The limits of thresholds: silica and the politics of science, 1935 to 1990

Our plain-English summary of The limits of thresholds: silica and the politics of science, 1935 to 1990, by G. Markowitz, D. Rosner (American Journal of Public Health, 1995).

The study

• Authors: G. Markowitz, D. Rosner
• Journal: American Journal of Public Health
• Published: 1995;85(2):253-62
• Type of article: Historical article, Review
• Read it: PubMed · Publisher · Free full text

Abstract

Since the 1930s threshold limit values have been presented as an objectively established measure of US industrial safety. However, there have been important questions raised regarding the adequacy of these thresholds for protecting workers from silicosis. This paper explores the historical debates over silica threshold limit values and the intense political negotiation that accompanied their establishment. In the 1930s and early 1940s, a coalition of business, public health, insurance, and political interests formed in response to a widely perceived "silicosis crisis." Part of the resulting program aimed at containing the crisis was the establishment of threshold limit values. Yet silicosis cases continued to be documented. By the 1960s these cases had become the basis for a number of revisions to the thresholds. In the 1970s, following a National Institute for Occupational Safety and Health recommendation to lower the threshold limit value for silica and to eliminate sand as an abrasive in blasting, industry fought attempts to make the existing values more stringent. This paper traces the process by which threshold limit values became part of a compromise between the health of workers and the economic interests of industry.

In plain English

This was a historical analysis, not a medical study. Using mostly archival sources, the authors traced how US silica dust threshold limit values were set from the 1930s into the 1990s. They reported that the medical committee of the industry-backed Air Hygiene Foundation suggested a limit of 5 million particles per cubic foot of air while writing that the knowledge to base such thresholds on was not available. Johns Manville had the same value adopted for asbestos, on the view that asbestosis was less serious than silicosis. Through the 1950s, the American Conference of Governmental Industrial Hygienists did not seriously reassess the silica value, though studies showed workers still getting silicosis. In 1974 NIOSH recommended a time-weighted average limit of 50 micrograms of free silica per cubic meter of air. It also recommended a ban on blasting abrasives containing more than 1% free silica, including silica sand. An industry group formed in 1975 to keep sand in use later claimed credit for delaying the proposals. The authors concluded that the standards were promoted as safety levels but reflected a compromise between workers' health and industry's economic interests.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Historical analysis of archival and published documents (not an epidemiological or clinical study)
• Who was studied: The history of US silica threshold limit values and the organizations involved: industry groups, government agencies (Bureau of Mines, NIOSH, OSHA), and industrial hygienists, from 1935 to about 1990. Sources were gathered largely during research for the authors' book Deadly Dust.
• Exposure: Crystalline silica dust, and the 5 million particles per cubic foot standard for it. The article also covers use of silica sand in abrasive blasting and the same 5-mppcf value as adopted for asbestos.
• Main results:
  • The Air Hygiene Foundation's Medical Committee, chaired by Lanza, recommended a 5-mppcf limit while stating that the knowledge to base thresholds on was lacking.
  • In their 1936 textbook, Drinker and Hatch wrote that the 5-mppcf standard had no published supporting data.
  • Johns Manville succeeded in having the 5-mppcf value adopted as the threshold limit value for asbestos, on the view that asbestosis was less serious than silicosis.
  • The American Conference of Governmental Industrial Hygienists did not seriously reassess the silica standard through the 1950s, even though studies continued to show workers getting silicosis.
  • A NIOSH-contracted survey of sandblasting operations in the early 1970s rated respiratory protection for workers as marginal to poor on average.
  • In 1974 NIOSH recommended limiting free silica exposure to a time-weighted average of 50 micrograms per cubic meter. It also recommended prohibiting silica sand, or other materials with more than 1% free silica, as an abrasive in blasting.
  • The Silica Safety Association, formed in 1975 by industry representatives, sought continued use of sand in blasting. In 1981 it claimed credit for delaying adoption of the proposals.
• The authors' conclusion: The authors concluded that silica standards were publicly promoted as safety levels. In their view, business representatives and technical experts understood the standards as compromises among competing interests. They also concluded that the historical record shows economic and social issues shaping questions of when silica is safe.
• Limitations the authors state:
  • The authors stated that the full history of the NIOSH proposal to revise the silica standard, and the controversy that followed, remains to be written.

Topics

Medical terms in this study, explained in our Lexicon: Threshold Limit Value (TLV), Silicosis.

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