The role of threshold limit values in U.S. air pollution policy

Our plain-English summary of The role of threshold limit values in U.S. air pollution policy, by J.C. Robinson, D.G. Paxman (American Journal of Industrial Medicine, 1992).

The study

• Authors: J.C. Robinson, D.G. Paxman
• Journal: American Journal of Industrial Medicine
• Published: 1992;21(3):383-96
• Cited by: 6 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

This paper analyzes the role of threshold limit values (TLVs) in national air pollution policy during the 1980s, a period in which the Environmental Protection Agency (EPA) sought to delegate to individual states the authority to evaluate and regulate airborne toxic substances. We focus on 20 carcinogens and 11 substances with non-genotoxic health effects that were regulated by local air toxics programs using TLVs. Data from EPA's National Air Toxics Information Clearinghouse indicate that maximum TLV-based Ambient Air Level guidelines (AALs) frequently exceed minimum TLV-based AALs by a factor of greater than 1,000. Cancer potency data from EPA's Integrated Risk Information System suggest significant risks remain at TLV-based AALs. Cancer risks at the median TLV-based AAL exceed 1,000 cases per million exposed persons for cadmium (1,040), nickel and its compounds (1,420), propylene oxide (1,550), coke oven emissions (1,860), benzene (2,500), arsenic and its compounds (7,300), N-nitrosodimethylamine (21,000), asbestos (21,500), and ethylene dibromide (55,000). We also summarize published studies that report non-genotoxic health effects in workers exposed at levels near the TLV for 11 substances whose AALs were based on TLVs. Contrary to the assumption frequently made by state air toxics program, TLVs cannot be taken to represent no observed effect levels (NOELs) for regulatory purposes.

In plain English

This paper was a policy analysis. The authors examined how U.S. state and city air toxics programs in the 1980s used workplace exposure limits called threshold limit values (TLVs) to set ambient air level (AAL) guidelines for the public. Using a 1989 EPA database of state and city programs and EPA cancer potency data, they looked at acrylonitrile, 19 other IARC-designated carcinogens and 11 substances with non-cancer effects. For acrylonitrile, permitted cancer risk across 15 programs ranged from 17 to 17,544 per million exposed. The highest and lowest TLV-based guidelines for a substance often differed more than 1,000-fold. For asbestos the ratio was over 500,000. At the median TLV-based guideline for asbestos, the calculated lifetime risk was 21,500 cancers per million exposed persons. The asbestos fiber type was not specified. The authors noted that these were risks the guidelines permitted, not actual risks to residents. Published studies reported non-cancer effects in workers exposed at or near the TLV for 11 substances. The authors concluded that TLVs cannot be assumed to be no observed effect levels.

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: Policy analysis using EPA databases (1989 National Air Toxics Information Clearinghouse and the Integrated Risk Information System cancer potency data), plus a summary of published studies of non-cancer effects in workers
• Who was studied: Ambient air level guidelines of U.S. state and city air toxics programs (1989 data) for substances regulated using TLVs by at least one program: acrylonitrile (15 programs), 19 other IARC-designated carcinogens including asbestos, and 11 substances with non-genotoxic effects
• Exposure: Community air exposure at levels permitted by TLV-based ambient air level guidelines, standardized to 24-hour averages. Programs routinely applied a safety factor of 4.2 to TLVs, often with further safety factors. Asbestos was one of the carcinogens; the fiber type was not specified.
• Main results:
  • Acrylonitrile: across 15 state and city programs, the allowable cancer risk ranged from 17 per million in Indianapolis and Kansas City to 17,544 per million in New York state
  • The ratio of the maximum to the minimum TLV-based guideline for the same substance frequently exceeded 1,000
  • Among the 19 carcinogens, the ratio of maximum to minimum TLV-based guideline ranged from 5 for beryllium to more than 500,000 for asbestos
  • For asbestos, 3 of 12 state and city guidelines were based on TLVs (ratio given in Table II as TLV-based guidelines over total guidelines)
  • Asbestos: the residual lifetime cancer risk at the median TLV-based guideline was 21,500 cancers per million exposed persons. Residual risk exceeded 1,000 per million for nine substances, including asbestos.
  • For 11 non-genotoxic substances, published studies reported adverse effects in workers exposed at or below the TLV (seven substances) or above but not far above the TLV (four substances)
• The authors' conclusion: The authors concluded that TLVs cannot be assumed to approximate no observed effect levels (NOELs) for regulatory purposes
• Limitations the authors state:
  • The risk estimates are the risks permitted under the guidelines, not the actual risks faced by residents, which depend on actual exposure levels
  • The maximum-to-minimum ratios are sensitive to a single very high or very low guideline
  • The actual degree of risk to community residents cannot be determined once the TLV has been divided by a safety factor (stated for the non-genotoxic substances)

Topics

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

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