Inhalation unit risk (IUR) of asbestos based on available science

Our plain-English summary of Inhalation unit risk (IUR) of asbestos based on available science, by A. Korchevskiy, J.O. Rasmuson, E.J. Rasmuson, et al (Inhalation Toxicology, 2020).

The study

• Authors: A. Korchevskiy, J.O. Rasmuson, E.J. Rasmuson, R.D. Strode
• Journal: Inhalation Toxicology
• Published: 2020;32(9-10):372-374
• Type of article: Letter
• Cited by: 3 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

In plain English

This letter to the editor presented calculations, not a new study of people. The authors built a spreadsheet calculator using the Peto-Nicholson model and 2009 US lifetables. They calculated inhalation unit risks with the Hodgson-Darnton and Berman-Crump methods, using updated Hodgson-Darnton chrysotile factors from a personal communication and Libby amphibole factors from other sources. An inhalation unit risk is excess cancer risk per PCM-equivalent fiber per cc of continuous lifetime exposure. They did this for crocidolite, amosite, Libby amphibole and chrysotile. For total cancers, the values were 1.50 and 1.83 for crocidolite, 0.60 and 1.83 for amosite, 0.13 by both methods for Libby amphibole, and 0.016 and 0.035 for chrysotile. Averaged across the two methods, chrysotile's value was 0.026, about 65 times lower than crocidolite's. The authors stated that the general US EPA asbestos value, 0.23, would overestimate risk for Libby amphibole and chrysotile and dramatically underestimate it for amosite and crocidolite. They concluded that risk assessors should consider the differences between fiber types. They noted limits from the number of, and variability between, available cohorts.

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: Letter to the editor presenting risk calculations made with a spreadsheet calculator (Peto-Nicholson model) using potency factors from published meta-analyses and other sources; no new data on people were collected
• Who was studied: No group of people was studied directly; lifetime risks were calculated using US population lifetables from 2009, with potency factors taken from earlier analyses of occupational cohorts
• Exposure: Four fiber types: crocidolite, amosite, Libby amphibole and chrysotile, expressed as continuous lifetime exposure in PCM-equivalent fibers per cc. Potency factors came from the Hodgson-Darnton and Berman-Crump methods. The Hodgson-Darnton chrysotile factors were updated with recent meta-analytical data for combined mining, manufacturing and textile cohorts, credited to a personal communication from Darnton. The Libby amphibole factors were taken from Moolgavkar et al. 2010 and Darnton (HSE) 2010.
• Main results:
  • When the calculator was tested on the original data from the 1986 US EPA publication, it reproduced the EPA's unit risk for mixed asbestos fibers: 0.23 for total cancer, including 0.09 for lung cancer and 0.14 for mesothelioma.
  • Crocidolite, per PCME f/cc: total cancer unit risk was 1.50 (Hodgson-Darnton linear method) and 1.83 (Berman-Crump method). Mesothelioma was 1.12 and 1.59. Lung carcinoma was 0.38 and 0.23.
  • Amosite, per PCME f/cc: total cancer unit risk was 0.60 (Hodgson-Darnton) and 1.83 (Berman-Crump). Mesothelioma was 0.22 and 1.59. Lung carcinoma was 0.38 and 0.23.
  • Libby amphibole: both methods gave a total cancer central tendency unit risk of 0.13, slightly higher than the US EPA estimate of 0.115.
  • Chrysotile, per PCME f/cc: total cancer unit risk was 0.016 (Hodgson-Darnton) and 0.035 (Berman-Crump). Mesothelioma was 0.004 and 0.002. Lung carcinoma was 0.012 and 0.033.
  • Averaging the two methods gave a central tendency unit risk for chrysotile of 0.026, about 65 times lower than the average value for crocidolite.
  • IRIS contains two asbestos unit risks: a general value of 0.23 for asbestos fibers and an upper-bound value of 0.17 for Libby amphibole. The authors stated that the general US EPA IRIS total asbestos value of 0.23 would overestimate cancer risk for Libby amphibole and chrysotile, but dramatically underestimate it for amosite and crocidolite.
  • In a hypothetical example, the authors calculated 13 excess cancer cases per 1,000,000 people per lifetime for lifetime exposure to 0.0005 f/cc of chrysotile. If 5% of that exposure were crocidolite (average unit risk 1.665), the estimate became 55 excess cases per 1,000,000 per lifetime.
• The authors' conclusion: The authors concluded that the differences in unit risk between fiber types are significant and have implications for public policy. They concluded that risk assessors, industrial hygienists and agencies should consider these differences.
• Limitations the authors state:
  • The calculations are limited by the number of cohorts available for each fiber type and by variability between cohorts, some of it caused by inaccurate exposure assessment.
  • Additional coefficients are needed for exposures shorter than a lifetime and for different ages of first exposure.
  • The mesothelioma potency factors assumed that all mesothelioma cases in the occupational cohorts were caused by asbestos. The authors said this is not true in general, though they estimated the potencies would change by less than 3 to 5% if background rates were subtracted.
  • The authors noted that some authors have suggested unit risks should account for fiber length and width.
  • Risk for specific exposure scenarios, especially in workplaces, is better calculated directly from potency factors and lifetables than from the average unit risks.

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