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

Federal RegisterJul 13, 2023

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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:

Proposed rule; request for comments; notice of public hearings.

SUMMARY:

The Mine Safety and Health Administration (MSHA) proposes to amend its existing standards to better protect miners against occupational exposure to respirable crystalline silica, a carcinogenic hazard, and to improve respiratory protection for all airborne hazards. MSHA has preliminarily determined that under the Agency's existing standards, miners at metal and nonmetal mines and coal mines face a risk of material impairment of health or functional capacity from exposure to respirable crystalline silica. MSHA proposes to set the permissible exposure limit of respirable crystalline silica at 50 micrograms per cubic meter of air (µg/m

) for a full shift exposure, calculated as an 8-hour time-weighted average, for all miners. MSHA's proposal would also include other requirements to protect miner health, such as exposure sampling, corrective actions to be taken when miner exposure exceeds the permissible exposure limit, and medical surveillance for metal and nonmetal miners. Furthermore, the proposal would replace existing requirements for respiratory protection and incorporate by reference

ASTM F3387-19 Standard Practice for Respiratory Protection.

The proposed uniform approach to respirable crystalline silica occupational exposure and improved respiratory protection for all airborne hazards would significantly improve health protections for all miners and lower the risk of material impairment of health or functional capacity.

DATES:

Written comments.

Written comments, including comments on the information collection requirements described in this preamble, must be received or postmarked by midnight Eastern Time on August 28, 2023.

Public Hearings.

MSHA will hold two public hearings on August 3, 2023 in Arlington, Virginia and August 21, 2023 in Denver, Colorado. For more information on the public hearings, see

SUPPLEMENTARY INFORMATION

ADDRESSES:

All submissions must include RIN 1219-AB36 or Docket No. MSHA-2023-0001. You should not include personal or proprietary information that you do not wish to disclose publicly. If you mark parts of a comment as “business confidential” information, MSHA will not post those parts of the comment. Otherwise, MSHA will post all comments without change, including any personal information provided. MSHA cautions against submitting personal information.

You may submit comments and informational materials, clearly identified by RIN 1219-AB36 or Docket Id. No. MSHA-2023-0001, by any of the following methods:

Federal E-Rulemaking Portal: https://www.regulations.gov.

Follow the online instructions for submitting comments.

Email: zzMSHA-comments@dol.gov.

Include “RIN 1219-AB36” in the subject line of the message.

Regular Mail:

MSHA, Office of Standards, Regulations, and Variances, 201 12th Street South, Suite 4E401, Arlington, Virginia 22202-5450.

Hand Delivery or Courier:

MSHA, Office of Standards, Regulations, and Variances, 201 12th Street South, Suite 4E401, Arlington, Virginia, between 9:00 a.m. and 5:00 p.m. Monday through Friday, except Federal holidays. Before visiting MSHA in person, call 202-693-9440 to make an appointment. Special health precautions may be required.

Facsimile:

202-693-9441. Include “RIN 1219-AB36” in the subject line of the message.

Information Collection Requirements.

Comments concerning the information collection requirements of this proposed rule must be clearly identified with “RIN 1219-AB36” or “Docket No. MSHA-2023-0001,” and sent to MSHA by one of the methods previously explained.

Docket.

For access to the docket to read comments and background documents, go to

https://www.regulations.gov.

The docket can also be reviewed in person at MSHA, Office of Standards, Regulations, and Variances, 201 12th Street South, Arlington, Virginia, between 9 a.m. and 5 p.m. Monday through Friday, except Federal holidays. Before visiting MSHA in person, call 202-693-9440 to make an appointment. Special health precautions may be required.

Email Notification.

To subscribe to receive an email notification when MSHA publishes rulemaking documents in the

Federal Register

, go to

https://public.govdelivery.com/accounts/USDOL/subscriber/new.

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:

MSHA will hold two public hearings to provide industry, labor, and other interested parties with an opportunity to present oral statements, written comments, and other information on the proposed rule. The public hearings will begin at 9 a.m. local time and end after the last presenter speaks on the following dates:

Date

Location

Contact

number

August 3, 2023

Mine Safety and Health Administration, 201 12th Street South, Room 7W202, Arlington, VA 22202

202-693-9440

August 21, 2023

Denver Federal Center, Building 25 Lecture Hall, West 6th Avenue and Kipling Street, Denver, CO 80225

202-693-9440

The public hearings will begin with an opening statement from MSHA, followed by an opportunity for members of the public to make oral presentations. Speakers and other attendees may present information to MSHA for inclusion in the rulemaking record. The hearings will be conducted in an informal manner. Formal rules of evidence or cross examination will not apply.

A verbatim transcript of each of the proceedings will be prepared and made a part of the rulemaking record. Copies of the transcripts will be available to the public. MSHA will make the transcript of the hearings available at

http://www.regulations.gov

and on MSHA's website at

https://arlweb.msha.gov/currentcomments.asp.

MSHA will accept post-hearing written comments and other appropriate information for the record from any interested party, including those not presenting oral statements, received by

midnight (Eastern Time) on August 28, 2023.

Pre-registration is not required to attend the hearings. Interested parties may attend the hearings virtually or in person. Interested parties who intend to present testimony at the hearings are asked to register in advance on MSHA's website (

http://www.msha.gov

). Speakers will be called in the order in which they signed up. Those who do not register in advance will have an opportunity to speak after all those who pre-registered have spoken. You may submit hearing testimony and documentary evidence, identified by docket number (MSHA-2023-0001), by any of the methods previously identified. Additional information on how to access the public hearings will be posted when available at

https://www.msha.gov/regulations/rulemaking.

The preamble to the proposed standard follows this outline:

I. Introduction

II. Request for Comments

III. Background

IV. Existing Standards and Implementation

V. Health Effects Summary

VI. Preliminary Risk Analysis Summary

VII. Section-by-Section Analysis

VIII. Technological Feasibility

IX. Summary of Preliminary Regulatory Impact Analysis and Regulatory Alternatives

X. Initial Regulatory Flexibility Analysis

XI. Paperwork Reduction Act

XII. Other Regulatory Considerations

XIII. References Cited in the Preamble

XIV. Appendix

Acronyms and Abbreviations

COPD chronic obstructive pulmonary disease

ESRD end-stage renal disease

FEV forced expiratory volume

FVC forced vital capacity

L/min liter per minute

mg milligram

mg/m

milligrams per cubic meter

mL milliliter

µg/m

micrograms per cubic meter

MNM metal and nonmetal

NMRD nonmalignant respiratory disease

PEL permissible exposure limit

PMF progressive massive fibrosis

RCMD respirable coal mine dust

REL recommended exposure limit

SiO

silica

TB tuberculosis

TLV® Threshold Limit Value

TWA time-weighted average

I. Introduction

With the passage of the Federal Mine Safety and Health Act of 1977 (Mine Act), Congress declared that “the first priority and concern of all in the coal or other mining industry must be the health and safety of its most precious resource—the miner[.]” 30 U.S.C. 801(a). In furtherance of that clear guiding principle, this proposed rule promotes MSHA's mission and statutory mandate to prevent death, illness, and injury from mining and promote safe and healthful workplaces for U.S. miners. This proposal provides the public with the opportunity to comment on the Agency's proposed uniform and streamlined regulatory approach to lowering miners' exposure to respirable crystalline silica and improving respiratory protection.

Exposure to silica dust causes adverse health effects, including silicosis (acute silicosis, accelerated silicosis, simple chronic silicosis, and progressive massive fibrosis (PMF)), nonmalignant respiratory diseases (NMRD) (

e.g.,

emphysema and chronic bronchitis), lung cancer, and renal diseases. Each of these effects is chronic, irreversible, and potentially disabling or fatal. Silica dust is generated in most mining activities, including cutting, sanding, drilling, crushing, grinding, sawing, scraping, jackhammering, excavating, and hauling materials that contain silica, and is found in all mines—underground and surface metal and nonmetal (MNM) and coal mines. In a mining context, silica exposures may occur in respirable dust together with exposures to other airborne contaminants and combustion biproducts.

MSHA's existing standards, established in the early 1970s, help protect miners from the most dangerous levels of exposure to respirable crystalline silica. However, since their promulgation, scientific understanding of respirable crystalline silica toxicity has advanced, and the National Institute for Occupational Safety and Health (NIOSH) has recommended a respirable crystalline silica exposure level of 50 µg/m

for workers. In 2016, the Occupational Safety and Health Administration (OSHA) established a permissible exposure limit (PEL) of 50 µg/m

in many industry sectors that it regulates.

To provide miners with exposure limits consistent with workers in other industries and NIOSH's recommendation, and to improve miners' health, MSHA proposes to lower its existing exposure limits to 50 µg/m

for respirable crystalline silica in MNM and coal mines. MSHA considered exposure limits below 50 µg/m

. However, MSHA believes, based on a review of the Agency's available silica sample data, that an exposure limit of 25 µg/m

may not be achievable for all mines. The proposed PEL would be expressed as a full-shift exposure, calculated as an 8-hour time-weighted average (TWA). Importantly, a uniform proposed PEL for all mines would make compliance simpler—especially for coal mines by eliminating the existing respirable dust standard when quartz is present.

To meet the requirements of the proposed PEL, mine operators would have to implement engineering controls, followed by administrative controls if supplementary protection is needed. Engineering controls, which are most effective, are designed to remove or reduce the hazard at the source and could include the installation of proper ventilation systems, use of water sprays or wetting agents to suppress airborne contaminants, installation of machine-mounted dust collectors to capture respirable crystalline silica and other contaminants, and the installation of control booths or environmental cabs to enclose equipment operators. Administrative controls, which are often less effective than engineering controls, are designed to change the way miners work. One example would be ensuring that miners safely clean dust off their work clothes so that they are not exposed to respirable dust after their shift ends.

MSHA's proposed rule would further protect all miners by requiring exposure sampling and corrective actions when miners' exposures exceed the proposed PEL, as well as periodic sampling when miners' exposure levels meet or exceed the proposed action level. The proposed rule also includes medical surveillance requirements for MNM miners (medical surveillance requirements already exist for coal miners). Proposed medical examinations would include chest X-rays, spirometry, symptom assessment, and occupational history and would be provided at no cost to the miner.

Finally, the proposed rule would incorporate by reference an updated respiratory protection standard, ASTM F3387-19, “

Standard Practice for Respiratory Protection

” (ASTM F3387-19), for respirable crystalline silica and all other regulated airborne contaminants. This voluntary consensus standard represents up-to-date advancements in respiratory protection technologies, practices, and techniques, including proper selection, use, and maintenance of respirators. The proposed incorporation of ASTM F3387-19 by reference would better protect all miners from airborne hazards. However, respiratory protection should only be relied upon as an exposure control measure in limited situations and on a temporary basis, and to supplement engineering controls, followed by administrative controls.

Taken together, all elements of the proposed rule are technologically and economically feasible. MSHA's 2014

final rule,

Lowering Miners' Exposure to Respirable Coal Mine Dust, Including Continuous Personal Dust Monitors

(Coal Dust Rule) improved health protections for coal miners by lowering exposure limits to respirable coal mine dust and establishing sampling requirements that included the use of a Continuous Personal Dust Monitor (79 FR 24813, May 1, 2014). Coal mine operators have generally achieved compliance with the respirable dust standards primarily by implementing or adjusting existing engineering controls. Coal mine operators' sampling data and MSHA's compliance data show that operators have lowered coal miners' exposures to respirable coal mine dust and to respirable crystalline silica. Data show that average exposures in coal mines are below the proposed PEL of 50 μg/m

, and therefore, corrective measures would often not be needed. Similarly, for MNM miners, MSHA data also show that most exposures to respirable crystalline silica are below the proposed PEL. However, at MNM and coal mines where elevated exposures are found, operators will be able to reduce exposures to the proposed PEL through some combination of properly maintaining existing engineering controls, implementing new engineering controls, and requiring safe work practices. Mines and laboratories will be able to meet exposure monitoring requirements with existing validated and widely used sampling and analytical methods. The proposed revision to the respiratory protection standard is technologically feasible because MSHA's existing respiratory protection requirements for selecting, fitting, using, and maintaining respiratory protection include similar requirements.

MSHA's Preliminary Risk Analysis (PRA) suggests that exposure consistent with a lower proposed PEL of 50 µg/m

would deliver many health benefits to miners who currently experience exposures above the proposed PEL by reducing the likelihood of respirable crystalline silica-related diseases. For those miners working only under the proposed PEL, MSHA estimates that the proposed rule would result in a total of 799 lifetime avoided deaths (63 in coal and 736 in MNM mines) and 2,809 lifetime avoided morbidity cases (244 in coal and 2,566 in MNM mines) over a 60-year period. MSHA expects full implementation and compliance to reduce lifetime mortality risk due specifically to silica exposures by 9.5 percent and to reduce silicosis morbidity risk by 41.9 percent. The latter statistic is particularly important to coal miners given surveillance findings noted by the National Academies of Sciences, Engineering, and Medicine that severe pneumoconiosis where respirable crystalline silica is likely an important contributor is presenting in relatively young miners, sometimes in their late 30's and early 40's.

MSHA's economic analysis estimates that the proposed respirable crystalline silica rule would cost an average of $56.1 million per year in 2021 dollars at an undiscounted rate, $57.6 million at a 3 percent discount rate, and $59.9 million at a 7 percent discount rate. Based on the results of the Preliminary Regulatory Impact Analysis (PRIA), MSHA estimates that the proposed rule's benefits would exceed its costs, with or without discount rates. Monetized benefits are estimated from avoidance of 410 deaths related to NMRD, silicosis, ESRD, and lung cancer and 1,420 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 $212.8 million at an undiscounted rate, $118.2 million at a 3 percent discount rate, and $36.3 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 or . . . adversely affect in a material way the economy, a sector of the economy, productivity, competition, jobs, the environment, public health or safely, or State, local, or tribal governments or communities.” The Office of Management and Budget has determined that the proposed rule is significant within the meaning of E.O. 12866 Section 3(f)(1).

The proposed rule would strengthen MSHA's existing regulatory framework. It would establish a uniform proposed PEL that provides all MNM and coal miners with the same exposure limits for respirable crystalline silica consistent with exposure limits that other U.S. workers currently receive in non-mining industries. It would update 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. The proposed rule would also include requirements for all MNM operators to provide medical surveillance in the form of a medical examination regime similar to what coal miners already receive. Cumulatively, the proposed provisions would lower miners' risk 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. Request for Comments

MSHA requests comments on the proposed rule and all relevant issues, including the review and conclusions of the health effects discussion, preliminary risk analysis, feasibility analysis, preliminary regulatory impact analysis and regulatory alternatives, and preliminary regulatory flexibility analysis. While MSHA invites comments on any aspect of its proposed rule and related documents, the Agency particularly seeks information and data in response to questions posed in this section and any other aspect of this proposed rule. Instructions for submitting and viewing comments are provided under the

DATES

heading. MSHA will consider all timely comments and may change the proposed rule based on such comments.

MSHA requests that commenters organize their comments, to the extent possible, around the following numbered questions. The Agency is interested in receiving responses to the listed questions and any information or data supporting the responses.

Health Effects

1. In the standalone, background document entitled “Health Effects of Respirable Crystalline Silica” and as summarized in Section V. Health Effects Summary of this preamble, MSHA has made a preliminary determination that miners' exposure to respirable crystalline silica presents a risk of material health impairment due to the risk of developing silicosis, NMRD, lung cancer, and renal disease, based on its extensive review of the health effects literature. MSHA requests comments on this preliminary determination and its literature review, which draws heavily from the review conducted by OSHA for its 2016 rulemaking. Are there additional adverse health effects that should be included or more recent literature that offers a different perspective? MSHA requests that commenters submit information, data, or additional studies or their citations. Please be specific regarding the basis for any recommendation to include additional adverse health effects.

Preliminary Risk Analysis

2. In the standalone, background document entitled “Preliminary Risk Analysis” and as summarized in Section VI. Preliminary Risk Analysis Summary

of this preamble, MSHA relied on risk models that OSHA used in support of its 2016 respirable crystalline silica final rule. Does the context of the MSHA rule suggest that the model would benefit from changes? If so, please describe both the justification for those changes and the likely impact on the final risk estimates. Are there additional studies or sources of data that MSHA should consider? What is the rationale for recommending the use of these additional studies or data?

3. MSHA's risk analysis of lung cancer mortality uses the exposure-response model from Miller and MacCalman (2010) instead of Steenland et al. (2001a), on which OSHA's risk assessment of lung cancer mortality was based. MSHA uses Miller and MacCalman (2010) for several reasons. First, it covers coal mining-specific cohort large enough (with 45,000 miners) to provide adequate statistical power to detect low levels of risk, and it covers an extended follow-up period (1959-2006). Second, the study provided data on cumulative exposure of cohort members and adjusted for or addressed confounders such as smoking and exposure to other carcinogens. Finally, it developed quantitative assessments of exposure-response relationships using appropriate statistical models or otherwise provided sufficient information that permitted MSHA to do so. The Agency is requesting comment on MSHA's reliance on the Miller and MacCalman (2010) study in assessing lung cancer mortality. Please provide any other studies or information that MSHA should take into account in determining the risk of lung cancer mortality among miners.

Technological Feasibility of the Proposed Rule

4. As discussed in Section VIII. Technological Feasibility of this preamble, MSHA has preliminarily determined that it is technologically feasible for mine operators to conduct air sampling and analysis and to achieve the proposed PEL using commercially available samplers. MSHA has also determined that these technologically feasible samplers are widely available, and a number of commercial laboratories provide the service of analyzing dust containing respirable crystalline silica. In addition, MSHA has determined that technologically feasible engineering controls are readily available, can control crystalline silica-containing dust particles at the source, provide reliable and consistent protection to all miners who would otherwise be exposed to respirable dust, and can be monitored. MSHA has also determined that administrative controls, used to supplement engineering controls, can further reduce and maintain exposures at or below the proposed PEL. Moreover, MSHA has preliminarily determined the proposed respiratory protection practices for respirator use are technologically feasible for mine operators to implement. MSHA requests comments on these preliminary conclusions. What methods have you used that proved effective in reducing miners' exposure to respirable crystalline silica in mining operations? Please explain how those methods were effective in reducing miners' exposures. To what extent do existing controls that reduce exposure to other airborne hazards (

e.g.,

coal dust, diesel particulate matter) already reduce exposures to respirable crystalline silica below the proposed PEL? To what extent does the proposed rule including the PEL facilitate MSHA's workplace health and safety goals? Please provide supporting information, such as quantitative data if available.

5. MSHA has determined that the proposed medical surveillance requirements for MNM are technologically feasible. MSHA requests comments on this preliminary conclusion. Please provide supporting information, such as quantitative data if available.

Preliminary Regulatory Impact Analysis and Regulatory Alternatives

6. In the standalone background document entitled “

Preliminary Regulatory Impact Analysis

” and as summarized in Section IX. Summary of Preliminary Regulatory Impact Analysis and Regulatory Alternatives of this preamble, MSHA developed estimated costs of compliance with the proposed rule and estimated monetized benefits associated with averted cases of respirable crystalline silica-related diseases. MSHA requests comments on the methodologies, baseline, assumptions, and estimates presented in the Preliminary Regulatory Impact Analysis. Please provide any data or quantitative information that may be useful in evaluating the estimated costs and benefits associated with the proposed rule.

7. MSHA considered two regulatory alternatives in developing the proposed rule discussed in Section IX. Summary of Preliminary Regulatory Impact Analysis and Regulatory Alternatives. In the regulatory alternatives presented, MSHA discussed alternatives to the proposed PEL, action level, sampling requirements, and semi-annual evaluations. MSHA requests comments on these and other regulatory alternatives and information on any other alternatives that the Agency should consider, including different average working-life spans and different average shift lengths. Please provide supporting information about how these alternatives could affect miners' protection from respirable crystalline silica exposure and affect mine operators' costs.

Initial Regulatory Flexibility Analysis

8. As summarized in Section X. Initial Regulatory Flexibility Analysis of this preamble, MSHA examined the impact of the proposed rule on small mines in accordance with the Regulatory Flexibility Act. MSHA estimated that small-entity controllers would be expected to incur, on average, additional regulatory costs equaling approximately 0.122 percent of their revenues (or $1,220 for every $1 million in revenues). MSHA is interested in how the proposed rule would affect small mines, including their ability to comply with the proposed requirements. Please provide information and data that supports your response. If you operate a small mine, please provide any projected impacts of the proposal on your mine, including the specific rationale supporting your projections.

Scope and Effective Date

9. MSHA is proposing a unified regulatory and enforcement framework for controlling miners' exposures to respirable crystalline silica for the mining industry. MSHA requests comments on this unified regulatory and enforcement framework. MSHA requests the views and recommendations of stakeholders regarding the scope of proposed part 60, which would include all surface and underground MNM and coal mines. MSHA requests comments on whether separate standards should be developed for the MNM mining industry and the coal mining industry. Please provide supporting information.

10. MSHA is proposing that the final rule would be effective 120 days after its publication in the

Federal Register

. This period is intended to provide mine operators time to evaluate existing engineering and administrative controls, update their respiratory protection programs, and prepare to comply with other provisions of the rule including recordkeeping requirements. Please provide your views on the proposed effective date. In your response, please include the rationale for your position.

Definitions

11. MSHA requests comments on the proposed action level. Stakeholders should provide specific information and data in support of or against a proposed action level. Stakeholders should include a discussion of how the use of a proposed action level would impact their mines, including the cost of monitoring respirable crystalline silica above the proposed action level, and other relevant information. Please provide supporting information.

12. MSHA requests comments on the proposed definition for “objective data.” Is it appropriate to allow mine operators to use objective data instead of a second baseline sample? Please provide supporting information.

Proposed Permissible Exposure Limit

13. MSHA is proposing a PEL for respirable crystalline silica of 50 μg/m

for a full-shift exposure, calculated as an 8-hour TWA for MNM and coal miners. MSHA has made a preliminary determination that the proposed PEL would reduce miners' risk of suffering material impairment of health or functional capacity over their working lives. MSHA seeks the views and recommendations of stakeholders on the proposed PEL. MSHA solicits comments on the approach of having a standalone PEL and whether to eliminate the reduced standard for total respirable dust when quartz is present at coal mines. Please provide evidence to support your response.

14. MSHA is proposing a PEL of 50 ug/m

and an action level of 25 μg/m

for respirable crystalline silica exposure. Which proposed requirements should be triggered by exposure at, above, or below the proposed action level? Please provide supporting information.

Methods of Compliance

15. MSHA requests comments on the proposed prohibition against rotation of miners as an administrative control. Please include a discussion of the potential effectiveness of this non-exposure approach and its impact on miners at specific mines. Please provide supporting information.

16. MSHA requests comments on the proposed requirement that mine operators must install, use, and maintain feasible engineering and administrative controls to keep miners' exposures to respirable crystalline silica below the proposed PEL. Please provide supporting information.

Proposed Exposure Monitoring

17. MSHA requests comments and information from stakeholders concerning the proposed approaches to monitoring exposures, and other approaches to accurately monitor miner exposure to respirable crystalline silica in MNM and coal mines. Please provide supporting information and data.

18. MSHA proposes to require mine operators to collect a respirable crystalline silica sample for a miner's regular full shift during typical mining activities. Many potential sources of respirable crystalline silica are present only when the mine is operating under typical conditions. MSHA requests comments on this requirement and whether to specify environmental conditions under which samples should be taken to ensure that samples accurately reflect actual levels of respirable crystalline silica exposure. In MSHA's experience, for example, environmental conditions such as precipitation (

e.g.,

rain or snow) or wind could affect the actual levels of respirable crystalline silica exposure at miners' normal or regular workplaces throughout their typical workday. Please provide supporting information and data.

19. MSHA recognizes that some mining facilities operate seasonally or intermittently and that cumulative exposures for miners at these facilities may be lower than that of miners working at year-round operations. MSHA requests comments on the exposure monitoring approach under proposed § 60.12, including the frequency of exposure monitoring necessary to safeguard the health of miners at seasonal or intermittent operations. Please provide supporting information and data.

20. MSHA is proposing that each mine operator perform baseline sampling within 180 days after the rule becomes effective to assess the respirable crystalline silica exposure of each miner who is or may reasonably be expected to be exposed to respirable crystalline silica. MSHA requests comments on this proposed baseline sampling requirement. MSHA also requests comment on the ability of service providers used by mines such as industrial hygiene suppliers and consultants, and accredited laboratories that conduct respirable crystalline silica analysis, to meet the demand created by the baseline sampling requirements within the proposed timeline. Please include alternative approaches that might be equally protective of miners that should be implemented for assessing a miner's initial exposure to respirable crystalline silica.

21. MSHA is proposing a requirement that mine operators qualitatively evaluate every 6 months any changes in production, processes, engineering controls, personnel, administrative controls, or other factors, beginning 18 months after the effective date. MSHA requests comments on the timing of the proposed semi-annual evaluation requirements, and in particular, whether miners would possibly be exposed unnecessarily to respirable crystalline silica levels above the PEL due to the gap between the effective date and the proposed requirements. Please provide supporting information.

22. MSHA has determined that most occupations related to extraction and processing would meet the “reasonably be expected” threshold for baseline sampling. MSHA recognizes that some miners may work in areas or perform tasks where exposure is not reasonably expected, if at all. MSHA solicits comments on the assumption that most miners are exposed to at least some level of respirable crystalline silica, and on the proposed requirement that these miners should be subject to baseline sampling. Please provide supporting information.

23. MSHA is proposing that mine operators would not be required to conduct periodic sampling if the baseline sampling result, together with another sampling result or objective data, as defined in proposed § 60.2, confirms miners' exposures are below the proposed action level. MSHA seeks comments on this proposal. Please provide supporting information and data.

24. MSHA is proposing that mine operators conduct periodic sampling within 3 months where the most recent sampling indicates miner exposures are at or above the proposed action level but at or below the proposed PEL and continue to sample within 3 months of the previous sampling until two consecutive samplings indicate that miner exposures are below the action level. MSHA solicits comments on the proposed frequency for periodic sampling, including whether the consecutive samples should be at least 7 days apart. Please provide supporting information and data.

25. MSHA is proposing that mine operators may discontinue periodic sampling when two consecutive samples indicate that miner exposures are below the proposed action level. MSHA requests comments on this proposal. Please provide supporting information and data.

26. MSHA is proposing that mine operators conduct semi-annual evaluations to evaluate whether any changes in production, processes, engineering controls, personnel, administrative controls, or other factors may reasonably be expected to result in

new or increased respirable crystalline silica exposures. Please provide comments on this proposal, as well as alternative approaches that would be appropriate for evaluating any potential new or increased respirable crystalline silica exposures. Please provide supporting information and data.

27. MSHA is proposing that miners' exposures are measured using personal breathing-zone air samples for MNM operations and occupational environmental samples collected in accordance with §§ 70.201(c), 71.201(b), or 90.201(b) for coal operations. MSHA requests comments on this proposal. Please provide supporting information and data.

28. MSHA is proposing the use of representative sampling. Where several miners perform the same task on the same shift and in the same work area, the mine operator may sample a representative fraction of miners to meet the proposed exposure monitoring requirements. MSHA seeks comments on the use of representative sampling. Please provide supporting information and data.

29. MSHA is proposing that mine operators use laboratories accredited to ISO/IEC 17025 “General requirements for the competence of testing and calibration laboratories,” where the accreditation has been issued by a body that is compliant with ISO/IEC 17011 “Conformity assessment—requirements for accreditation bodies accrediting conformity assessment bodies.” MSHA solicits comments on this proposal. Are there additional requirements that should be incorporated into this proposal to ensure accurate sample analysis methods? Please provide supporting information and data.

30. MSHA seeks comments on the proposal that mine operators ensure that laboratories evaluate all respirable crystalline silica samples using respirable crystalline silica analytical methods specified by MSHA, NIOSH, or OSHA. Are there additional requirements that should be incorporated into this proposal to ensure accurate sample analysis? Please provide supporting information and data.

31. MSHA seeks comments and information on mine operator and stakeholder experience using NIOSH's rapid field-based quartz monitoring (RQM) monitors for determining miners' exposures to respirable crystalline silica. Please provide any information and data.

Proposed Medical Surveillance for Metal and Nonmetal Miners

32. MSHA is proposing to require medical surveillance for MNM miners. Medical surveillance is already required for coal miners under 30 CFR 72.100 and has played an important role in tracking the burden of pneumoconiosis in coal miners but is not currently required for MNM miners. MSHA's proposal would require MNM mine operators to provide each miner new to the mining industry with an initial medical examination and a follow-up examination no later than 3 years after the initial examination, at no cost to the miner. It would also require MNM mine operators to provide examinations for all miners at least every 5 years, which would be voluntary for miners. Is there an alternative strategy or schedule, such as voluntary initial or follow-up examinations, tying the medical surveillance requirement to miners reasonably expected to be exposed to any level of silica or to the action level that would be more appropriate for new MNM miners? Should the rule make each 5-year examination mandatory? Should the 5-year examination be mandatory for coal mine operators as well? Please provide data or cite references to support your position.

33. MSHA's proposed medical surveillance requirements for MNM miners do not include some requirements that are in MSHA's existing medical surveillance requirements for coal mine operators in 30 CFR 72.100. For example, § 72.100 requires coal mine operators to use NIOSH-approved facilities for medical examinations. Should MNM operators be required to use NIOSH-approved facilities for medical examinations? Coal mine operators also are required to submit for approval to NIOSH a plan for providing miners with the examinations specified. This is because NIOSH administers medical surveillance for coal miners with requirements for coal operators, but not MNM operators, in NIOSH standards (42 CFR part 37). Should the plan requirements be extended to MNM operators? However, the proposed requirements also include some requirements for MNM operators that are not included for coal operators. For example, the proposed provisions require operators of MNM mines to provide MNM miners with periodic medical examinations performed by physicians or other licensed health care professionals (PLHCP) or specialists including a history and physical examination focused on the respiratory system, a chest X-ray, and a spirometry test. The proposed rule also requires a written medical opinion be provided by the PLHCP or specialist to the mine operator regarding the miner's ability to wear a respirator. MSHA seeks comment on the differences between the medical surveillance requirements for MNM operators in this proposed rule and the existing medical surveillance requirements for coal mine operators in § 72.100. MSHA also seeks comment on how best to collect health surveillance data from PLHCPs and specialists to track MNM miners' health, for example how to know when pneumoconiosis cases occur. MSHA seeks comments on alternative approaches to scheduling periodic medical surveillance. MSHA proposes to require operators to keep medical surveillance information for the duration of a miner's employment plus 6 months. The Agency seeks comments on this proposed requirement and on any alternative recordkeeping schedules that would be appropriate. Please provide supporting information.

34. MSHA's proposed medical surveillance requirements for MNM miners would require operators of MNM mines to provide miners with periodic medical examinations performed by PLHCP or specialists, including a history and physical examination focused on the respiratory system, a chest X-ray, and a spirometry test. MSHA seeks comment on whether use of any new diagnostic technology (

e.g.,

high-resolution computed tomography) for the purposes of medical surveillance should be used.

35. MSHA's proposed medical surveillance requirements would require that the MNM mine operator provide a mandatory follow-up examination to the miner no later than 3 years after the miner's initial medical examination. If a miner's 3-year follow-up examination shows evidence of a respirable crystalline silica-related disease or decreased lung function, the operator would be required to provide the miner with another mandatory follow-up examination with a specialist within 2 years. For examinations that show evidence of disease or decreased lung function, MSHA seeks comment on how, and to whom, test results should be communicated.

36. MSHA requests comments as to whether the proposed provisions should include a medical removal option for MNM miners who have developed evidence of silica-related disease that is equivalent to the transfer rights and exposure monitoring provided to coal miners in 30 CFR part 90 (part 90). Under part 90, any coal miner who has evidence of the development of pneumoconiosis based on a chest X-ray or other medical examinations has the

option to work in an area of the 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 part 90, coal miners are entitled to retention of pay rate, future actual wage increases, and future work assignment, shift and respirable dust protection. MSHA seeks comment on whether this medical removal option should be provided to MNM miners. What would be the economic impact of providing MNM miners a medical removal option? Please provide supporting information and data.

Proposed Respiratory Protection Standard

37. MSHA requests comments concerning the temporary, non-routine use of respirators and whether there are other instances or occupations in which the Agency should allow the use of respirators as a supplemental control. Please discuss any impacts on particular mines and mining conditions and the cost of air-purifying respirators, if applicable. MSHA also solicits comments on the proposed requirement that affected miners wear respiratory protection to maintain protection during temporary and non-routine use of respirators. Please provide supporting information.

38. MSHA is proposing to incorporate by reference ASTM F3387-19, published in 2019. Whenever respiratory protective equipment is needed, mine operators would be required to follow practices for program administration, standard operating procedures, medical evaluations, respirator selection, training, fit testing, and maintenance, inspection, and storage in accordance with the requirements of ASTM F3387-19. Beyond these elements, MSHA is proposing to provide operators the flexibility to select the elements in ASTM F3387-19 that are applicable to their practices of respirator use at their mines. Should mine operators have the flexibility to choose the ASTM F3387-19 elements that are appropriate for their mine-specific hazards because the need for respirators may vary due to the variability of mining processes, activities, airborne hazards, and commodities mined? What, specifically, do you think should factor into the determination of what is applicable? MSHA seeks comments on its proposed approach and the impact it would have on mine operators and on miners' life and health.

39. ASTM F3387-19 identifies a variety of respiratory protection practice elements. MSHA proposes to require certain minimally acceptable program elements: program administration; standard operating procedures; medical evaluations; respirator selection; training; fit testing; and maintenance, inspection, and storage. Please comment on whether these are the appropriate elements to require, or if there are any other elements of ASTM F3387-19 that should be minimally included in any respiratory protection program. MSHA also welcomes comments on whether it would be appropriate to require the standard in its entirety. Please identify those elements that would ensure that approved respirators are selected, fitted, used, cleaned, and maintained so that the life and health of miners are safeguarded. MSHA also seeks data and information on the impact these changes would have on mine operators, especially smaller operators. What would be the economic impact if all or parts of ASTM F3387-19 were required respirator program elements? Please be specific with your response and provide details on respirator use at your mine to include information and data on mining processes and environmental conditions; level of exposures to airborne contaminants; frequency and duration of exposures; type and amount of work or physical labor, including frequency and duration; and medical evaluation on respirator use, if applicable.

Recordkeeping Requirements

40. MSHA is proposing to require recordkeeping for records of evaluations, records of samplings, records of corrective actions, and written determination records received from a PLHCP. The proposed rule's recordkeeping requirements are discussed in the Section-by-Section Analysis section of this Preamble. MSHA seeks comment on the utility of these recordkeeping requirements as well as the costs of making and maintaining these records. Please provide supporting information.

Training Requirements

41. MSHA requests the views and recommendations of stakeholders regarding whether training requirements for miners should be included in proposed part 60. Please provide supporting information and data.

Conforming Changes

42. MSHA requests comments on the proposed conforming changes to remove the reduced coal dust standard from 30 CFR and the potential impact on coal mines and miners and on whether to retain the reduced standard for part 90 miners. Please provide supporting information.

43. MSHA is not proposing to adopt a similar approach as the OSHA Table 1 for the construction industry, where MSHA would prescribe specific exposure control methods for task-based work practices when working with materials containing respirable crystalline silica. See 29 CFR 1926.1153(c)(1). MSHA requests comments on specific tasks and exposure control methods appropriate for a Table 1-approach for the mining industry that also would adequately protect miners from risk of exposure to respirable crystalline silica. Please provide specific rationale and supporting information, including data on how such an approach would be implemented.

III. Background

The purpose of this proposed rule is to reduce miners' risk of developing occupational lung disease and other diseases caused by exposure to respirable crystalline silica and to better protect all miners from occupational exposure to airborne hazards. In promulgating mandatory standards dealing with toxic materials or harmful physical agents, MSHA is required 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 . . .” 30 U.S.C. 811(a)(6)(A).

A. Statutory Authority

The statutory authority for this proposal 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

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, as appropriate, mandatory health standards to address toxic materials or harmful physical agents. Under Section 101(a), standards must protect lives and prevent injuries in mines and be “improved” over any standard that it replaces or revises. Moreover, “the Mine Act does not contain the ‘significant risk' threshold requirement . . . from the OSH Act.”

Nat'l Mining Ass'n

United Steel Workers,

985 F.3d 1309, 1319 (11th Cir. 2021); see also

Nat'l Min. Ass'n

Mine Safety & Health Admin.,

116 F.3d 520, 527-28 (D.C. Cir. 1997) (contrasting the OSH Act at 29 U.S.C. 652 with the Mine Act at 30 U.S.C. 811(a) 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” (emphasis in original)).

The Secretary must set standards to assure, based on the best available evidence, that no miners 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.

However, 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. Instead, “when MSHA itself weighs the evidence before it, it does so in light of its congressional mandate.”

Id.

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). In general, section 103(h) requires that every mine operator establish and maintain records, make reports, and provide this information, if required by the Secretary.

Id.

Also, 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 proposal to lower the exposure limits for respirable crystalline silica and adopt an integrated monitoring approach across all mining sectors and to update the existing respiratory protection requirements would fulfill Congress' direction by preventing miners from suffering material impairment of health or functional capacity caused by exposure to respirable crystalline silica and other airborne contaminants.

B. Respirable Crystalline Silica Hazard and Mining

Silica is a common component of rock composed of silicon and oxygen (chemical formula SiO

), 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—several different structures with the same chemical composition. The most common form of crystalline silica found in nature is quartz, but cristobalite and tridymite may also be found 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 is 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 occurs 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 may therefore lead to miner exposure.

Inhaled small particles of silica dust can be deposited throughout the lungs. A large number of crystalline silica particles can reach and remain in the deep lung (

i.e.,

alveolar region), although some small particles are 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 for prolonged periods result in a variety of cellular responses that may lead to pulmonary disease. 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 III-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.

Table III-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)).

Commodities such as sand, gravel, silica, and/or stone for example are used in road building, concrete construction, 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).

EP13JY23.000

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, as well as other components (IARC, 1997). These activities include the general mining activities previously mentioned (

e.g.,

cutting, sanding, drilling, crushing, and hauling materials), as well as roof bolter operations, continuous mining machine operations, longwall mining, and other activities. Table III-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).

IV. Existing Standards and Implementation

MSHA has maintained health standards to protect MNM and coal miners from excessive exposure to respirable crystalline silica for decades. MSHA's existing standards, established in the early 1970s, limit miners' exposures to respirable crystalline silica. These standards require mine operators to monitor occupational exposures to respirable crystalline silica and 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 respiratory protection in limited situations and on a temporary basis. The existing standards for MNM and coal mines differ in some respects, including exposure limits and monitoring. 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 them in recent years.

A. Existing Standards—Metal and Nonmetal Mines

MSHA's existing standards for exposure to airborne contaminants, including respirable crystalline silica, in MNM mines are found in 30 CFR part 56, subpart D (Air Quality and Physical Agents), and 30 CFR part 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

) for each respirable dust sample. The TLV® for quartz is calculated by dividing the percent of respirable quartz plus 2, into the number 10. The TLV® for cristobalite and the TLV® for tridymite, respectively, are calculated by multiplying the same mass formula by one-half using the percentages of either cristobalite or tridymite found in the sample. Thus, 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

or 100 µg/m

for quartz, to less than 0.05 mg/m

or 50 µg/m

for cristobalite, and to less than 0.05 mg/m

or 50 µg/m

for tridymite. Throughout the remainder of this preamble, the concentrations of respirable dust and respirable crystalline silica are expressed in µg/m

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, examining dust control system and ventilation system maintenance, and reviewing 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 to harmful airborne contaminants and the effectiveness of existing controls in reducing such 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 these efforts were successful. Re-surveying should be done as frequently as necessary to ensure that the implemented control measures remain adequate. MSHA's determination of whether a mine operator has surveyed frequently enough is based on several factors, including whether sampling results comply with the permissible exposure limit, whether there have been changes in the mining operation or process, and whether controls such as local exhaust ventilation systems need routine or special maintenance.

Exposure Controls.

MSHA's existing standards for controlling a miner's exposure to harmful airborne contaminants (§§ 56.5005 and 57.5005) require, if feasible, prevention of contamination, removal by exhaust ventilation, or dilution with uncontaminated air. The use of respiratory protective equipment is also allowed under specified circumstances such as when engineering controls are being developed or are not feasible. When respiratory protective equipment is used, the operator must have a respiratory protection program consistent with the requirements of

American National Standards Practices for Respiratory Protection ANSI Z88.2-1969.

Consistent with widely accepted industrial hygiene principles and NIOSH's recommendations, MSHA requires the use of engineering controls, supplemented by administrative controls, in its enforcement for the control of occupational exposure to respirable crystalline silica and other airborne contaminants (NIOSH, 1974). Engineering controls designed to remove or reduce the hazard at the source are the most effective. Examples of engineering controls include the installation of proper ventilation systems, use of water sprays or wetting agents to suppress airborne contaminants, installation of machine-mounted dust collectors to capture respirable crystalline silica and other contaminants, and the installation of control booths or environmental cabs to enclose equipment operators.

Although 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. In applying administrative controls, mine operators can direct miners to perform certain activities in specific manners. For instance, as an administrative control, operators can specify adequate housekeeping procedures for miners to clean spills or handle contaminated clothing which could reduce occupational exposure to airborne contaminants, including respirable crystalline silica.

In addition, respiratory protective equipment can be used in controlling miners' exposures to airborne contaminants, including respirable crystalline silica, on a temporary basis or under non-routine, limited conditions. The use of respiratory protection is, however, considered to be a supplement, not an alternative to any engineering or administrative control, in reducing or eliminating a miner's exposure to airborne contaminants including respirable crystalline silica.

Under the existing standards in §§ 56.5005 and 57.5005, in circumstances where engineering controls are not yet developed or where it is necessary for miners to enter hazardous atmospheres to establish controls or to perform non-routine maintenance or investigation, a miner using appropriate respiratory protection “may work for reasonable periods of time” in concentrations of airborne contaminants which exceed exposure limits. 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 airborne contaminants. Whenever respiratory protection is used, MNM mine operators are required to have a respirator program consistent with the requirements specified in ANSI Z88.2-1969.

B. Existing Standards—Coal Mines

Under existing standards, there is no separate standard for respirable crystalline silica for coal mines. 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 coal miners. Under these respirable dust standards, coal miners' exposures to respirable quartz are indirectly regulated through reductions in the overall respirable dust standard.

Under its existing respirable coal mine dust standards, MSHA defines quartz as crystalline silicon dioxide (SiO

), which includes not only quartz but also two other polymorphs, cristobalite and tridymite.

Therefore, 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.

Quartz is defined in 30 CFR 70.2, 71.2, and 90.2 as crystalline silicon dioxide (SiO

) 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

, reported as an equivalent concentration as measured by the Mining Research Establishment (MRE) instrument. This 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

, 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

. The applicable dust standard, when the equivalent concentration of respirable crystalline silica exceeds 100 µg/m

, is computed by dividing the percent of quartz into the number 10.

The result of this calculation becomes the exposure limit for respirable coal mine dust (RCMD), for the sections of the mine represented by the sample. Various sections within a mine may have different reduced 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. 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

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 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. In addition, respirable dust sampling must be representative of respirable dust exposures during a normal production shift. Also, sampling 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, from the start of their work day to the end of their work day, in their normal work locations.

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.

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. Similar to the MNM standards, engineering and environmental controls include the use of dust collectors, water sprays, and ventilation controls. 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

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

. If operators exceed the reduced 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. Corrective actions can include increasing air flow, improving ventilation controls, repairing and maintaining existing dust suppression controls, adding water sprays or other controls, cleaning dust filters or collectors more frequently, or repositioning the miner away from the dust source.

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 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

MSHA collects respirable dust samples at mines and analyzes them for respirable crystalline silica to determine whether the respirable crystalline silica exposure limits are met and whether exposure controls are adequate. This section describes the respirable dust samples collected at MNM and coal mines in recent years and presents the results of the sample data analyses.

1. Respirable Dust Sample Collection

This subsection offers a brief description of how MSHA samples for respirable crystalline silica under the existing standards. Upon their arrival at mines, MSHA inspectors determine which areas of the mine and which miners to select for respirable dust sampling. At MNM mines, the MSHA inspector often determines sampling locations based on sample results from previous inspections and on the inspector's onsite observations of work practices and work areas. At coal mines, the MSHA inspector conducts sampling among the occupations or from the work areas that are specified for operator sampling under 30 CFR parts 70, 71, and 90. Generally speaking, MSHA inspectors collect respirable dust samples from the common occupations during typical and normal activities at the mine and from the positions that are commonly known to have the highest concentration of respirable dust.

After identifying which miners and which areas at the mine will be sampled for respirable dust, MSHA inspectors place gravimetric samplers on the selected miners or at the selected locations. Gravimetric samplers consist of a portable air-sampling pump connected to a particle-size separator (

i.e.,

cyclone) and collection medium (

i.e.,

filter). MSHA inspectors use Dorr-Oliver 10-mm nylon cyclones operated at a 1.7 liters per minute (L/min) flow rate for MNM mine sampling and at a 2.0 L/min flow rate (reported as MRE-equivalent concentrations) for coal mine sampling.

For the entire duration of the work shift, the gravimetric sampler captures air from the breathing zone of each selected miner or occupation and from each selected work area.

This type of sampling equipment was developed to separate the airborne particles by size in a manner similar to the size-selective deposition and retention characteristics of the human respiratory system. It is important to note that size-selective sampling does not measure the deposition of respirable particles in the lung. Rather, it provides a measure of the particulate mass available for deposition to the deep lung during breathing (Raabe and Stuart, 1999).

MSHA inspectors use the full-shift sampling approach. When miners work longer than an 8-hour shift, which is common, those miners are sampled continuously throughout the extended work shifts. 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, the inspectors send the cassettes containing the full-shift respirable dust samples to the MSHA Laboratory for analysis.

2. Respirable Dust Sample Analysis

The MSHA Laboratory analyzes inspectors' respirable dust samples, following its standard operating procedures (SOPs) summarized below.

Any samples that are broken, torn, or visibly wet are voided and removed before analysis. Once weighing of the samples is completed, samples are again screened based on mass gain and examined for validity. 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.

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.

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): X-ray diffraction (XRD) for respirable dust samples from MNM mines, and Fourier transform infrared spectroscopy (FTIR) for respirable coal mine dust samples.

The XRD method uses X-rays to distinguish and measure the structure, composition, and physical properties of a sample. The FTIR method relies on the absorption of infrared light to determine the composition of a sample. 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. The percentage of silica is used to calculate MSHA's PELs for quartz and cristobalite, in accordance with §§ 56.5001 and 57.5001. 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. For coal mines, the percentage of quartz is used to calculate the reduced dust standard when the quartz concentration exceeds 100 µg/m

(MRE).

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.

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.

It is worth noting how MSHA calculates full-shift exposure to respirable crystalline silica (and other airborne contaminants). When a miner who works an 8-hour shift is sampled, the miner's 8-hour TWA exposure is calculated as follows:

EP13JY23.001

However, for work shifts that last longer than 8 hours, a coal miner's full-shift exposure is calculated differently than an MNM miner's full-shift exposure. In accordance with § 70.2, the coal miner's extended full-shift exposure has, since 2014, been calculated in the following way:

EP13JY23.002

For the MNM miner, MSHA calculates extended full-shift exposure according to the following formula:

EP13JY23.003

For respirable dust samples from MNM mines, 480 minutes is used in the denominator regardless of the actual sampling time. Contaminants collected over extended shifts (

e.g.,

600-720 minutes) are calculated as if they had been collected over 480 minutes. MSHA has used this calculation approach (also known as “shift-weighted average”) since the 1970s.

Under the shift-weighted average approach, exposures for work schedules greater than 8 hours are proportionately adjusted to allow direct comparison with the 8-hour PEL. The ACGIH TLVs® adopted by MSHA are based on exposure periods of no more than 8 hours per day and 40 hours per week, with 16 hours of recovery time between shifts.

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

This section presents 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 that met the minimum mass gain criteria were analyzed for respirable crystalline silica.

The vast majority of the 46,585 valid samples that were excluded from the analysis in this rulemaking did not meet the mass gain criteria described earlier and therefore the lab did not determine their silica concentration. Further information on the valid respirable dust samples that are excluded from the analysis in this rulemaking can be found in Appendix A of the preamble.

The respirable crystalline silica concentration is calculated using the measured mass of each of the polymorphs and the air sampling volume. As discussed above, the existing PEL for quartz in MNM mines is approximately equivalent to 100 µg/m

for a full-shift exposure, calculated as an 8-hour TWA, while the existing PELs for cristobalite and tridymite, respectively, are approximately equivalent to 50 µg/m

for a full-shift exposure, calculated as an 8-hour TWA.

If more than one polymorph is present the equation used to calculate the TLV® for respirable dust containing quartz is modified per Appendix C of the 1973 ACGIH TLV® Handbook, and the equation is modified as follows: 10/[(% quartz + 2) + 2 (% cristobalite + 2)].

1. Annual Results of MNM Respirable Crystalline Silica Samples

Table IV-1 below shows the variation between 2005 and 2019 in: (1) the numbers 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

. 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

. 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.

BILLING CODE 4520-43-P

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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-2 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

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).

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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:

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-3 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-3 also shows the number and percentage of the samples exceeding the existing respirable crystalline silica PEL of 100 µg/m

. 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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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

, 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

for respirable crystalline silica. In addition, about 80 percent of samples taken from stone cutting operators did not exceed the existing PEL, which historically has had high exposures to respirable dust and respirable crystalline silica;

nevertheless, this occupation continues to experience the highest overexposures relative to other MNM occupations. 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.

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).

MSHA believes that improved technology, engineering controls, and better training contributed to the reductions in exposures for miners who work in occupations exposed to the highest levels of respirable crystalline silica. 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' exposure at or below the existing limit of 100 µg/m

E. Respirable Crystalline Silica Sampling Results—Coal Mines

To examine coal mine operators' compliance with existing respirable crystalline silica standards, MSHA analyzed 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 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 (Coal Dust Rule) (79 FR 24813, May 1, 2014) went into effect. At that time, the exposure limits for RCMD

were lowered from 2.0 mg/m

to 1.5 mg/m

(MRE equivalent) at underground and surface coal mines, and from 1.0 mg/m

to 0.5 mg/m

(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 these 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.

Of those samples, 63,127 samples that met the minimum mass gain criteria and had no other disqualifying issues were analyzed for respirable quartz and quartz concentrations were determined. 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.

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

(MRE equivalent) for a full shift, calculated as a TWA.

Overexposure rates (the percent of samples above the exposure limit, on average across all coal mining occupations) 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).

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

BMRC-MRE = 1.29 mg/m

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.

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-4, 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).

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

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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-5, 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.

EP13JY23.008

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,

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 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.

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-6 displays the number and percent of respirable coal mine dust samples with quartz greater than the existing exposure limit for each occupational category.

EP13JY23.009

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.

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BILLING CODE 4520-43-C

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.

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.

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 can 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

(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 is contained in the standalone document, entitled

Effects of Occupational Exposure to Respirable Crystalline Silica on the Health of Miners

(Health Effects document), which has been placed in the rulemaking docket for the MSHA silica rulemaking (RIN 1219-AB36, Docket ID no. MSHA-2023-0001) and is available on MSHA's website.

The purpose of the Agency's scientific review is to present MSHA's preliminary findings on the nature of the hazards presented by exposure to respirable crystalline silica and to present the basis for the Preliminary

Risk Analysis (PRA) to follow. (A PRA summary is presented in Section VI of this preamble and a standalone document entitled

Preliminary Risk Analysis

has been placed in the rulemaking docket for the MSHA silica rulemaking (RIN 1219-AB36, Docket ID no. MSHA-2023-0001) and is available on MSHA's website.) MSHA reviewed a wide range of health research 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. After discussing the toxicity of respirable crystalline silica, MSHA's review of the literature covers the following topics:

(1) Silicosis;

(2) NMRD, excluding silicosis;

(3) Lung cancer and cancer at other sites;

(4) Renal disease; and

(5) Autoimmune diseases.

To develop this literature review, MSHA expanded upon OSHA's (2013b) review of the health effects literature to support its final respirable crystalline silica rule (81 FR 16286, March 25, 2016). MSHA also 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, morbidity (having a disease or a symptom of disease) and mortality (disease resulting in death) analyses, progression and pathology evaluations, death certificate and autopsy reviews, medical surveillance data, health hazard assessments,

in vivo

(animal) and

in vitro

toxicity data, and other toxicological reviews. These sources are cited throughout this summary and are listed in the References section of the Health Effects document. Additionally, these sources appear in the rulemaking docket.

MSHA's literature review is based on a weight-of-evidence approach, in which studies 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. Factors MSHA considered in this weight-of-evidence analysis include: size of the cohort studied and power of the study to detect a sufficiently low level of disease risk, duration of follow-up of the study population, potential for study bias (such as selection bias or healthy worker effects), and adequacy of underlying exposure information for examining exposure-response relationships. Of the studies examined in the Health Effects document, studies were deemed suitable for inclusion in the PRA if there was adequate quantitative information on exposure and disease risks and the study was judged to be of sufficiently high quality according to the above criteria.

The 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. Based on its extensive review of health research literature, MSHA has preliminarily determined that occupational exposure to respirable crystalline silica causes silicosis (acute silicosis, accelerated silicosis, simple chronic silicosis, and PMF), NMRD (including COPD), and lung cancer, and it also causes end-stage renal disease (ESRD). In addition, MSHA believes that respirable crystalline silica exposure is causally related to the development of some autoimmune disorders through inflammation pathways. Each of these effects is exposure-dependent, chronic, irreversible, and potentially disabling or fatal. 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 and functional capacity. Based on the assessment of health effects of respirable crystalline silica, MSHA preliminarily concludes that the proposed rule, which would lower the exposure limits in MNM and coal mining to 50 µg/m

and establish an action level of 25 µg/m

for a full-shift exposure, calculated as an 8-hour TWA, would reduce the risk of miners developing silicosis, NMRD, lung cancer, and renal disease.

A. 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 it is deposited in mines or mills. Respirable crystalline silica particles may be irregularly shaped and variable in size. Inhaled respirable crystalline silica can be deposited throughout the lungs. Some pulmonary clearance of particles deposited in the deep lung (

i.e.,

alveolar region) may occur, but a large number of 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 do not undergo metabolism into less toxic compounds. This is important biologically and physiologically, as 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. Researchers believe that the size and surface characteristics 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 (OSHA, 2013b; ATSDR, 2019).

Inflammation 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. The elevated production of ROS/RNS would result in oxidative stress and lung injury that stimulates alveolar macrophages, ultimately resulting in fibroblast activation and pulmonary fibrosis. The prolonged recruitment of macrophages and PMN causes a 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, production of rheumatoid factor, anti-nuclear antibodies, and release of other immune complexes (Parks

et al.,

1999, Haustein and Anderegg, 1988; Green and Vallyathan, 1996). Respirable crystalline silica exposure has also been associated with nonmalignant renal disease 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 (Brown and Donaldson, 1996; Castranova, 2004; Fubini

et al.,

2004; Nolan

et al.,

1981; Shi

et al.,

1989, 1998).

B. Diseases

1. Silicosis

Silicosis is a progressive occupational disease that has long been identified as a cause of lung disease in miners. Based on its review of the literature, MSHA has preliminarily determined that exposure to respirable crystalline silica causes silicosis (acute silicosis, accelerated silicosis, simple chronic silicosis, and PMF) in MNM and coal miners, which is a significant cause of serious morbidity and early mortality in this occupational cohort (Mazurek and Attfield, 2008; Mazurek and Wood, 2008a, 2008b; Mazurek

et al.,

2015, 2018).

When respirable crystalline silica particles accumulate in the lungs, they cause an inflammatory reaction, leading to lung damage and scarring. Silicosis can continue to develop even after silica exposure has ceased. 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 MNM miners can lead to all three forms of silicosis (acute, accelerated, and chronic). These forms differ in the rate of exposure, pathology (

i.e.,

the 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 (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 it 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 can suffocate the patient. The radiographic (X-ray) appearance and results of microscopic examination of acute silicosis are like those of idiopathic pulmonary alveolar proteinosis.

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 10 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 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 chest X-ray films usually underestimate the true prevalence of silicosis (Craighead and Vallathol, 1980; Hnizdo

et al.,

1993; Rosenman

et al.,

1997; Cohen and Velho, 2002). However, 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 a chronic cough, sputum production, shortness of breath, and reduced pulmonary function.

Accelerated silicosis includes both inflammation and fibrosis and is associated with intense respirable crystalline silica exposure. Accelerated silicosis usually manifests over a period of 3 to 10 years (Cowie and Becklake, 2016), but it can develop in as little as 2 to 5 years if exposure is sufficiently intense (Davis, 1996). Accelerated silicosis may have features of both chronic and acute silicosis (

i.e.,

alveolar proteinosis in addition to X-ray evidence of fibrosis). Although the symptoms are similar to 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.

Among coal miners, silicosis is usually found in conjunction with simple coal worker's pneumoconiosis (CWP) (Castranova and Vallyathan, 2000) because of their exposures to RCMD that contains respirable crystalline silica. 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, see Figure 2, 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 (Attfield

et al.,

1994; Cohen

et al.,

2016, 2019, 2022; Davis

et al.,

1979; Douglas

et al.,

1986; Fernie and Ruckley, 1987; Green

et al.,

1989, 1998b; Ruckley

et al.,

1981, 1984; Vallyathan

et al.,

2011). Autopsy studies in British coal miners indicated that the more advanced the disease, the more mixed coal mine dust 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 from the United States (Cohen

et al.,

2016, 2022), sandblasters (Abraham and Wiesenfeld, 1997; Hughes

et al.,

1982), 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

Two classification methods used to characterize the radiographic findings of silicosis in chest X-rays are described in this literature review: the International Labour Office (ILO) Standardized System and the Chinese categorization system.

The “Radiological Diagnostic Criteria of Pneumoconiosis and Principles for Management of Pneumoconiosis” (GB5906-86) (Chen

et al.,

2001; Yang

et al.,

2006).

To describe the presence and severity of pneumoconiosis from chest X-rays or digital radiographic images, the ILO developed a standardized system to classify the opacities identified (ILO, 1980, 2002, 2011, 2022). The ILO system grades the size, shape, and profusion (frequency) of opacities in the lungs. The density of opacities is classified on a 4-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 and categories 1 to 3 reflect increasing profusion of opacities and a concomitant increase in severity of disease.

MSHA's analysis of silicosis studies uses NIOSH's surveillance case definition to determine the presence of silicosis. 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 where 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.

On March 26, 1969, Charles C. Johnson, Jr., Administrator, Consumer Protection and Environmental Health Service, PHS, U.S. Department of Health, Education, and Welfare, testified before the General Subcommittee on Labor and presented remarks of the Surgeon General. They are referenced in the 91st Congress House of Representatives Report, 1st Session No. 91-563, Federal Coal Mine Health and Safety Act, October 13, 1969 (

https://arlweb.msha.gov/SOLICITOR/COALACT/69hous.htm

However, some studies in MSHA's literature review use the Chinese categorization scheme, which includes four categories of silicosis: a suspected case (0+), stage I, stage II, or stage III. 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). Under this scheme, a panel of three radiologists determines the presence and severity of radiographic changes consistent with pneumoconiosis.

b. Progression and Associated Impairment

Progression of silicosis is shown 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 uncertainty in scoring films, some investigators count progression as advancing two or more subcategories, such as 1/0 to 1/2.

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 more recent literature (Dumavibhat

et al.,

2013; Mohebbi and Zubeyri, 2007; Wade

et al.,

2011) not previously reviewed by OSHA (2013b).

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 (Almberg

et al.,

2020; Cochrane

et al.,

1956; Hall

et al.,

2020b; Hurley

et al.,

1987; Kimura

et al.,

2010; Maclaren

et al.,

1985). In addition, although lung function impairment is highly correlated with chest X-ray films indicating silicosis, researchers cautioned 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 (Hessel

et al.,

1988 study of gold miners; Miller and MacCalman, 2010 study of coal miners; Miller

et al.,

1998 study of coal miners; Ng

et al.,

1987a study of granite 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 (

i.e.,

the product of the concentration times duration of exposure, which is summed over time) (Hessel

et al.,

1988; Ng

et al.,

1987a; Miller and MacCalman, 2010; Miller

et al.,

1998). 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. It 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 on 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

(1-3 mg/m

), and for a brief period, concentrations exceeded 10,000 µg/m

(10 mg/m

) for one job. Some of these high exposures were associated with accelerated disease progression.

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

, >2 mg/m

) increased the silicosis risk by three-fold (compared to the risk of cumulative exposure alone) (see the

separate

Preliminary Risk Analysis

document).

The risks of increased rate of progression, predicted by Buchanan

et al.

(2003) have been seen in coal miners (

e.g.,

Cohen

et al.,

2016; Laney

et al.,

2010, 2017; Miller

et al.,

1998), 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 (Mohebbi and Zubeyri, 2007; NIOSH 2000a,b; Ogawa

et al.,

2003a). Accordingly, it is important to limit higher exposures to respirable crystalline silica in order to minimize the risk of rapid progressive pneumoconiosis (RPP) in miners.

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 a 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.

c. Occupation-Based Epidemiological Studies

MSHA reviewed the occupation-based epidemiological literature (

i.e.,

studies that examine health outcomes among workers and their potential association with conditions in the workplace). MSHA's review included the occupation-based literature OSHA cited in developing its respirable crystalline silica standard (OSHA, 2013b). Overall, OSHA found substantial evidence suggesting that occupational exposure to respirable crystalline silica increases the risk of silicosis, and MSHA concurs with this conclusion. MSHA also reviewed additional occupation-based literature specific to respirable crystalline silica exposure in MNM and coal miners and preliminarily 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 2 months.

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

(0.48 mg/m

), 45 percent of those diagnosed with simple silicosis showed radiological progression of disease 2 to 10 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).

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). 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 simple 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 second (FEV

) percent predicted, 1.0 percent (95 percent CI, 0.6 percent-1.3 percent) forced vital capacity (FVC) percent predicted, and 0.6 percent (95 percent CI, 0.4 percent-0.8 FEV

/FVC).

Overall, MSHA preliminarily agrees with OSHA's conclusion that substantial evidence suggests that occupational exposure to respirable crystalline silica increases the risk of silicosis. MSHA also preliminarily concludes that respirable crystalline silica exposure increases the risk of silicosis morbidity and early mortality among miners.

d. Surveillance Data

In addition to occupation-based epidemiological studies, MSHA reviewed surveillance studies, which provide and interpret data to facilitate the prevention and control of disease, and preliminarily finds that the prevalence of silicosis generally increases with duration of exposure (work tenure). However, the available statistics may underestimate silicosis-related morbidity and mortality in miners. For example, the following have been reported: (1) misclassification of causes of death (

e.g.,

as TB, chronic bronchitis, emphysema, or

cor pulmonale

); (2) errors in recording occupation on death certificates; and (3) misdiagnosis of disease (Windau

et al.,

1991; Goodwin

et al.,

2003; Rosenman

et al.,

2003, Blackley

et al.,

2017). Furthermore, chest X-ray findings may lead to missed silicosis cases when fibrotic changes in the lung are not yet visible on chest X-rays. In other words, silicosis may be present but not yet detectable by chest X-ray, or may be more severe than indicated by the assigned profusion score (Craighead and Vallyathan, 1980; Hnizdo

et al.,

1993; Rosenman

et al.,

1997).

e. Pulmonary Tuberculosis

Finally, in addition to the relationship between silica exposure and silicosis, studies indicate a relationship between silica exposure, silicosis, and pulmonary TB. OSHA reviewed these and concluded that silica exposure and silicosis increase the risk of pulmonary TB (Cowie, 1994; Hnizdo and Murray, 1998; teWaterNaude

et al.,

2006). MSHA agrees with this conclusion.

Although early descriptions of dust diseases of the lung did not distinguish between TB and silicosis and most fatal cases described in the first half of the 20th century were likely a combination of silicosis and TB (Castranova

et al.,

1996), more recent findings have demonstrated that respirable crystalline silica exposure, even without silicosis, increases the risk of infectious (

i.e.,

active) pulmonary TB (Sherson and

Lander, 1990; Cowie, 1994; Hnizdo and Murray, 1998; teWaterNaude

et al.,

2006). These co-morbid conditions hasten the development of respiratory impairment and increased mortality risk even beyond the risk in unexposed persons with active TB (Banks, 2005).

Ng and Chan (1991) hypothesized that silicosis and TB “act synergistically” (

i.e.,

are more than additive) to increase fibrotic scar tissue (leading to massive fibrosis) or to enhance susceptibility to active mycobacterial infection. The authors found that lung fibrosis is common to both diseases, and that both diseases decrease the ability of alveolar macrophages to aid in the clearance of dust or infectious particles.

These findings are also supported by new studies (Ndlovu

et al.,

2019; Oni and Ehrlich, 2015) published since OSHA's review (2013b). Oni and Ehrlich (2015) reviewed a case of silico-TB in a former gold miner with ILO category 2/2 silicosis. Ndlovu

et al.

(2019) found that in a study sample of South African gold miners who had died from causes other than silicosis between 2005 and 2015, 33 percent of men (n = 254) and 43 percent of women (n = 29) at autopsy were found to have TB, whereas 7 percent of men (n = 54) and 3 percent of women (n = 4) were found to have pulmonary silicosis.

Overall, MSHA agrees with OSHA's conclusion that silica exposure increases the risk of pulmonary TB and that pulmonary TB is a complication of chronic silicosis.

2. Nonmalignant Respiratory Disease (Excluding Silicosis)

In addition to causing silicosis (acute silicosis, accelerated silicosis, simple chronic silicosis, and PMF), exposure to respirable crystalline silica causes other NMRD. NMRD includes emphysema and chronic bronchitis, which are both diagnoses within the category of COPD. Patients with COPD may have chronic bronchitis, emphysema, or both (ATS, 2010a).

Based on its review of the literature, MSHA preliminarily concludes that exposure to respirable crystalline silica increases the risk for mortality from NMRD. The following summarizes MSHA's review of the literature.

a. Emphysema

Emphysema involves the destruction of lung architecture in the alveolar region, causing airway obstruction and impaired gas exchange. In its literature review, OSHA (2013b) concluded that exposure to respirable crystalline silica can increase the risk of emphysema, regardless of whether silicosis is present. OSHA also concluded that this is the case for smokers and that smoking amplifies the effects of respirable crystalline silica exposure, increasing the risk of emphysema. MSHA reviewed the studies cited by OSHA and agrees with its conclusion. The studies reviewed are summarized below.

Becklake

et al.

(1987) determined that a miner who had worked in a high dust environment for 20 years had a greater chance of developing emphysema than a miner who had never worked in a high dust environment. In a retrospective cohort study, Hnizdo

et al.

(1991a) used autopsy lung specimens from 1,553 white gold miners to investigate the types of emphysema caused by respirable crystalline silica and found that the occurrence of emphysema was related to both smoking and dust exposure. This study also found a significant association between emphysema (both panacinar and centriacinar emphysema types) and length of employment for miners working in high dust occupations. A separate study by Hnizdo

et al.

(1994) on life-long non-smoking South African gold miners found that the degree of emphysema was significantly associated with the degree of hilar gland nodules, which the authors suggested might serve as a surrogate for respirable crystalline silica exposure. While Hnizdo

et al.

(2000) conversely found that emphysema prevalence was decreased in relation to dust exposure, the authors suggested that selection bias was responsible for this finding.

The findings of several cross-sectional and case-control studies discussed in the OSHA (2013b) Health Effects Literature were more mixed. For example, de Beer

et al.

(1992) found an increased risk for emphysema; however, the reported odds ratio (OR) was smaller than previously reported by Becklake

et al.

(1987).

The OSHA (2013b) Health Effects Literature also recognized that several of the referenced studies (Becklake

et al.,

1987 Hnizdo

et al.,

1994) found that emphysema might occur in respirable crystalline silica-exposed workers who did not have silicosis and suggested a causal relationship between respirable crystalline silica exposure and emphysema. Experimental (animal) studies found that emphysema occurred at lower respirable crystalline silica exposure concentrations than fibrosis in the airways or the appearance of early silicotic nodules (Wright

et al.,

1988). These findings tended to support human studies that respirable crystalline silica-induced emphysema can occur absent signs of silicosis.

Green and Vallyathan (1996) reviewed several studies of emphysema in workers exposed to silica and found an association between cumulative dust exposure and death from emphysema. The IARC (1997) also reviewed several studies and concluded that exposure to respirable crystalline silica increases the risk of emphysema. Finally, NIOSH (2002b) concluded in its Hazard Review that occupational exposure to respirable crystalline silica is associated with emphysema. However, some epidemiological studies suggested that this effect might be less frequent or absent in non-smokers.

Overall, MSHA agrees with OSHA that exposure to respirable crystalline silica causes emphysema even in the absence of silicosis.

b. Chronic Bronchitis

Chronic bronchitis is long-term inflammation of the bronchi, increasing the risk of lung infections. This condition develops slowly by small increments and “exists” when it reaches a certain stage (

i.e.,

the presence of a productive cough sputum production for at least 3 months of the year for at least 2 consecutive years) (ATS, 2010b).

OSHA considered many studies that examined the association between respirable crystalline silica exposure and chronic bronchitis, concluding the following: (1) exposure to respirable crystalline silica causes chronic bronchitis regardless of whether silicosis is present; (2) an exposure-response relationship may exist; and (3) smokers may be at an increased risk of chronic bronchitis compared to non-smokers. MSHA has reviewed the literature and agrees with OSHA's conclusions.

Miller

et al.

(1997) reported a 20 percent increased risk of chronic bronchitis in a British mining cohort compared to the disease occurrence in the general population. Using British pneumoconiosis field research data, Hurley

et al.

(2002) calculated estimates of mixed-RCMD-related disease in British coal miners at exposure levels that were common in the late 1980s and related their lung function and development of chronic bronchitis with their cumulative dust exposure. The authors estimated that by the age of 58, 5.8 percent of these men would report breathlessness for every 100 gram-hour/m

dust exposure. The authors also estimated the prevalence of chronic bronchitis at age 58 would be 4 percent per 100 gram-hour/m

of dust exposure. These miners averaged over 35 years of tenure in mining and a cumulative respirable dust exposure of 132 gram-hour/m

Cowie and Mabena (1991) found that chronic bronchitis was present in 742 of

1,197 (62 percent) black South African gold miners, and Ng

et al.

(1992b) found a higher prevalence of respiratory symptoms, independent of smoking and age, in Singaporean granite quarry workers exposed to high levels of dust (rock drilling and crushing) compared to those exposed to low levels of dust (maintenance and transport workers). However, Irwig and Rocks (1978) compared symptoms of chronic bronchitis in silicotic and non-silicotic South African gold miners and did not find as clear a relationship as did the above studies, concluding that the symptoms were not statistically more prevalent in the silicotic miners, although prevalence was slightly higher.

Sluis-Cremer

et al.

(1967) found that dust-exposed male smokers had a higher prevalence of chronic bronchitis than non-dust exposed smokers in a gold mining town in South Africa. Similarly, Wiles and Faure (1977) found that the prevalence of chronic bronchitis rose significantly with increasing dust concentration and cumulative dust exposure in South African gold miners of smokers, nonsmokers, and ex-smokers. Rastogi

et al.

(1991) found that female grinders of agate stones in India had a significantly higher prevalence of acute bronchitis, but they had no increase in the prevalence of chronic bronchitis compared to controls matched by socioeconomic status, age, and smoking. However, the study noted that respirable crystalline silica exposure durations were very short, and control workers may also have been exposed to respirable crystalline silica.

Studies examining the effect of years of mining on chronic bronchitis risk were mixed. Samet

et al.

(1984) found that prevalence of symptoms of chronic bronchitis was not associated with years of mining in a population of underground uranium miners, even after adjusting for smoking. However, Holman

et al.

(1987) studied gold miners in West Australia and found that the prevalence of chronic bronchitis, as indicated by ORs (controlled for age and smoking), was significantly increased in those that had worked in the mines for over 1 year, compared to lifetime non-miners. In addition, while other studies found no effect of years of mining on chronic bronchitis risk, those studies often qualified this result with possible confounding factors. For example, Kreiss

et al.

(1989) studied 281 hard-rock (molybdenum) miners and 108 non-miner residents of Leadville, Colorado. They did not find an association between the prevalence of chronic bronchitis and work in the mining industry (Kreiss

et al.,

1989); however, it is important to note that the mine had been temporarily closed for 5 months when the study began, so miners were not exposed at the time of the study.

The American Thoracic Society (ATS) (1997) published a review finding chronic bronchitis to be common among worker groups exposed to dusty environments contaminated with respirable crystalline silica. NIOSH (2002b) also published a review finding that occupational exposure to respirable crystalline silica has been associated with bronchitis; however, some epidemiological studies suggested this effect might be less frequent or absent in non-smokers.

Finally, Hnizdo

et al.

(1990) found an independent exposure-response relationship between respirable crystalline silica exposure and impaired lung function. For miners with less severe impairment, the effects of smoking and dust together were additive. However, for miners with the most severe impairment, the effects of smoking and dust were synergistic (

i.e.,

more than additive).

Overall, MSHA agrees with OSHA's conclusion that exposure to respirable crystalline silica causes chronic bronchitis regardless of whether silicosis is present and that an exposure-response relationship may exist.

c. Pulmonary Function Impairment

Pulmonary function impairment, generally defined as reduction below the lower limit of normal predicted by reference equations (and in older literature as less than 80 percent predicted) of diffusion capacity for carbon monoxide (DLCOcSB), total lung capacity (TLC), FVC, or FEV

is also a common condition of NMRD. Based on its review of the evidence in numerous longitudinal and cross-sectional studies and reviews, OSHA concluded that there is an exposure-response relationship between respirable crystalline silica and the development of impaired lung function. OSHA also concluded that the effect of tobacco smoking on this relationship may be additive or synergistic, and workers who were exposed to respirable crystalline silica but did not show signs of silicosis may also have pulmonary function impairment. MSHA has reviewed the studies cited by OSHA and agrees with their conclusions.

OSHA reviewed several longitudinal studies regarding the relationship between respirable crystalline silica exposure and pulmonary function impairment. To evaluate whether exposure to silica affects pulmonary function in the absence of silicosis, the studies focused on workers who did not exhibit progressive silicosis.

Among both active and retired Vermont granite workers exposed to an average quartz dust exposure level of 60 µg/m

, researchers found no exposure-related decreases in pulmonary function (Graham

et al.,

1981, 1994). However, Eisen

et al.

(1995) found significant pulmonary decrements among a subset of granite workers who left work and consequently did not voluntarily participate in the last of a series of annual pulmonary function tests (termed “dropouts”). This group experienced steeper declines in lung function compared to the subset of workers who remained at work and participated in all tests (termed “survivors”), and these declines were significantly related to dust exposure. Exposure-related changes in lung function were also reported in a 12-year study of granite workers (Malmberg

et al.,

1993), in two 5-year studies of South African miners (Hnizdo, 1992; Cowie, 1998), and in a study of foundry workers whose lung function was assessed between 1978 and 1992 (Hertzberg

et al.,

2002). Similar reductions in FEV

(indicating an airway obstruction) were linked to respirable crystalline silica exposure.

Each of these studies reported their findings in terms of rates of decline in any of several pulmonary function measures (

e.g.,

FEV

, FVC, FEV

/FVC). To put these declines in perspective, Eisen

et al.

(1995) reported that the rate of decline in FEV

seen among the dropout subgroup of Vermont granite workers was 4 ml per 1,000 µg/m

-year (4 ml per mg/m

-year) of exposure to respirable granite dust. By comparison, FEV

declines at a rate of 10 ml/year from smoking one pack of cigarettes daily. From their study of foundry workers, Hertzberg

et al.

(2002) reported a 1.1 ml/year decline in FEV

and a 1.6 ml/year decline in FVC for each 1,000 µg/m

-year (1 mg/m

-year) of respirable crystalline silica exposure after controlling for ethnicity and smoking. From these rates of decline, they estimated that exposure to 100 µg/m

of respirable crystalline silica for 40 years would result in a total loss of FEV

and FVC that was less than, but still comparable to, smoking a pack of cigarettes daily for 40 years. Hertzberg

et al.

(2002) also estimated that exposure to the existing MSHA standard (100 µg/m

) for 40 years would increase the risk of developing abnormal FEV

or FVC by factors of 1.68 and 1.42, respectively.

OSHA reviewed cross-sectional studies that described relationships between lung function loss and respirable crystalline silica exposure or

exposure measurement surrogates (

e.g.,

tenure). The results of these studies were similar to those longitudinal studies already discussed. In several studies, respirable crystalline silica exposure was found to reduce lung function of:

• White South African gold miners (Hnizdo

et al.,

1990),

• Black South African gold miners (Cowie and Mabena, 1991; Irwig and Rocks, 1978),

• Respirable crystalline silica-exposed workers in Quebec (Bégin

et al.,

1995),

• Rock drilling and crushing workers in Singapore (Ng

et al.,

1992b),

• Granite shed workers in Vermont (Theriault

et al.,

1974a, 1974b),

• Aggregate quarry workers and coal miners in Spain (Montes

et al.,

2004a, 2004b),

• Concrete workers in the Netherlands (Meijer

et al.,

2001),

• Chinese refractory brick manufacturing workers in an iron-steel plant (Wang

et al.,

1997),

• Chinese gemstone workers (Ng

et al.,

1987b),

• Hard-rock miners in Manitoba, Canada (Manfreda

et al.,

1982) and in Colorado (Kreiss

et al.,

1989),

• Pottery workers in France (Neukirch

et al.,

1994),

• Potato sorters in the Netherlands (Jorna

et al.,

1994),

• Slate workers in Norway (Suhr

et al.,

2003), and

• Men in a Norwegian community with years of occupational exposure to respirable crystalline silica (quartz) (Humerfelt

et al.,

1998).

The OSHA (2013b) Health Effects Literature recognized that many of these studies found that pulmonary function impairment: (1) can occur in respirable crystalline silica-exposed workers without silicosis, (2) was still observable when controlling for silicosis in the analysis, and (3) was related to the magnitude and duration of respirable crystalline silica exposure, rather than to the presence or severity of silicosis. Many other studies in the OSHA (2013b) Health Effects Literature have also found a relationship between respirable crystalline silica exposure and lung function impairment, including IARC (1997), the ATS (1997), and Hnizdo and Vallyathan (2003).

MSHA reviewed the studies and agrees with OSHA's finding that there is an exposure-response relationship between respirable crystalline silica and the impairment of lung function. MSHA also agrees with OSHA's finding that the effect of tobacco smoking on this relationship may be additive or synergistic, and that workers who were exposed to respirable crystalline silica, but did not show signs of silicosis, may also have pulmonary function impairment.

3. Carcinogenic Effects

a. Lung Cancer

Lung cancer, an irreversible and usually fatal disease, is a type of cancer that forms in lung tissue. Agreeing with the conclusion of other government and public health organizations that respirable crystalline silica is a “known human carcinogen,” MSHA has preliminarily found that the scientific literature supports that respirable crystalline silica exposure significantly increases the risk of lung cancer mortality among miners. This determination is consistent with the conclusions of other government and public health organizations, including the IARC (1997b, 2012), the NTP (2000, 2016), NIOSH (2002b), the ATS (1997), and the American Conference of Governmental Industrial Hygienists (ACGIH®, (2010)). The Agency's determination is supported by epidemiological literature, encompassing more than 85 studies of occupational cohorts from more than a dozen industrial sectors including: granite/stone quarrying and processing (Carta

et al.,

2001; Attfield and Costello, 2004; Costello

et al.,

1995; Guénel

et al.,

1989a,b), industrial sand (Sanderson

et al.,

2000; Hughes

et al.,

2001; McDonald

et al.,

2001, 2005; Rando

et al.,

2001; Steenland and Sanderson, 2001), MNM mining (Steenland and Brown, 1995a; deKlerk and Musk, 1998; Roscoe

et al.,

1995; Hessel

et al.,

1986, 1990; Hnizdo and Sluis-Cremer, 1991; Reid and Sluis-Cremer, 1996; Hnizdo

et al.,

1997; Chen

et al.,

1992; McLaughlin

et al.,

1992; Chen and Chen, 2002; Chen

et al.,

2006; Schubauer-Berigan

et al.,

2009; Hua

et al.,

1994; Meijers

et al.,

1991; Finkelstein 1998; Chen

et al.,

2012; Liu

et al.,

2017a; Wang

et al.,

2020a,b; Wang

et al.,

2021), coal mining (Meijers

et al.,

1988; Miller

et al.,

2007; Miller and MacCalman, 2010; Miyazaki and Une, 2001; Graber

et al.,

2014a,b; Tomaskova

et al.,

2012, 2017, 2020, 2022; Kurth

et al.,

2020), pottery (Winter

et al.,

1990; McLaughlin

et al.,

1992; McDonald

et al.,

1995), ceramic industries (Starzynski

et al.,

1996), diatomaceous earth (Checkoway

et al.,

1993, 1996, 1997, 1999; Seixas

et al.,

1997; Rice

et al.,

2001), and refractory brick industries (cristobalite exposures) (Dong

et al.,

1995).

The strongest evidence comes from the worldwide cohort and case-control studies reporting excess lung cancer mortality among workers exposed to respirable crystalline silica in various industrial sectors, confirmed by the 10-cohort pooled case-control analysis by Steenland

et al.

(2001a), the more recent pooled case-control analysis of seven European countries by Cassidy

et al.

(2007), and two national death certificate registry studies (Calvert

et al.,

2003 in the United States; Pukkala

et al.,

2005 in Finland).

Recent studies examined lung cancer mortality among coal and non-coal miners (Meijers

et al.,

1988, 1991; Starzynski

et al.,

1996; Miyazaki and Une, 2001; Tomaskova

et al.,

2012, 2017, 2020, 2022; Attfield and Kuempel, 2008; Graber

et al.,

2014a, 2014b; Kurth

et al.,

2020; NIOSH, 2019a). These studies also discuss the associations between RCMD and respirable crystalline silica exposures with lung cancer in coal mining populations. Furthermore, these newer studies are consistent with the conclusion of OSHA's final Quantitative Risk Assessment (QRA) (2016a) that respirable crystalline silica is a human carcinogen. MSHA preliminarily concludes that miners, both MNM and coal miners, are at risk of developing lung cancer due to their occupational exposure to respirable crystalline silica.

In addition, based on its review of the literature, MSHA has preliminarily determined that radiographic silicosis is a marker for lung cancer risk. Reducing exposure to levels that lower the silicosis risk would reduce the lung cancer risk to exposed miners (Finkelstein, 1995, 2000; Brown, 2009). MSHA has also found that, based on the available epidemiological and animal data, respirable crystalline silica causes lung cancer (IARC, 2012; RTECS, 2016; ATSDR, 2019). Miners who inhale respirable crystalline silica over time are at increased risk of developing silicosis and lung cancer (Greaves, 2000; Erren

et al.,

2009; Tomaskova

et al.,

2017, 2020, 2022).

Toxicity studies provide additional evidence of the carcinogenic potential of respirable crystalline silica. Studies using DNA exposed directly to freshly fractured respirable crystalline silica demonstrate the direct effect respirable crystalline silica had on DNA breakage. Cell culture research has investigated the processes by which respirable crystalline silica disrupt normal gene expression and replication. Studies have demonstrated that chronic inflammatory and fibrotic processes resulting in oxidative and cellular damage may lead to neoplastic changes in the lung (Goldsmith, 1997). In addition, the biologically damaging physical characteristics of respirable crystalline silica and its direct and indirect

genotoxicity (Schins

et al.,

2002; Borm and Driscoll, 1996) support MSHA's preliminary determination that respirable crystalline silica is an occupational carcinogen.

b. Cancers of Other Sites

In addition to lung cancer, OSHA reviewed studies examining the relationship between silica exposure and cancers at other sites. MSHA notes that OSHA reviewed these mortality studies (

e.g.,

cancer of the larynx and the digestive system, including the stomach and esophagus) and found that studies suggesting a dose-response relationship were too limited in terms of size, study design, or potential for confounding variables to be conclusive. OSHA also pointed to the NIOSH (2002b) silica (respirable crystalline silica) hazard review, which concluded that no association has been established between respirable crystalline silica exposure and excess mortality from cancer at other sites. MSHA has reviewed these studies and agrees with OSHA's conclusion. The following summarizes the studies reviewed with inconclusive findings.

(1) Laryngeal Cancer

Three lung cancer studies (Checkoway

et al.,

1997; Davis

et al.,

1983; McDonald

et al.,

2001) included in OSHA's health literature review suggest an association between respirable crystalline silica exposure and increased mortality from laryngeal cancer. However, a small number of cases were reported and researchers were unable to determine a statistically significant effect. Therefore, there is little evidence of an association based on these studies.

(2) Gastric (Stomach) Cancer

OSHA reviewed several studies in its 2013b health literature review to assess a potential relationship between respirable crystalline silica exposures and stomach cancers. OSHA's literature review noted observations made previously by Cocco

et al.

(1996) and in the NIOSH respirable crystalline silica hazard review (2002b), which found that most epidemiological studies of respirable crystalline silica and stomach cancer did not sufficiently adjust for the effects of confounding factors. In addition, some of these studies were not properly designed to assess a dose-response relationship (

e.g.,

Finkelstein and Verma, 2005; Moshammer and Neuberger, 2004; Selikoff, 1978; Stern

et al.,

2001) or did not demonstrate a statistically significant dose-response relationship (

e.g.,

Calvert

et al.,

2003; Tsuda

et al.,

2001). For these reasons, MSHA determined these studies were inconclusive in the context of this rulemaking.

(3) Esophageal Cancer

OSHA considered several studies that examined the relationship between respirable crystalline silica exposures and esophageal cancer and found that the studies were limited in terms of size, study design, or potential for confounding variables. Three nested case-control studies of Chinese workers demonstrated a dose-response association between increased risk of esophageal cancer mortality and respirable crystalline silica exposure (Pan

et al.,

1999; Wernli

et al.,

2006; Yu

et al.,

2005). Other studies (Tsuda

et al.,

2001; Xu

et al.,

1996a) also indicated elevated rates of esophageal cancer mortality with respirable crystalline silica exposure. However, OSHA noted that confounding factors due to other occupational exposures was possible. Additionally, two large national mortality studies in Finland and the United States did not show a positive association between respirable crystalline silica exposure and esophageal cancer mortality (Calvert

et al.,

2003; Weiderpass

et al.,

2003). MSHA agrees with OSHA's conclusion that the literature does not support attributing increased esophageal cancer mortality to exposure to respirable crystalline silica.

(4) Other Sites

NIOSH (2002b) conducted a health literature review of the health effects potentially associated with respirable crystalline silica exposure, which identified only infrequent reports of statistically significant excesses of deaths for other cancers. Cancer studies have been reported in the following organs/systems: salivary gland, liver, bone, pancreas, skin, lymphopoietic or hematopoietic, brain, and bladder (see NIOSH, 2002b for full bibliographic references). However, the findings were not observed consistently among epidemiological studies, and NIOSH (2002b) concluded that no association has been established between these cancers and respirable crystalline silica exposure. OSHA concurred with NIOSH that these isolated reports of excess cancer mortality were insufficient to determine the role of respirable crystalline silica exposure.

Overall, OSHA concluded that evidence of an association between silica exposure and cancer at sites other than the lungs is not sufficient. MSHA agrees with OSHA's conclusion.

4. Renal Disease

Renal disease is characterized by the loss of kidney function, and in the case of ESRD, the need for a regular course of long-term dialysis or a kidney transplant. MSHA reviewed a wide variety of longitudinal and mortality epidemiological studies, including case series, case-control, and cohort studies, as well as case reports, and preliminarily concludes that respirable crystalline silica exposure increases the risk of morbidity and/or mortality related to ESRD. However, MSHA notes that the available literature on respirable crystalline silica exposures and renal disease in coal miners is less conclusive than the literature related to MNM miners.

Epidemiological studies have found statistically significant associations between occupational exposure to respirable crystalline silica and chronic renal disease (

e.g.,

Calvert

et al.,

1997), sub-clinical renal changes, including proteinuria and elevated serum creatinine (

e.g.,

et al.,

1992a; Hotz

et al.,

1995; Rosenman

et al.,

2000), ESRD morbidity (

e.g.,

Steenland

et al.,

1990), ESRD mortality (Steenland

et al.,

2001b, 2002a), and Wegener's granulomatosis (Nuyts

et al.,

1995) (severe injury to the glomeruli that, if untreated, rapidly leads to renal failure). The pooled analysis conducted by Steenland

et al.

(2002a) is particularly convincing because it involved a large number of workers from three combined cohorts and had we

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