ACGIH TLVs Lack Scientific Integrity & Transparency – The Mica NIC

The American Conference of Governmental Industrial Hygienists (ACGIH®) is a non-profit corporation established in 1938, to advance occupational and environmental health.  The corporation’s motto, included in its logo, hubristically announces:  “Defining the Science of Occupational and Environmental Health.”

Philosophers of science may demur from “the” in “the Science,” as well as from the intellectual arrogance in suggesting that this private organization has any such ability to commandeer the complex social nature of scientific knowledge. And yet, in the small area of setting permissible exposure limits to potential environmental or occupational toxic substances, the ACGIH is in the business of “defining” safety. Starting in 1941, the group started to review and recommend “exposure limits.” In 1956, the group coined (literally and figuratively) the term “threshold limit values” (TLVs®), and started to publish documentation for its recommended values.

From the beginning, the ACGIH has asserted that TLVs® are not standards; rather they are guidelines for use, with other information, in determining safe levels of workplace and environmental exposure. The ACGIH maintains that its TLVs are based upon published, peer-reviewed scientific studies in industrial hygiene, toxicology, occupational medicine, and epidemiology, without consideration for economic or technical feasibility.

Beginning in the 1980s, “the Lobby”[1] started to throw brushback pitches at the ACGIH to bully the organization out of positions that the Lobby thought were too comforting to manufacturing industry.[2]  The result was a dramatic shift in the ACGIH’s perspective. The bullying created a “white-hat” bias that operates as a one-way ratchet to push the ACGIH always to lower TLVs, regardless whether there was a scientific warrant for doing so. Efforts to curb ACGIH overreach by litigation have generally failed. The TLVs have becoming increasingly controversial and non-evidence-based.[3]

What follows is what a hypothetical stakeholder might submit in response to a recent ACGIH Notice of Intended change for its TLV for mica dust. Like other mineral dusts, mica when inhaled in large quantities over long time periods, causes a pneumoconiosis. Documenting a “reasonable safe” level requires studies with adequate quantification of exposure. I will leave the reader to decide whether the ACGIH has that evidence in hand, based upon the following.

[1]  F.D.K. Liddell, “Magic, Menace, Myth and Malice,” 41 Ann. Occup. Hyg. 3, 3 (1997); seeThe Lobby Lives – Lobbyists Attack IARC for Conducting Scientific Research” (Feb. 19, 2013).

[2]  Barry I. Castleman & Grace E. Ziem, “Corporate influence on threshold limit values,” 13 Am. J. Indus.  Med. 531 (1988); Grace Ziem & Barry I. Castleman, “Threshold limit values: historical perspectives and current practice,” 31 J. Occup. Med. 910 (1989); S.A. Roach & S.M. Rappaport, “But they are not thresholds:  a critical analysis of the documentation of Threshold Limit Values,” 17 Am. J. Indus. Med. 727 (1990).

[3]  Philip E. Karmel, “The Threshold Limit Values Controversy,” N.Y. L. J. (Jan. 3, 2008).


These comments are in response to the proposed change in the ACGIH® TLV® for Mica, as explained in the ACGIH “Mica: TLV® Chemical Substances Draft Documentation, Notice of Intended Change” (“NIC”).  For the reasons stated below, the change in the Mica TLV-TWA (time-weighted average) in the NIC is not warranted and the existing Mica TLV should not be changed.

The ACGIH® TLVs® are important non-governmental standards, largely because a number of government entities incorporate TLVs by reference into regulations and thus give TLVs the force of law.[4]  For example, some states and Canadian provinces simply adopt TLVs as state or provincial occupational exposures levels, and some states have established “maximum allowable ambient concentrations” or similar limits on “toxic air contaminants” based entirely or in part on TLVs.  The U.S. Mine Safety and Health Administration (MSHA) uses the 1973 ACGIH TLV for crystalline silica (quartz) as a legally enforceable permissible exposure level.[5]  The U.S. Occupational Health and Safety Administration (OSHA) Hazard Communication Standard requires that ACGIH TLVs be disclosed in required Safety Data Sheets.  The process by which the ACGIH develops TLVs is critically important, and furthermore, given the regulatory and legal significance of the ACGIH TLVs, the ACGIH has the burden to support proposed changes in TLVs by an adequate process, which includes transparency and evidence sufficient to support any proposed change.

The flaws in the ACGIH TLV setting process are well known and the subject of several publications, most recently in paper titled “142 ACGIH Threshold Limit Values® established from 2008-2018 lack consistency and transparency.”

The following specific comments address the ACGIH’s process disclosed in the ACGIH’s NIC in support of its proposed change for the TLV-TWA for mica, which illustrate the process problem — the ACGIH NIC for the proposed Mica TLV-TWA change does not support the change proposed by the ACGIH.   Again, the ACGIH TLVs have regulatory and legal significance; therefore, the ACGIH should not make TLV changes arbitrarily and capriciously.  Instead, the changes should be made pursuant to a transparent process, and the ACGIH should support the proposed changes with the weight of the available evidence, and the evidence in support of and the reasons for the proposed change should be publicly disclosed.  It has not done that in this case, and its own NIC makes it clear that it has not:

  1. There is no evidence that “mica is an important cause of disabling occupational pneumoconiosis” as stated in the NIC. 

The NIC provides no citation or other supporting evidence for this conclusion; it merely states the conclusion as “fact” and a premise for the proposed change.[6] The NIC fails to estimate the number of workers currently potentially exposed to mica in the U.S. (or elsewhere), what industries these workers work in, what forms of mica these workers may be exposed to, what levels of respirable mica these workers may be exposed to, and to what extent pneumoconiosis caused by the inhalation of respirable mica exists.[7]

  1. The NIC proposes to materially lower the TLV for mica, but, other than noting that there are “nine different major species”, does not adequately address the mineralogical differences between the different species of mica, makes no attempt to assess the potential adverse health effects for the different species of mica, does not examine the “dose-response” data (as inadequate as it is) for the different species of mica, and so on. 

The “Chemical and Physical Properties” section of the NIC suggests the wide variety of materials that fall within the general term “mica”.  In spite of this, the ACGIH appears to have ignored differences and concluded that the TLV for “mica” as a general category of substances should be applicable to all forms of mica, with no support for this conclusion in the NIC.[8]

  1. The human studies (sic) cited in the NIC are inadequate to support a decision to change the mica TLV and do not support the mica TLV proposed.

The first cited study involved four employees in a muscovite milling plant, with an alleged exposure to respirable mica (as muscovite) dust between 1.86 and 5.77 mg/m3.

The second cited study involved a (one) South African man who worked in a mica milling factory.  As noted in the NIC, “[q]uantitative exposure data were not reported.”

The third cited study involved a (one) 65-year old who worked in the rubber industry for 40 years, where he was exposed to numerous dusts, including mica.  There was no exposure data reported.

The fourth study involved a (one) 62-year old woman allegedly exposed to “pure mica” for seven years; no quantitative exposure data were available.

The NIC cites the case of a worker who bagged mica flake for 36 years.  In this case, there were, apparently, two dust samples taken – one at the time of a medical exam of the worker at age 54, total dust of 0.2 mg/m3 — and one taken 17 years earlier – 0.7 mg/m3.  The bulk mica samples disclosed 7.1% to 8.4% silica (presumably, respirable crystalline silica as quartz).  The reference to the silica content of the “bulk samples” suggests that there was no analysis of the material collected in the two air samples taken.

The NIC cites the case of two British men who worked as “grinders of imported muscovite,” one starting in 1957. The “workplace dust concentrations were not quantified.”

The next paper cited in the NIC was from 1940 and involved employees who were exposed to the dust caused by “mica-scrap” grinding.  There was actually an attempt to quantify mica exposures (the data from before 1940), but it was done by particle count.  The NIC notes that the available information regarding mica health effects may be “limited by potential uncertainty converting from mppcf (million particles per cubic foot) to mg/m3 (which might not apply to all dust exposure scenarios).”  The difficulties associated with converting from mppcf to mg/m3 are well known in the cases of minerals far more extensively studied than mica (e.g., crystalline silica as quartz).  In addition to the conversion factor issue, there are other concerns raised by relying upon a paper published in 1940 to support a TLV today, such as, the quality of the sampling, the quality of the chest x-rays, and issues with the classification of the chest x-rays.  With that said, the NIC noted that “[n]one of the workers exposed at less than 10 mppcf (1.8 mg/m3), irrespective of employment duration, developed pneumoconiosis.”

The remaining studies cited in the NIC are similar.  But, to close this section of the comments, I will refer to the last study, a study of 71 South African workers employed in mica milling.  Twelve personal and static samples were taken during the course of the study.  The results of the personal samples indicated a range of respirable dust (or was it mica?) between 0.4 to 1.68 mg/m3.  The radiologic examination disclosed that 19 of the 71 workers had changes consistent with one or more of asbestos, silica and/or mica.  “The specific dust concentrations to which the individuals presenting with lung changes were exposed were not reported.”

The ACGIH is proposing a reduction in the mica TLV based on the studies as described in the NIC.  We submit that this is a process and transparency problem – there is simply no way to conclude that a reduction in the mica TLV is warranted based on the Human Studies (the “evidence”) cited in the NIC.  In most cases, the Human Studies are simply case reports involving one, two, or a few people, with no quantitative exposure data.  The studies with exposure data are inadequate, i.e., date from before 1940, and among other things measured exposure as mppcf, with one study literally including two samples.  Given the legal and regulatory significance of ACGIH TLVs, the evidence cited in support of the change in the mica TLV, and the reasonable conclusion that can be drawn from the evidence cited, should exceed some threshold and meet some burden.  The evidence in the NIC is grossly insufficient to support the proposed change.

  1. The NIC for mica does not disclose any evidence to support the proposed TLV of a TWA of 0.1 mg/m3

The comparison to OSHA’s notice of proposed rulemaking for occupational exposure to respirable crystalline silica (RCS) is instructive.  The supporting documentation sets forth a preliminary quantitative risk assessment outlining life-time risks for various disease end points associated with occupational exposure to RCS at various levels.  The preliminary quantitative risk assessment disclosed all of the underlying studies and methodology, sufficient to allow a reader to understand the basis for the risk assessment conclusions and agree or disagree with the conclusions.  Based on the risks (and other factors not considered by the ACGIH) set forth in the documentation, OSHA proposed a PEL (permissible exposure level) for RCS.

By contrast, the ACGIH simply contends for its proposed mica TLV: “Consequently, a TLV-TWA of 0.1 mg/m3 measured as respirable fraction (containing no asbestos and <1% crystalline silica) is recommended.”  The materials preceding “[c]consequently”, which in normal reading would be expected to support the conclusion following, are not a risk assessment or anything similar to one, and in no way even superficially support the conclusion – the recommendation – stated.[9]  Therefore, the ACGIH proposed a TLV-TWA of 0.1. The NIC materials do not support a TLV of 0.1 mg/m3, any more than they support a TLV of 0.0001 or 10.  It cannot be stated too emphatically that the NIC is devoid of any evidence to support any TLV, including the recommended TLV-TWA of 0.1 mg/m3.

Of course, the ACGIH TLV process is not a federal rulemaking.  And readers should be aware of the ACGIH Position Statement regarding TLVs (“TLVs … are not quantitative levels of risk at different exposure levels…”). But the same Position Statement notes that “TLVs®…represent conditions under which ACGIH® believes that nearly all workers may be repeatedly exposed without adverse health effects.”  So, presumably, the ACGIH concluded that its proposed mica TLV was that level, and yet there is simply no evidence in the NIC to support that conclusion.  Given the regulatory and legal significance of TLVs, the process of establishing TLVs should have some basis in science and evidence.


ACGIH® TLV/BEI® Position Statement, available at:

ACGIH® TLV/BEI® Policy Statement, available at:

30 C.F.R. § 56.5001 (MSHA exposure to airborne contaminants)

29 C.F.R. § 1910.1200 (OSHA Hazard Communications)

D. Davies & R. Cotton, “Mica pneumoconiosis,” 40 Br. J. Indus. Med. 22 (1983)

Subhabrata Moitra, “Mica pneumoconiosis: a neglected occupational lung disease – letter,” 6 The Lancet Respir. Med. e39 (2018), available at:

Notice of Proposed Rule Making (NPRM) for Occupational Exposure to Respirable Crystalline Silica, 56 Fed. Reg. 56273 (Sept. 12, 2013)

Knut R. Skulburg, “Mica pneumoconiosis – a literature review,” 11 Scand. J. Work & Envt’l Health 65 (1985)

Carl J. Smith & Thomas A. Perfetti, “142 ACGIH Threshold Limit Values® (TLV®s) established from 2008-2018 lack consistency and transparency,” 3 Toxicol. Research & Application 1 (2019)

[4] ACGIH states that TLVs are not intended to be legal standards, but the ACGIH recognizes the broad use of TLVs and should reasonably anticipate that TLVs will be used in ways beyond the scope of the legal disclaimers that the ACGIH publishes.

[5] 30 C.F.R. 56.5001

[6] The problems cited by Moitra — illegally operated mica mines exploiting vulnerable populations to work without protections in India and some African countries — speak to the need to eliminate illegal mining and protect vulnerable populations from exploitation, not the adequacy or inadequacy of any TLV.

[7] By comparison, see the OSHA documentation for the NPRM for Occupational Exposure to Respirable Crystalline Silica published September 12, 2013.  The citation to the 1985 Skulberg article (in “Major Sources of Exposure”) is inadequate on its face in 2020; the article summarizes world mica use from 1905-1981, and provides no information regarding use post-1981.  Likewise, the citation to the “Campaign for Safe Cosmetics” does not provide information on occupational exposure.

[8] The case of crystalline silica again provides a useful contrast.  There is no ACGIH TLV for “crystalline silica,” which as a general term includes many polymorphs.

[9] Actually, the materials that precede “consequently” explicitly refute the conclusion stated by the ACGIH (the TLV-TWA of 0.1 mg/m3) — “the published literature has established an association between mica exposure and pneumoconiosis typically at concentrations in the range of 1-6 mg/m3,”  and “no cases were observed among workers exposed to mica dusts at concentrations of 1.8 mg/m3 or less….”

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