Is Short-Fibered Asbestos Dust a Biological Hazard?

Our plain-English summary of Is Short-Fibered Asbestos Dust a Biological Hazard?, by P. Gross (1974).

The study

• Author: P. Gross
• Published: Archives of Environmental Health, Volume 29. August 1974
• Type of article: Journal article
• Our copy: from Johns-Manville's own medical library. The article is still under copyright, so we describe it here rather than post it.

In plain English

This 1974 commentary by Paul Gross reviewed animal research on whether asbestos fibers shorter than 5 microns cause disease. The author cited studies in which finely ground chrysotile or crocidolite caused only a macrophage reaction or no fibrosis, while longer fibers caused fibrosis or, in hamsters, pleural cancer. In an unpublished study of the author's own, short chrysotile injected into the airways of 10 rats caused no more than a macrophage reaction. The author disputed one laboratory's report of tumors in rats given short chrysotile in the abdomen, citing the high dose and tumors from magnesium hydroxide and fibrous glass. Unpublished data from other laboratories showed no tumors or gastrointestinal lesions in 10 rats fed a 5% chrysotile diet for 21 months. The author concluded this should lead to abandoning the concept that mesotheliomas and gastrointestinal cancers arise from swallowing asbestos dust cleared from the lungs. A published letter cited a 1965 guinea pig study reporting fibrosis from very small particles. The author replied that long fibers were also present in that study.

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• Design: Narrative review and commentary on animal experiments from several laboratories, with brief mention of unpublished studies, followed by a published letter and the author's reply
• Who was studied: Animals in the reviewed experiments (rats, guinea pigs, monkeys, hamsters). This includes the author's own unpublished study of 10 rats and an unpublished feeding study from other laboratories of 10 weanling male rats with 5 pair-fed littermate controls.
• Exposure: Short-fibered asbestos dust, defined as fibers less than 5 microns long (chrysotile, crocidolite, amosite), compared with longer fibers. Routes were injection into the airways, abdomen or pleura, inhalation, and feeding.
• Main results:
  • In a study by Klosterkotter that the author cited, chrysotile and crocidolite ground to an average fiber length of less than 5 microns produced no fibrosis when injected into the airways or abdomen.
  • In a study by Webster that the author cited, monkeys inhaling finely ground crocidolite (fibers under 5 microns) showed only a macrophage reaction in the lungs.
  • In a study by Smith et al that the author cited, hamsters injected into the pleura with finely ground chrysotile developed no pleural cancer, while hamsters injected with longer chrysotile fibers did.
  • In the author's own unpublished study, short-fibered chrysotile injected into the airways of 10 rats caused no more than a macrophage reaction.
  • The author described a contrary report from the Pott laboratory: 100 mg of chrysotile with fibers under 3 microns, injected into the abdomen of rats, caused cancers, nearly 80% of them sarcomas.
  • The author disputed that report. The author noted that the same laboratory listed tumor production of better than 60% with magnesium hydroxide and 55% with fibrous glass, against 40% with chrysotile.
  • In an unpublished feeding experiment from other laboratories, 10 weanling male rats were fed a diet containing 5% chrysotile by weight for 21 months, with 5 pair-fed littermates as controls.
  • At autopsy in that feeding experiment, neither group showed gross abnormality, and no tumor or other gastrointestinal lesion was seen under the microscope.
  • A published letter from George W. Comstock pointed to a 1965 paper by Holt, Mills, and Young reporting that very small asbestos particles caused fibrosis in guinea pig lungs.
  • The author replied that long fibers were also present in the dust used in the Holt study.
• The authors' conclusion: The author concluded that the negative findings for short fibers, together with the negative rat-feeding studies, should lead to abandoning the concept that mesotheliomas and gastrointestinal cancers arise from swallowing asbestos dust cleared from the lungs. The author also wrote that these findings should ease alarm about mineral fibers found in beverages and drinking water. In the body of the article the author put this more narrowly. There the feeding results 'should cast some doubts' on the concept that peritoneal mesotheliomas and increased gastrointestinal cancers in occupationally asbestos-exposed people arise from swallowing fibers cleared from their lungs.
• Limitations the authors state:
  • In the reply to the letter, the author stated that what determines asbestos toxicity is not known, and that the author makes no claim to know it.
  • The author acknowledged that some fibers ground to under 5 microns long are thicker than 0.25 microns and so visible under a light microscope. The author therefore suggested speaking only of 'short' fibers rather than 'submicroscopic' ones.

Topics

Medical terms in this study, explained in our Lexicon: Chrysotile, Crocidolite, Macrophage, Fibrosis, Mesothelioma.

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