review process and preparation of the standalone Health Effects document.

As described in greater detail in MSHA's standalone Health Effects document, the scientific understanding of how respirable crystalline silica causes adverse health effects has evolved greatly in the more than 45 years since the Mine Act was passed in 1977. MSHA's review of the literature indicates that under the existing standards found in 30 CFR parts 56, 57, 70, 71, and 90, miners are still developing preventable diseases that are material impairments of health or functional capacity. Regulatory action to reduce occupational exposures that cause these diseases is necessary to ensure no miner suffers material impairment of health or functional capacity, as required by section 101(a)(6)(A) of the Mine Act.

Based on an extensive review of health effects literature, MSHA determines that occupational exposure to respirable crystalline silica causes silicosis (acute silicosis, accelerated silicosis, chronic silicosis, and progressive massive fibrosis (PMF)), NMRD (including COPD), lung cancer, and end-stage renal disease (ESRD). Each of these effects is exposure-dependent, potentially chronic, irreversible, potentially disabling, and can be fatal. In addition, MSHA's review of the health effects literature has shown that respirable crystalline silica exposure is causally related to the development of some autoimmune disorders through inflammatory pathways. Current health information cited in the final standalone Health Effects document indicates that miners are suffering material impairment of health or functional capacity due to their occupational exposures to respirable crystalline silica. MSHA's review of respirable crystalline silica health effects concludes that the final rule, which lowers the exposure limits in MNM and coal mining to 50 µg/m
3
and establishes an action level of 25 µg/m
3
for a full-shift exposure, calculated as an 8-hour TWA, will reduce the risk

of miners developing silicosis, NMRD, lung cancer, and renal disease.

B. Toxicity of Respirable Crystalline Silica

Respirable crystalline silica is released into the environment during mining or milling processes, thus creating an airborne hazard. The particles may be freshly generated or re-suspended from surfaces on which they are deposited in mines or mills. Respirable crystalline silica particles may be irregularly shaped and variable in size. These particles may be inhaled by miners and can be deposited throughout the lungs. Some pulmonary clearance of particles deposited in the alveolar region (deep lung) may occur, but many particles can be retained and initiate or advance the disease process. The toxicity of these retained particles is amplified because the particles are not water-soluble and are not metabolized into less toxic compounds. This is important because insoluble dusts may remain in the lungs for prolonged periods, resulting in a variety of cellular responses that can lead to pulmonary disease (ATSDR, 2019). Respirable crystalline silica particles that are cleared from the lungs by the lymphatic system are distributed to the lymph nodes, blood, liver, spleen, and kidneys, potentially accumulating in these other organ systems and causing renal disease and other adverse health effects (ATSDR, 2019).

Physical characteristics relevant to the toxicity of respirable crystalline silica primarily relate to its size and surface characteristics, both of which play important roles in how respirable crystalline silica causes tissue damage. Any factor that influences or modifies these physical characteristics may alter the toxicity of respirable crystalline silica by affecting the mechanistic processes (ATSDR, 2019).

Inflammatory pathways affect disease development in various systems and tissues in the human body. For instance, it has been proposed that lung fibrosis caused by exposure to respirable crystalline silica results from a cycle of cell damage, oxidant generation, inflammation, scarring, and ultimately fibrosis. This has been reported by: Nolan
et al.
(1981), Shi
et al.
(1989, 1998), Lapp and Castranova (1993), Brown and Donaldson (1996), Parker and Banks (1998), Castranova and Vallyathan (2000), Castranova (2004), Fubini
et al.
(2004), Hu
et al.
(2017), Benmerzoug
et al.
(2018), and Yu
et al.
(2020).

Respirable crystalline silica entering the lungs could cause damage by a variety of mechanisms, including direct damage to lung cells. In addition, activation or stimulation by respirable crystalline silica of alveolar macrophages (after phagocytosis) and/or alveolar epithelial cells may lead to: (1) release of cytotoxic enzymes, reactive oxygen species (ROS), reactive nitrogen species (RNS), inflammatory cytokines and chemokines; (2) eventual cell death with the release of respirable crystalline silica; and (3) recruitment and activation of polymorphonuclear leukocytes (PMNs) and additional alveolar macrophages (Castranova and Vallyathan, 2000; Castranova, 2004; Hamilton
et al.,
2008). The elevated production of ROS/RNS could result in oxidative stress and lung injury that stimulate alveolar macrophages, ultimately resulting in fibroblast activation and pulmonary fibrosis (Li
et al.,
2018; Feng
et al.,
2020). The prolonged recruitment of macrophages and PMN causes persistent inflammation, regarded as a primary step in the development of silicosis.

The strong immune response in the lung following exposure to respirable crystalline silica may also be linked to a variety of extra-pulmonary adverse effects such as hypergammaglobulinemia (overproduction of more than one class of immunoglobulins by plasma cells), production of rheumatoid factor, anti-nuclear antibodies, and release of other immune complexes (Haustein and Anderegg, 1998; Green and Vallyathan, 1996; Parks
et al.,
1999). Respirable crystalline silica exposure has also been associated with ESRD through the initiation of immunological injury to the glomerulus of the kidney (Calvert
et al.,
1997).

Proposed mechanisms involved in respirable crystalline silica-induced carcinogenesis have included: direct DNA damage, inhibition of the p53 tumor suppressor gene, loss of cell cycle regulation; stimulation of growth factors, and production on oncogenes (Nolan
et al.,
1981; Shi
et al.,
1989, 1998; Brown and Donaldson, 1996; Castranova, 2004; Fubini
et al.,
2004).

Three commenters expressed concerns about the findings of the health effects literature review and their relevance to the sorptive minerals industry (Document ID 1446, Attachment 1; 1442; 1419). The SMI and Essential Minerals Association (EMA) stated that MSHA has an incomplete understanding of the latest available scientific research (Document ID 1446, Attachment 1; 1442). Asserting that occluded quartz in sorptive clays is not fractured (either in the clay formation in which it exists or during the mining and processing of the material to form sorptive mineral-based products), the SMI concluded that occluded quartz in sorptive clays does not pose the health risk posed by fractured quartz (Document ID 1446, Attachment 1). Discussing at length studies it recommended MSHA include in its health effects literature review, SMI and EMA said that much of this research was previously considered by OSHA (2013b) and that it had led to OSHA's decision to exempt sorptive clays from coverage under OSHA's silica standard. SMI also noted that additional research since OSHA's revised silica standard was promulgated has advanced the question of how quartz causes disease and the difference in risk potential between fractured and unfractured and occluded quartz. Asserting that, without consideration of the additional research provided, the proposed standard would not be based on the best available evidence and would not reflect the latest available scientific data in the field, this commenter discussed Mine Act statutory provisions and case law that it asserted demonstrate the high level of scientific evidence and scrutiny required of MSHA when setting health and safety standards.

A more detailed response to SMI's overall comment can be found in
Section VIII.A. General Issues
of this preamble. In response to the suggestion to consider additional studies, MSHA reviewed the suggested references and added some to the final standalone Health Effects document (Creutzenberg
et al.,
2008; Borm
et al.,
2018; Pavan
et al.,
2019). MSHA also notes that some of these studies were already cited in the version of the standalone Health Effects document published alongside the proposed rule (
e.g.,
Donaldson and Borm, 1998; Fubini, 1998; Bruch
et al.,
2004; Fubini
et al.,
2004). Overall, many of the studies suggested by the commenter have argued that occluded or aged quartz is less toxic but have not suggested that occluded or aged quartz is not toxic or carries no risk of disease. MSHA agrees that there is some evidence to suggest that occluded silica is less toxic than unoccluded silica (Wallace
et al.,
1996), but there is no evidence that occlusion and the initial reduced toxicity persist following deposition and retention of the crystalline silica particles in the lungs. Similarly, animal studies involving respirable crystalline silica suggest that the aged form has lower toxicity than the freshly fractured form; however, the aged form still retains toxicity (Shoemaker
et al.,
1995; Vallyathan
et al.,
1995; Porter
et al.,
2002c). From these studies, MSHA concludes that

exposure to the crystalline silica present in sorptive minerals poses a risk of material impairment of health or functional capacity to miners.

Others appeared to be irrelevant to the scope of the rule, such as those focused on amorphous silica, microscopy techniques, or workshop discussions (
e.g.,
Mercer
et al.,
2018; Weber
et al.,
2018; Driscoll and Borm, 2020). MSHA notes that none of the suggested animal studies included acute or chronic inhalation exposures to aged or occluded respirable crystalline silica. One suggested review, Poland
et al.
(2023) described a 2020 animal inhalation study (nose-only) which did not include exposures to aged or occluded respirable crystalline silica; the 2020 study was conducted using amorphous silica and the data were compared to a 1988 animal study that included whole-body (as opposed to nose-only) exposures to respirable crystalline silica.
17

Since this 2020 surface area comparison study described by Poland
et al.
(2023) focused on amorphous silica, which is not a part of this rulemaking, it was deemed unsuitable for inclusion in MSHA's final standalone Health Effects document. Other animal studies discussing aged or occluded respirable crystalline silica suggested used either intratracheal instillation or oropharyngeal aspiration, which do not reflect the behavior of particles that enter the lungs via inhalation, including lung clearance (Foster
et al.,
2001; Wong, 2007; Driscoll and Borm, 2020).
Section VIII.A. General Issues
of this preamble responds more fully to these comments. In its response, MSHA notes that several studies of occluded or fractured quartz discussed their methods, including careful handling of occluded samples, but did not include analysis of occluded quartz that was analyzed with less than careful handling. This is not applicable to real-world conditions; MSHA's experience with mining and processing of sorptive minerals includes the use of grinding and milling processes.

17
These two studies (1988 and 2020) described by Poland
et al.
(2023) had limited comparability for a variety of reasons; they differ in: (1) rat strains (types of rats), (2) exposure durations, (3) recovery periods, as well as (4) types of inhalation exposure, among others.

After reviewing the available literature, MSHA concludes that miners working in the sorptive minerals industry are exposed to respirable crystalline silica. OSHA (2013b) concluded that while there was considerable evidence that several environmental influences can modify surface activity to either enhance or diminish the toxicity of silica, the available information was insufficient to determine in any quantitative way how these influences may affect disease risk to workers in any particular workplace setting (81 FR at 16311). MSHA agrees with OSHA (2013b) that there is evidence to support that surface activity of respirable crystalline silica may play a role in producing disease. However, mining is significantly different from other industries regulated by OSHA, for instance, in that it involves milling, grinding and removal of overburden. While the available information is insufficient to determine how these influences may affect disease risk to miners in any quantitative way and in any mining sector. MSHA is permitted “ `to err on the side of overprotection by setting a fully adequate margin of safety.' ”
Kennecott Greens Creek Min. Co.
v.
Mine Safety & Health Admin.,
476 F.3d 946, 952 (D.C. Cir. 2007) (quoting
Nat'l Min. Ass'n
v.
Mine Safety & Health Admin.,
116 F.3d 520, 528 (D.C. Cir. 1997)).

C. Diseases

1. Silicosis

Silicosis is a material impairment of health or functional capacity, as defined by the Mine Act, and refers to a group of lung diseases caused by the inhalation of respirable crystalline silica. See 30 U.S.C. 811(a)(6)(A). Silicosis is a progressive, occupational disease, in which accumulation of respirable crystalline silica particles causes an inflammatory reaction in the lung. This reaction leads to lung damage and scarring and, in some cases, progresses to disability and death. Respirable crystalline silica has long been identified as a cause of lung diseases in miners, and adverse health effects were noted and described as early as 1550 by Georgius Agricola (Agricola, as translated by Banner in 1950). Based on the review of the literature, MSHA has determined that exposure to respirable crystalline silica causes silicosis in MNM and coal miners and that it is a significant cause of premature morbidity and mortality (Mazurek and Attfield, 2008; Mazurek and Wood, 2008a,b; Mazurek
et al.,
2015, 2018).

When respirable crystalline silica accumulates in the lungs, it causes an inflammatory reaction, leading to lung damage and scarring. Silicosis can continue to develop even after silica exposure has ceased (Hughes
et al.,
1982; Ng
et al.,
1987a; Hessel
et al.,
1988; Kreiss and Zhen, 1996; Miller
et al.,
1998; Yang
et al.,
2006). It is not reversible, and there is only symptomatic treatment, including bronchodilators to maintain open airways, oxygen therapy, and lung transplants in the most severe cases (Cochrane
et al.,
1956; Ng
et al.,
1987a; Lee
et al.,
2001; Mohebbi and Zubeyri, 2007; Kimura
et al.,
2010; Laney
et al.,
2017; Almberg
et al.,
2020; Hall
et al.,
2022). Respirable crystalline silica exposure in miners can lead to all three forms of silicosis (acute, accelerated, and chronic). These forms differ in the rate of exposure, pathology (structural and functional changes produced by the disease), and latency period from exposure to disease onset.

Acute silicosis is an aggressive inflammatory process following intense exposure to respirable crystalline silica for “periods measured in months rather than years” (Cowie and Becklake, 2016). It causes alveolar proteinosis, an accumulation of lipoproteins in the alveoli of the lungs. This restructuring of the lungs leads to symptoms such as coughing and difficult or labored breathing, and often progresses to profound disability and death due to respiratory failure or infectious complications. In addition, symptoms often advance even after exposure has stopped, primarily due to the massive amount of protein debris and fluid that collects in the alveoli, which leads to the impairment of gas exchange (oxygen) in the lungs and respiratory distress of the patient. The X-ray appearance and results of microscopic examination of acute silicosis are like those of idiopathic (having an unknown cause) pulmonary alveolar proteinosis.

Accelerated silicosis includes both inflammation and fibrosis and is associated with intense respirable crystalline silica exposure. Accelerated silicosis usually manifests over a period of three to ten years (Cowie and Becklake, 2016), but it can develop in as little as two to five years if exposure is sufficiently intense (Davis, 1996). Accelerated silicosis may have features of both chronic and acute silicosis, with alveolar proteinosis in addition to X-ray evidence of fibrosis, seen as small opacities or the large opacities of PMF. Although the symptoms are like those of chronic silicosis, the clinical and radiographic progression of accelerated silicosis evolves more rapidly, and often leads to PMF, severe respiratory impairment, and respiratory failure. Accelerated silicosis can progress with associated morbidity and mortality, even if exposure ceases. Accelerated silicosis is frequently fatal.

Chronic silicosis is the most frequently observed form of silicosis in the United States today (Banks, 2005; OSHA, 2013b; Cowie and Becklake,

2016). It is also the most common form of silicosis diagnosed in miners. Chronic silicosis is a fibrotic process that typically follows less intense respirable crystalline silica exposure of ten or more years (Becklake, 1994; Balaan and Banks, 1998; NIOSH, 2002b; Kambouchner and Bernaudin, 2015; Cowie and Becklake, 2016; Rosental, 2017; ATSDR, 2019; Barnes
et al.,
2019; Hoy and Chambers, 2020). It is identified histopathologically by the presence of the silicotic islet or nodule that is an agent-specific fibrotic lesion and is recognized by its pathology (Balaan and Banks, 1998). Chronic silicosis develops slowly and creates rounded whorls of scar tissue that progressively destroy the normal structure and function of the lungs. In addition, the scar tissue opacities become visible by chest X-ray or computerized tomography (CT) only after the disease is well-established and the lesions become large enough to view. As a result, surveys based on identification of small and large opacity disease on chest X-ray films usually underestimate the true prevalence of silicosis (Craighead and Vallyathan, 1980; Hnizdo
et al.,
1993; Rosenman
et al.,
1997; Cohen and Velho, 2002). The lesions eventually advance and result in lung restriction, reduced lung volumes, decreased pulmonary compliance, and reduction in the gas exchange capabilities of the lungs (Balaan and Banks, 1998). As the disease progresses, affected miners may have chronic cough, sputum production, shortness of breath, and reduced pulmonary function.

Among coal miners, silicosis is usually found in conjunction with simple coal workers' pneumoconiosis (CWP) because of the miners' exposures to RCMD that also contains respirable crystalline silica (Castranova and Vallyathan, 2000). Coal miners also face an added risk of developing mixed-dust pneumoconiosis (MDP) (includes the presence of coal dust macules), mixed-dust fibrosis (MDF), and/or silicotic nodules (Honma
et al.,
2004; Green, 2019). The autopsy studies on coal miners that MSHA reviewed support a pathological relationship between mixed-RCMD or respirable crystalline silica exposures and PMF, silicosis, and CWP (Davis
et al.,
1979; Ruckley
et al.,
1981, 1984; Douglas
et al.,
1986; Fernie and Ruckley, 1987; Green
et al.,
1989, 1998b; Attfield
et al.,
1994; Vallyathan
et al.,
2011; Cohen
et al.,
2016, 2019, 2022). Autopsy studies in British coal miners indicated that the more advanced the disease, the more mixed-RCMD components were retained in the lung tissue (Ruckley
et al.,
1984; Douglas
et al.,
1986). Green
et al.
(1998b) determined that of 4,115 coal miners with pneumoconiosis autopsied as part of the National Coal Workers' Autopsy Study (NCWAS), 39 percent had mixed dust nodules and 23 percent had silicotic nodules.

PMF or “complicated silicosis” has been diagnosed in both coal and MNM miners exposed to dusts containing respirable crystalline silica. Recent literature on the pathophysiology of PMF supports the importance of crystalline silica as a cause of PMF in silica-exposed workers such as coal miners (Cohen
et al.,
2016, 2022), sandblasters (Hughes
et al.,
1982; Abraham and Wiesenfeld, 1997), industrial sand workers (Vacek
et al.,
2019), hard rock miners (Verma
et al.,
1982, 2008), and gold miners (Carneiro
et al.,
2006a; Tse
et al.,
2007b).

a. Classifying Radiographic Findings of Silicosis

The studies reviewed by MSHA used one of two established methods for identifying findings of pneumoconiosis: the International Labour Office (ILO) Classification System or the Chinese categorization system, each of which is described below. In addition, the NIOSH case definition of silicosis used in surveillance systems relies on the ILO system.

The ILO developed a standardized system to classify the radiographic appearances of pneumoconiosis identified in chest X-rays films or digital chest radiographic images (ILO, 1980, 2002, 2011, 2022). One aspect of the ILO system involves grading the size, shape, and profusion (density) of opacities in the lungs. The density of opacities is classified on a four-point major category scale (category 0, 1, 2, or 3), with each major category divided into three subcategories, giving a 12-point scale between 0/− and 3/+. Differences between ILO categories are subtle. For each subcategory, the top number indicates the major category that the profusion most closely resembles, and the bottom number indicates the major category that was given secondary consideration. For example, film readers may assign classifications such as 1/0, which means the reader classified it as category 1, but category 0 (normal) was also considered (ILO, 2022). Major category 0 indicates the absence of visible opacities consistent with pneumoconiosis and categories 1 to 3 reflect increasing profusion of opacities and a concomitant increase in severity of disease.

However, some studies in MSHA's literature review used the Chinese system of X-ray classification based on the “Radiological Diagnostic Criteria of Pneumoconiosis and Principles for Management of Pneumoconiosis” (GB5906-86). This includes four categories of pneumoconiosis findings: a suspected case (0+), stage I, stage II, or stage III. Under this scheme, a panel of three radiologists determines the presence and severity of radiographic changes consistent with pneumoconiosis. The four categories correspond to ILO profusion category 0/1, category 1, category 2, and category 3, respectively. A suspected case of silicosis (0+) in a dust-exposed worker refers to a dust response in the lung and its corresponding lymph nodes, or a scale and severity of small opacities that fall short of the level observed in a stage I case of silicosis (Chen
et al.,
2001; Yang
et al.,
2006).

MSHA's analysis of silicosis studies uses NIOSH's surveillance case definition to determine the presence of silicosis. As described further in the final standalone Health Effects document, NIOSH defines the presence of silicosis in terms of the ILO system and considers a small opacity profusion score of 1/0 or greater to indicate pneumoconiosis (NIOSH, 2014b). This definition originated from testimony before Congress regarding the 1969 Coal Act in which the Public Health Service recommended that miners be removed from dusty environments as soon as they showed “minimal effects” of dust exposure on a chest X-ray (
i.e.,
pinpoint, dispersed micro-nodular lesions). MSHA interprets “minimal effects” to mean an X-ray ILO profusion score of category 1/0 or greater. This is also consistent with Hnizdo
et al.
(1993), which recommended that, due to the low sensitivity of chest x-rays for detecting silicosis, radiographs consistent with an ILO category of 0/1 or greater be considered indictive of silicosis among workers exposed to a high concentration of silica-containing dust.

b. Progression and Associated Impairment

MSHA reviewed studies referenced by OSHA (2013b) that examined the relationship between exposure and progression, as well as between X-ray findings and pulmonary function. Additionally, MSHA considered literature not previously reviewed by OSHA (2013b) (Mohebbi and Zubeyri, 2007; Wade
et al.,
2011; Dumavibhat
et al.,
2013).

Progression of silicosis is recognized when there are changes or worsening of the opacities in the lungs, and sequential chest radiographs are

classified higher by one or more subcategories (
e.g.,
from 1/0 to 1/1) because of changes in the location, thickness, or extent of lung abnormalities and/or the presence of calcifications. The higher the category number, the more severe the disease. Due to the variability in film technique and classification of films, some investigators count progression as advancing two or more subcategories, such as 1/0 to 1/2.

Overall, the studies indicate that progression is more likely with continued exposure, especially high average levels of exposure. Progression is also more likely for miners with higher ILO profusion classifications. As discussed previously, progression of disease may continue after miners are no longer exposed to respirable crystalline silica (Cochrane
et al.,
1956; Maclaren and Soutar, 1985; Hurley
et al.,
1987; Kimura
et al.,
2010; Almberg
et al.,
2020; Hall
et al.,
2020b). In addition, although lung function impairment is highly correlated with chest X-ray films indicating silicosis, researchers caution that respirable crystalline silica exposure could impair lung function before it is detected by X-ray.

Of the studies in which silicosis progression was documented in populations of workers, four included quantitative exposure data that were based on either existing exposure levels or historical measurements of respirable crystalline silica (Ng
et al.,
1987a study of granite miners; Hessel
et al.,
1988 study of gold miners; Miller
et al.,
1998 study of coal miners; Miller and MacCalman, 2010 study of coal miners). In some studies, episodic exposures to high average concentrations were documented and considered in the analysis. These exposures were strong predictors of more rapid progression beyond that predicted by cumulative exposure alone. Otherwise, the variable most strongly associated in these studies with progression of silicosis was cumulative respirable crystalline silica exposure (the product of the concentration times duration of exposure, which is summed over time) (Ng
et al.,
1987a; Hessel
et al.,
1988; Miller
et al.,
1998; Miller and MacCalman, 2010). In the absence of concentration measurements, duration of employment in specific occupations known to involve exposure to high levels of respirable dust has been used as a surrogate for cumulative exposure to respirable crystalline silica. Duration of employment has also been found to be associated with the progression of silicosis (Ogawa
et al.,
2003a).

Miller
et al.
(1998) examined the impact of high quartz exposures on silicosis disease progression in 547 British coal miners from 1990 to 1991 and evaluated chest X-ray changes after the mines closed in 1981. The study reviewed chest X-rays taken during health surveys conducted between 1954 and 1978 and data from extensive exposure monitoring conducted between 1964 and 1978. For some occupations, exposure was high because miners had to dig through a sandstone stratum to reach the coal. For example, quarterly mean respirable crystalline silica (quartz) concentrations ranged from 1,000 to 3,000 µg/m
3
and for a brief period, concentrations exceeded 10,000 µg/m
3
for one job. Some of these high exposures were associated with accelerated disease progression in these miners.

Buchanan
et al.
(2003) reviewed the exposure history and chest X-ray progression of 371 retired miners and found that short-term exposures (i.e
.,
“a few months”) to high concentrations of respirable crystalline silica (
e.g.,
>2,000 µg/m
3
) increased the silicosis risk by three-fold (compared to the risk of cumulative exposure alone) (see the standalone FRA document).

The risks of increased rate of progression predicted by Buchanan
et al.
(2003) have been seen in coal miners (Miller
et al.,
1998; Laney
et al.,
2010, 2017; Cohen
et al.,
2016), metal (Hessel
et al.,
1988; Hnizdo and Sluis-Cremer, 1993; Nelson, 2013), and nonmetal miners such as silica plant and ground silica mill workers, whetstone cutters, and silica flour packers (NIOSH, 2000a,b; Ogawa
et al.,
2003a; Mohebbi and Zubeyri, 2007). Accordingly, it is important to limit higher exposures to respirable crystalline silica to minimize the risk of rapid progressive pneumoconiosis (RPP) in miners. RPP is the development of progressive massive fibrosis (PMF) and/or an increase in small opacity profusion greater than one subcategory over five years or less (Antão
et al.,
2005).

The results of many surveillance studies conducted by NIOSH as part of the Coal Workers' Health Surveillance Program indicate that the pathology of pneumoconiosis in coal miners has changed over time, in part due to increased exposure to respirable crystalline silica. The studies of Cohen
et al.
(2016, 2022) indicate that RPP develops due to increased exposure to respirable crystalline silica among contemporary coal miners as compared to historical coal miners. Through the examination of pathologic materials from 23 contemporary (born in or after 1930) and 62 historical coal miners (born between 1910 and 1930) with severe pneumoconiosis, who were autopsied as part of NCWAS, Cohen
et al.
(2022) found a significantly higher proportion of silica-type PMF among contemporary miners (57 percent vs. 18 percent, p <0.001). They also found that mineral dust alveolar proteinosis (MDAP) was more common in the current generation of miners and that the lung tissues of contemporary coal miners contained a significantly greater percentage and concentration of silica particles than those of past generations of miners.

Many studies found an association between pulmonary function decrements and ILO profusion category 2 or 3. Additionally, the review of the literature indicated a decreased lung function among workers who were exposed to respirable crystalline silica. MSHA therefore concludes that respirable crystalline silica exposure may impair lung function in some instances before silicosis can be detected by chest X-rays.

c. Occupation-Based Epidemiological Studies

MSHA reviewed the occupation-based epidemiological literature, which examines health outcomes among workers and their potential association with conditions in the workplace. In addition, MSHA reviewed additional occupation-based literature specific to respirable crystalline silica exposure in MNM and coal miners and concludes that respirable crystalline silica exposure increases the risk of silicosis morbidity and early mortality.

One study examined the acute and accelerated silicosis outbreak that occurred during and after construction of Hawk's Nest Tunnel in West Virginia from 1930 to 1931. There, an estimated 2,500 men worked in a tunnel drilling rock consisting of 90 percent silica or more. The study later estimated that at least 764 of the 2,500 workers (30.6 percent) died from acute or accelerated silicosis (Cherniack, 1986). There was also high turnover among the tunnel workers, with an average length of employment underground of only about two months.

MSHA's review included the occupation-based literature cited by OSHA (2013b) in developing its respirable crystalline silica standard (OSHA, 2016a). Overall, MSHA found substantial evidence suggesting that occupational exposure to respirable crystalline silica increases the risk of silicosis. This conclusion is consistent with OSHA's conclusion.

In a population of granite quarry workers (mean length of employment:

23.4 years) exposed to an average respirable crystalline silica concentration of 480 µg/m
3
, 45 percent of those diagnosed with simple silicosis showed radiological progression of disease two to ten years after diagnosis (Ng
et al.,
1987a). Among a population of gold miners, 92 percent showed progression after 14 years (Hessel
et al.,
1988). Chinese factory workers and miners who were categorized under the Chinese system of X-ray classification as “suspected” silicosis cases (analogous to ILO 0/1) had a progression rate to stage I (analogous to ILO major category 1) of 48.7 percent, with an average interval of about 5.1 years (Yang
et al.,
2006).

The risk of silicosis, and particularly its progression, carries with it an increased risk of reduced lung function. Strong evidence has shown that lung function deteriorates more rapidly in miners exposed to respirable crystalline silica, especially in those with silicosis (Hughes
et al.,
1982; Ng and Chan, 1992; Malmberg
et al.,
1993; Cowie, 1998). The rates of decline in lung function are greater where disease shows evidence of radiologic progression (Bégin
et al.,
1987; Ng
et al.,
1987a; Ng and Chan, 1992; Cowie, 1998). Additionally, the average deterioration of lung function exceeds that in smokers (Hughes
et al.,
1982).

Blackley
et al.
(2015) found progressive lung function impairment across the range of radiographic profusion of simple CWP in a cohort of 8,230 coal miners that participated in the Enhanced Coal Workers' Health Surveillance Program from 2005 to 2013. There, 269 coal miners had category 1 or 2 chronic CWP. This study also found that each increase in profusion score was associated with decreases in various lung function parameters: 1.5 percent (95 percent CI, 1.0 percent-1.9 percent) in forced expiratory volume in one sec

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Source: Frix Law Library, https://www.frixlaw.com/law-library/documents/fr%3A2024-06920. Public record. Not legal advice.