An evaluation of the risks of lung cancer and mesothelioma from exposure to amphibole cleavage fragments

Our plain-English summary of An evaluation of the risks of lung cancer and mesothelioma from exposure to amphibole cleavage fragments, by J.F. Gamble, G.W. Gibbs (Regulatory Toxicology and Pharmacology, 2008).

The study

• Authors: J.F. Gamble, G.W. Gibbs
• Journal: Regulatory Toxicology and Pharmacology
• Published: 2008;52(1 Suppl):S154-86
• Type of article: Review
• Cited by: 74 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Amphiboles are hydrated mineral silicates five of which occur in asbestiform habits as asbestos grunerite (amosite) asbestos, riebeckite (crocidolite) asbestos, anthophyllite asbestos, tremolite asbestos and actinolite asbestos] and non-asbestiform habits (grunerite, riebeckite, anthophyllite, tremolite and actinolite). The asbestiform varieties are characterized by long, thin fibers while non-asbestiform varieties such as cleavage fragments form short fibers with larger widths. The U.S. regulatory method for counting asbestos fibers (aspect ratio > or = 3:1, length > or = 5 microm) does not distinguish between asbestos and cleavage fragments. The method biases toward increased counts of non-asbestiform cleavage fragments compared to long, thin asbestos fibers. One consequence of this regulatory approach is that workers can be erroneously classified as exposed to concentrations of asbestos (asbestiform amphiboles) above the U.S. 0.1 f/mL exposure standard when in fact they are not exposed to asbestos at all but non-asbestiform amphibole cleavage fragments. Another consequence is that the known carcinogenic effects of asbestos may be falsely attributed to non-asbestiform amphibole cleavage fragments of the same mineral. The purpose of this review is to assess whether amphibole cleavage fragments pose the same risk of lung cancer and mesothelioma characteristic of amphibole asbestos fibers. We identified three groups of workers exposed to non-asbestiform amphiboles: two groups exposed to grunerite (Homestake gold miners and taconite miners) and one group exposed to industrial talc containing non-asbestiform tremolite and anthophyllite in St. Lawrence County, NY. In addition to assessing strength of association and exposure-response trends in the non-asbestiform amphibole cohorts, comparisons were also made with cohorts exposed to the asbestiform counterpart (positive control) and cohorts exposed to the mineral (e.g. talc) that does not contain amphiboles (negative controls). The cohorts exposed to non-asbestiform amphiboles had no excesses of lung cancer or mesothelioma. Similar results were observed in the negative control groups, in stark contrast to the excess risks of asbestos-related disease found in the asbestos cohorts. The only possible exception is the twofold increased risk of lung cancer where exposure was to industrial talc containing cleavage fragments of tremolite and anthophyllite. However, this risk is not considered attributable to the talc or amphibole cleavage fragments for several reasons. A similar increased risk of lung cancer was found in Vermont talc workers, studied in the same time period. Their exposure was to relatively pure talc. There was no relationship between lung cancer mortality and exposure measured as mg/m(3)years and years worked. A case-control study reported that all the lung cancer cases were smokers (or former smokers) and attributed the excess to smoking. There were two mesothelioma cases among the NY State talc workers exposed to cleavage fragments of tremolite and anthophyllite, but talc is not a plausible cause because of too short latency and potential for previous asbestos exposure. The positive controls of tremolite asbestos and anthophyllite asbestos exposed workers showed excess risks of both lung cancer and mesothelioma and positive exposure-response trends. St. Lawrence, NY talc does not produce mesotheliomas in animals while amphibole asbestos does. In sum, the weight of evidence fully supports a conclusion that non-asbestiform amphiboles do not increase the risk of lung cancer or mesothelioma.

In plain English

This was a review of published studies. The authors compared workers exposed to non-asbestiform amphiboles (cleavage fragments) with workers exposed to amosite, tremolite asbestos or anthophyllite asbestos, and with talc or hematite workers whose ore lacked amphiboles. The cleavage-fragment workers were gold and taconite miners exposed to non-asbestiform grunerite, and New York talc workers exposed to non-asbestiform tremolite and anthophyllite. In the combined gold and taconite cohorts, no mesothelioma was linked to exposure; there were 192 lung cancer deaths against 211.9 expected. In the combined amosite cohorts, 1.2% of deaths were mesothelioma; there were 224 lung cancer deaths against 81 expected. New York talc workers had 31 lung cancer deaths against 13 expected. But in the latest study, risk fell as cumulative dust exposure rose, and in a case-control study all 22 cases were smokers or ex-smokers. In rat injection studies, tremolite asbestos samples from Italy, California, Swansea and South Korea produced mesothelioma in 70 to 100% of animals; non-asbestiform tremolite produced near zero. The authors concluded that non-asbestiform amphiboles do not increase lung cancer or mesothelioma risk, noting limited exposure estimates.

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 of published epidemiological and experimental studies. Cohorts exposed to non-asbestiform amphiboles were compared with asbestos-exposed cohorts (positive controls) and with cohorts exposed to ores without amphiboles (negative controls). The article states it was funded by the National Stone and Gravel Association.
• Who was studied: The non-asbestiform cohorts were published cohorts of Homestake gold miners (South Dakota), Reserve and Erie–Minntac taconite miners (Minnesota), and St. Lawrence County / Gouverneur, New York talc workers. The positive comparisons were amosite miners and insulation factory workers, Libby, Montana vermiculite workers, and Finnish anthophyllite asbestos miners. The negative comparisons were Minnesota hematite miners and Vermont, Italian, French and Austrian talc miners and millers.
• Exposure: Non-asbestiform amphibole cleavage fragments: grunerite–cummingtonite (with actinolite and hornblende) in gold and taconite ore, and non-asbestiform tremolite and anthophyllite in New York industrial talc. These were compared with grunerite (amosite) asbestos, tremolite asbestos (Libby vermiculite) and anthophyllite asbestos.
• Main results:
  • Mesothelioma: in the combined amosite asbestos cohorts, 19% of persons had died and 1.2% of deaths were from mesothelioma. In the combined non-asbestiform cohorts, 23% had died and no mesothelioma was linked to the exposure.
  • Lung cancer, combined non-asbestiform grunerite cohorts (Homestake gold, Reserve and Erie taconite): 192 observed against 211.9 expected (SMR 0.91).
  • Lung cancer, combined grunerite (amosite) asbestos cohorts: 224 observed against 81 expected (SMR 2.77).
  • Homestake gold miners (Steenland and Brown, 1995): lung cancer was not associated with cumulative exposure, and the nested case-control analysis showed a negative trend.
  • In the amosite insulation factory (Seidman et al., 1986), lung cancer risk was increased even at cumulative exposures below 6 f/mL years.
  • New York talc workers (Honda et al., 2002), lung cancer SMR overall: 31 observed against 13 expected (2.32, 95% CI 1.57 to 3.29).
  • New York talc workers (Honda et al., 2002), internal comparison by cumulative talc dust exposure: the relative risk of lung cancer in the highest exposure category was 0.5 (0.2 to 1.3).
  • New York talc workers, miners versus millers: the lung cancer excess was among miners (SMR 3.94, 18 observed) and not among millers (SMR 1.28, 7 observed).
  • Nested case-control study of New York talc workers (Gamble, 1993; one of this review's authors): 22 cases and 66 controls. All cases were smokers or ex-smokers, compared with 73% of controls.
  • Two mesothelioma deaths among New York talc workers: Honda et al. considered neither likely to be caused by talc exposure. One had a 15-year latency; the other was a draftsman during mill construction.
  • Vermont talc workers (talc without asbestos, negative comparison): non-significant 1.63-fold lung cancer excess overall, significant among miners (SMR 4.35, 5 observed) but not millers (SMR 1.02, 2 observed).
  • Finnish anthophyllite asbestos miners: four mesothelioma cases, a 46-fold increased SIR (95% CI 12.2 to 115).
  • Libby, Montana vermiculite workers exposed to tremolite asbestos: lung cancer SMR 2.40 (1.74 to 3.22); mesothelioma 4.2% of deaths.
  • Rat injection studies: asbestiform tremolite samples from Italy, California, Swansea and South Korea produced mesothelioma incidences of 70 to 100%.
  • Rat injection studies: two Scottish tremolite samples contained relatively few asbestiform fibers, and exposed rats differed little from controls whether or not the tremolite was asbestiform.
  • Rat injection studies: mesothelioma incidence was near zero for non-asbestiform tremolite and talc fibers.
• The authors' conclusion: The authors concluded that the weight of evidence supports the view that non-asbestiform amphiboles do not increase the risk of lung cancer or mesothelioma.
• Limitations the authors state:
  • Fiber exposure estimates in both the amosite asbestos and the non-asbestiform grunerite studies may be limited.
  • Comparisons with outside populations generally cannot be adjusted for differences in smoking and lifestyle.
  • The Vermont talc study had few deaths.
  • Cumulative exposures differed greatly between the New York talc cohort and the French and Austrian talc cohorts.
  • The South Carolina vermiculite cohort is too small for confident conclusions about lack of risk.
  • No systematic animal studies have compared cleavage fragments and asbestos fibers of the same lengths.

Topics

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

Medical terms in this study, explained in our Lexicon: Lung Cancer, Mesothelioma, Amphibole Group, Cleavage Fragment, Silicate, Asbestiform, Grunerite, Amosite.

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