State-of-the-science assessment of non-asbestos amphibole exposure: is there a cancer risk?

Our plain-English summary of State-of-the-science assessment of non-asbestos amphibole exposure: is there a cancer risk?, by C. Williams, L. Dell, R. Adams, et al (Environmental Geochemistry and Health, 2013).

The study

• Authors: C. Williams, L. Dell, R. Adams, T. Rose, D. Van Orden
• Journal: Environmental Geochemistry and Health
• Published: 2013;35(3):357-77
• Type of article: Review
• Cited by: 17 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

The distinction between amphibole asbestos fibers and non-asbestos amphibole particles has important implications for assessing potential cancer risks associated with exposure to amphibole asbestos or amphibole-containing products. Exposure to amphibole asbestos fibers can pose a cancer risk due to its ability to reside for long periods of time in the deep lung (i.e., biopersistence). In contrast, non-asbestos amphibole particles are usually cleared rapidly from the lung and do not pose similar respiratory risks even at high doses. Most regulatory and public health agencies, as well as scientific bodies, agree that non-asbestos amphiboles possess reduced biological (e.g., carcinogenic) activity. Although non-asbestos amphibole minerals have been excluded historically from Federal regulations, non-asbestos structures may be counted as asbestos fibers on the basis of dimensional criteria specified in analytical protocols. Given the potential to mischaracterize a non-asbestos structure as a "true" asbestos fiber, our objective was to assess whether exposure to non-asbestos amphiboles that may meet the dimensional criteria for counting as a fiber pose a cancer risk similar to amphibole asbestos. We reviewed analytical methods as well as the mineralogical, epidemiological, and toxicological literature for non-asbestos amphiboles. No evidence of demonstrable cancer effects from exposure to non-asbestos amphiboles that may be counted as fibers, under certain assessment protocols, was found. Data gaps (industrial hygiene data for amphibole-exposed cohorts), inconsistencies (analytical laboratory methods/protocols used to count fibers), and sources of potential bias from misclassification of exposure were identified.

In plain English

This article was a review: the authors searched PubMed from 1966 onward and several toxicology databases. They reviewed laboratory methods, mineralogy, epidemiology and animal studies of non-asbestos amphiboles that size and shape counting rules can count as asbestos fibers. Two Minnesota taconite studies they reviewed reported no mesotheliomas. One found 15 respiratory cancer deaths against 18 expected. In a New York talc cohort (Honda et al.), lung cancer deaths were elevated overall (SMR = 2.32, 95% CI 1.57–3.29, 31 deaths). However, internal analyses showed an inverse association with estimated cumulative dust exposure. Five more mesothelioma deaths among cohort members were reported after 1989, but a company-sponsored medical review raised doubts about the diagnosis in all but one. In a rat injection study (Davis et al.), mesothelioma occurred in 36/36, 32/33 and 35/36 rats given tremolite asbestos, and 24/36, 4/33 and 2/35 given non-asbestos tremolite. Those investigators attributed the non-asbestos results to trace asbestos contamination. The authors concluded there was no convincing evidence of increased lung cancer or mesothelioma risk from non-asbestos amphiboles in taconite, gold or talc mining and milling.

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: Review (described by the authors as a systematic review) of published and unpublished literature on amphibole mineralogy, laboratory analysis, epidemiology and animal carcinogenicity
• Who was studied: Studies found through PubMed (1966 to the time of writing) and the Toxicology Data Network databases. These included epidemiological studies of taconite miners in Minnesota, gold miners (Homestake mine and Ontario), and talc miners and millers (upstate New York, Vermont, Norway), plus ten animal carcinogenicity studies in rats, hamsters and mice.
• Exposure: Amphibole minerals in a non-asbestiform habit (for example acicular or prismatic particles and cleavage fragments) that meet regulatory fiber-counting criteria: longer than 5 micrometers with a length-to-width ratio of 3:1 or more. Minerals covered included cummingtonite-grunerite, tremolite, actinolite, ferroactinolite and talc. Chrysotile and short amphibole particles were excluded.
• Main results:
  • In the reviewed study of 5,751 Reserve Mining Company taconite employees (Higgins et al. 1983), 15 respiratory cancer deaths were observed against 18 expected.
  • In the reviewed Erie and Minntac taconite cohort of 3,431 men (Cooper et al.), the SMR for respiratory cancers using Minnesota rates was 97 (95% CI 75–123), and no mesotheliomas were reported.
  • The Minnesota Department of Health reported 63 mesotheliomas among 72,000 former iron mine workers. Of 17 miners diagnosed with mesothelioma who were reviewed for job history, 11 had jobs with probable exposure to commercial asbestos, 3 possible, 1 no apparent exposure, and 2 had missing information.
  • In the reviewed Homestake gold miner study (Steenland and Brown 1995), no association was reported between lung cancer mortality and quantitative measures of dust exposure.
  • In the reviewed upstate New York talc cohort (Honda et al. 2002), lung cancer mortality was significantly elevated overall, based on 31 deaths.
  • In the same talc cohort, internal analyses showed lung cancer risk decreasing as estimated cumulative respirable dust exposure increased (an inverse association).
  • In the same talc cohort, two mesothelioma deaths occurred by 1989, and five additional mesothelioma deaths among cohort members were reported after 1989. A company-sponsored medical review, cited by the authors, raised doubts about the diagnosis in all but one of these additional cases.
  • In the reviewed rat intraperitoneal injection study by Davis et al. (1991), mesothelioma occurred in 36/36, 32/33 and 35/36 rats given tremolite asbestos samples (California, Korea, Swansea).
  • In the same Davis et al. study, rats given non-asbestos tremolite samples had mesothelioma in 24/36 (Italian), 4/33 (Carr Brae) and 2/35 (Shinness). Those investigators attributed this to trace contamination with asbestos amphiboles.
  • The National Toxicology Program inhalation study of non-asbestos talc was the only reviewed animal study with a positive carcinogenic response. Lung adenomas and carcinomas were found in 13/50 female rats at the highest dose versus 0/50 controls. There was no evidence of carcinogenic activity in mice.
  • The authors' synthesis of the animal studies was that tremolite asbestos was markedly more carcinogenic than non-asbestos tremolite.
• The authors' conclusion: The authors concluded that there is no convincing evidence of increased risk of lung cancer or mesothelioma among people exposed to non-asbestos amphiboles in mining and milling of taconite ore, gold ore or talc. They called for future studies to identify both the mineral and its form rigorously.
• Limitations the authors state:
  • The epidemiology studies lacked quantified measures of asbestos fibers for individual study subjects.
  • Follow-up of the taconite cohorts was too short to detect small increases in lung cancer or mesothelioma risk.
  • The talc cohort studies were small, and mortality for that cohort was last updated through 1989.
  • Studies were affected by incorrect or incomplete characterization of fiber types, so workers may have been misclassified as fiber-exposed.
  • Apart from the NTP talc study, the animal studies did not use inhalation, the most relevant route of exposure. No new animal studies were found after 1993.
  • Study quality was not used to exclude studies, and many early studies had methodological flaws.

Topics

The National Library of Medicine indexes this study under subjects our site covers: Lung cancer.

Medical terms in this study, explained in our Lexicon: Amphibole Group, Amphibole Asbestos, biopersistence, Carcinogenic, Industrial Hygiene.

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