Deposition and clearance of chrysotile asbestos

Our plain-English summary of Deposition and clearance of chrysotile asbestos, by A. Churg (The Annals of Occupational Hygiene, 1994).

The study

• Author: A. Churg
• Journal: The Annals of Occupational Hygiene
• Published: 1994;38(4):625-33, 424-5
• Cited by: 58 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Studies of human lungs indicate that, for virtually all types of exposure, the relative proportion of amphibole asbestos retained in the lung far exceeds the proportion in the original dust and, conversely, the relative proportion of chrysotile is far less than that in the original dust. Although amphiboles appear to accumulate in lung in proportion to exposure and chrysotile does not, failure of chrysotile deposition is probably not the reason for the disproportionate retention of amphibole fibres. The available data suggest that chrysotile is deposited in the parenchyma but is cleared extremely rapidly, with the vast bulk of fibres removed from human lungs within weeks to months after inhalation; by comparison, amphibole clearance half-lives are of the order of years to decades. The mechanisms of preferential chrysotile clearance remain uncertain, but fragmentation of chrysotile into short fibres, possibly accompanied by extremely rapid dissolution of such fibres, appears to be important in this process. Chrysotile fibres do penetrate to the periphery of the lung, so that differences in mesothelial pathogenicity of chrysotile and amphiboles in regard to mesothelioma are not caused by failure of chrysotile to reach the pleura. The theory that the tremolite contaminant rather than the chrysotile itself is the cause of 'chrysotile-induced' disease (especially mesothelioma) is consistent with the available human data, but the contrary ideas that disease is caused either by the total transient burden of inhaled chrysotile fibres or by a small, sequestered, long-retained fraction of chrysotile fibres still need to be excluded.

In plain English

This 1993 workshop paper reviewed largely human data on how chrysotile and amphibole asbestos are deposited in and cleared from the lung. The author reported that human lungs almost always retained amphiboles preferentially. In chrysotile miners and millers from Thetford Mines, Quebec, tremolite made up roughly 75% of fibres in a typical autopsy lung but only a few per cent of the ore. In 94 such workers, only tremolite increased significantly with exposure years. Chrysotile did reach the subpleural lung. In guinea pigs given an 8:1 chrysotile:amosite mixture, the lung ratio fell to about 2:1 within 1 month. The author proposed that this preferential clearance is substantially complete within weeks to months, with a small residual burden possibly clearing slowly. The author cited Oberdorster's estimated 90-100-day half-time and judged fragmentation into short fibres, perhaps with rapid chemical breakdown, the most likely mechanism. The author's group earlier found no correlation between chrysotile and any disease in Thetford workers after adjusting for tremolite. The author concluded that the tremolite theory of 'chrysotile-induced' disease was not inconsistent with the data but needed lung studies of putatively pure chrysotile 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: Narrative review of published (largely human) data, presented as a workshop paper; includes the author's own earlier human and guinea pig studies
• Who was studied: No single study population. The review drew on human lung studies of the general population, of workers in industries using predominantly chrysotile, and of workers with mixed amphibole and chrysotile exposure, together with animal studies. It also included data on 94 chrysotile miners and millers from Thetford Mines, Quebec, analysed in the author's laboratory.
• Exposure: Chrysotile asbestos and its tremolite contaminant, compared with the amphiboles amosite and crocidolite
• Main results:
  • Across human lung studies, amphiboles were preferentially retained compared with chrysotile, whatever the nominal exposure.
  • In Thetford chrysotile miners and millers, tremolite made up roughly 75% of the fibre burden (by number) in a typical autopsy lung, although it comprises only a few per cent of the ore.
  • In a summary of 8 reports on workers in industries using predominantly chrysotile, chrysotile averaged 47% (range 19-80%) and tremolite 46% (range 10-81%) of total lung fibres.
  • In 94 chrysotile miners and millers, tremolite fibre concentration increased with years of exposure. Chrysotile showed no significant increase.
  • In chrysotile miners and millers, the author's group found no consistent differences in chrysotile fibre concentration or fibre size between central and subpleural lung tissue.
  • In guinea pigs given an amosite and chrysotile mixture by intratracheal instillation, the chrysotile:amosite ratio in lung tissue fell over 1 month from the original 8:1 to about 2:1.
  • As one possible explanation, the author proposed that preferential chrysotile clearance is substantially complete in weeks to months. After that, only slow clearance of a small burden of residual fibres, possibly from a sequestered compartment, would occur.
  • The review cited Oberdorster's (1994) estimate, calculated from various published data, that chrysotile clearance half-times are on the order of 90-100 days. By contrast, amosite and crocidolite half-lives were measured in years to decades.
  • In a cited study of nearly 2000 fibres from workers, there was only 20% magnesium leaching even after 20 years. The author judged that magnesium leaching alone is unlikely to play a significant role in chrysotile clearance.
  • The author judged that fragmentation into short fibres, and perhaps rapid chemical degradation of such short fibres, appear to be the most likely mechanism(s) of preferential chrysotile clearance. The author noted that virtually no human data allow direct examination of this theory.
  • In the author's group's 1993 study of Thetford chrysotile miners and millers, high tremolite burdens were associated with disease, including mesothelioma and airway fibrosis. After adjustment for tremolite, chrysotile concentration was not correlated with any disease.
• The authors' conclusion: The author concluded that the theory that tremolite is the real agent of chrysotile-induced disease, particularly mesothelioma, is not inconsistent with the available data. It would be on firmer ground if lung fibre burden analyses were done on cohorts with putative pure chrysotile exposure and known disease rates. The author also judged that failure to reach the pleura does not explain chrysotile's weak mesothelial carcinogenicity. Fragmentation into short fibres, perhaps with rapid chemical degradation, was judged the most likely clearance mechanism.
• Limitations the authors state:
  • The human data on chrysotile clearance are fragmentary.
  • Most cohorts have little exact information on the proportions of amphibole and chrysotile in the inhaled dust.
  • Virtually no human data allow direct examination of the fragmentation theory.
  • In most animal studies showing disease from chrysotile, the tremolite content of the dust or lungs was not established.

Topics

The National Library of Medicine indexes this study under subjects our site covers: Amosite, Chrysotile, Crocidolite, Tremolite.

Medical terms in this study, explained in our Lexicon: Deposition, Chrysotile, Amphibole Asbestos, Amphibole Group, Parenchyma, Mesothelioma, Pleura, Tremolite.

Related studies

• Inferences on the kinetics of asbestos deposition and clearance among chrysotile miners and millers — M.M. Finkelstein, A. Dufresne (1999)
• Rapid short-term clearance of chrysotile compared with amosite asbestos in the guinea pig — A. Churg, J.L. Wright, et al. (1989)
• Deposition, clearance, and translocation of chrysotile asbestos from peripheral and central regions of the rat lung — P.G. Coin, V.L. Roggli, et al. (1992)
• Pulmonary mineral fibers after occupational and environmental exposure to asbestos in the Russian chrysotile industry — A. Tossavainen, E. Kovalevsky, et al. (2000)
• Deposition pattern and clearance pathways of inhaled chrysotile asbestos — A.R. Brody, L.H. Hill (1981)
• Evaluation of the deposition, translocation and pathological response of brake dust with and without added chrysotile in comparison to crocidolite asbestos following short-term inhalation: interim results — D.M. Bernstein, R. Rogers, et al. (2014)

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