Asbestos exposure indices

Our plain-English summary of Asbestos exposure indices, by M. Lippmann (Environmental Research, 1988).

The study

• Author: M. Lippmann
• Journal: Environmental Research
• Published: 1988;46(1):86-106
• Type of article: Review
• Cited by: 131 articles, as counted by NIH iCite
• Read it: PubMed · Publisher
• In J-M's library: Johns-Manville's company medical library held a copy (J-M no. 004419).

Abstract

The ability of inhaled asbestos to produce asbestosis, lung cancer, and mesothelioma in both humans and animals is well established, and asbestos exposures in the occupational and general community environment are recognized as significant hazards. However, it has not been possible to establish realistic and credible dose-response relationships, primarily because of our inability to define which constituents of the aerosols produce or initiate the pathological responses. It is generally acknowledged that the responses are associated with the fibers rather than the nonfibrous silicate mineral of the same chemical composition. Available data from experimental studies in animals exposed by injection and inhalation to fibers of defined size distributions are reviewed, alone with data from studies of fiber distributions in lungs of exposed humans in relation to the effects associated with the retained fibers. It is concluded that asbestosis is most closely related to the surface area of retained fibers, that mesothelioma is most closely associated with numbers of fibers longer than approximately 5 microns and thinner than approximately 0.1 micron, and that lung cancer is most closely associated with fibers longer than approximately 10 microns and thicker than approximately 0.15 micron. The implications of these conclusions on methods for fiber sampling and analyses are discussed.

In plain English

This 1988 article reviewed animal experiments using fibers of known sizes and studies of fibers retained in workers' lungs. It asked which fiber features relate to asbestosis, mesothelioma and lung cancer. The author concluded that the standard optical count of fibers longer than 5 micrometers could not adequately index any of these hazards. In reviewed work by Timbrell and colleagues, fibrosis tracked the surface area of retained amphibole fibers regardless of amphibole type. In Davis and colleagues' rat inhalation study, short amosite produced no malignant cancers in 42 rats, while long amosite produced 3 adenocarcinomas, 4 squamous carcinomas and 1 undifferentiated carcinoma in 40 rats. The author proposed a separate index for each disease. For asbestosis from amphiboles, it was the surface area of fibers 0.15 to 2 micrometers in diameter and longer than about 2 micrometers; the author said this index was less well established for chrysotile. For mesothelioma it was the number of fibers longer than 5 and thinner than 0.1 micrometers. For lung cancer, proposed tentatively, it was the number longer than 10 and thicker than 0.15 micrometers.

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 animal experiments and human lung fiber-burden studies, presented at a scientific meeting in 1987
• Who was studied: No new subjects. The review covered animal studies (rats, guinea pigs, rabbits) exposed by injection, instillation or inhalation to fibers of defined sizes, and studies of fibers retained in the lungs of exposed workers (for example South African amosite and crocidolite mineworkers, Finnish Paakkila anthophyllite workers, and Wittenoom, Australia workers).
• Exposure: Asbestos fibers of different types and sizes: chrysotile, amosite, crocidolite, anthophyllite and tremolite. Erionite and glass fibers were also discussed for comparison. Fibers were characterized by length and diameter.
• Main results:
  • The author judged that the current occupational index (phase-contrast optical count of fibers with an aspect ratio greater than 3 and a length greater than 5 micrometers) could not adequately index asbestosis, mesothelioma or lung cancer.
  • In the reviewed study by Timbrell et al. (1987) of lung samples from amphibole mineworkers, the ability of fibers to produce fibrosis did not depend on amphibole type once it was expressed as the total surface area of long retained fibers per unit weight of lung tissue.
  • For amphiboles, the author proposed that a better index of asbestosis hazard is the total surface area of fibers with diameters between 0.15 and 2 micrometers and lengths greater than about 2 micrometers.
  • Among the Paakkila (Finland) anthophyllite-exposed population, no mesotheliomas were observed despite very high exposures. Timbrell's review of mines worldwide found a good correlation with mesothelioma incidence if the threshold fiber diameter was set at 0.1 micrometer.
  • In the reviewed intraperitoneal injection study in rats (Davis et al., 1986b), short amosite produced 1 mesothelioma among 24 rats, and UICC amosite produced 30 mesotheliomas among 32 rats.
  • In the same injection study, long amosite produced 20 mesotheliomas among 21 rats. From this and Timbrell's findings, the author concluded that the critical fibers for mesothelioma have lengths between 5 and 10 micrometers.
  • In the reviewed rat inhalation study by Davis et al. (1986b), short amosite produced no malignant cancers in 42 rats. Long amosite produced 3 adenocarcinomas, 4 squamous carcinomas and 1 undifferentiated carcinoma in 40 rats.
  • Across the rat inhalation studies reviewed, the author found no clear-cut influence of fiber diameter on tumor yield, but read the results as suggesting that lung cancer incidence increases with both fiber length and diameter.
  • The author noted that animal and human data differ on chrysotile and mesothelioma. Animal data indicate chrysotile produces as much or more mesothelioma than the amphiboles, while human data more often implicate amphiboles, even when exposure was mainly to chrysotile.
  • The author proposed a different critical fiber length for each disease: 2 micrometers for asbestosis, 5 micrometers for mesothelioma, and 10 micrometers for lung cancer.
• The authors' conclusion: The author concluded that each disease relates to a different fiber characteristic. Asbestosis is most closely related to the surface area of retained fibers. Mesothelioma is most closely associated with the number of fibers longer than about 5 micrometers and thinner than about 0.1 micrometer. Lung cancer is most closely associated with fibers longer than about 10 micrometers and thicker than about 0.15 micrometer. The author recommended exposure indices for sampling based on these characteristics.
• Limitations the authors state:
  • The proposed asbestosis index is less well established for chrysotile than for the amphiboles.
  • The calibration of the MALS analysis method is less well established for chrysotile than for the amphiboles.
  • The lung cancer index was offered only as a tentative proposal.
  • The article addressed only which fibers to measure, not what concentration limits should apply to them.

Topics

Medical terms in this study, explained in our Lexicon: Asbestos Exposure, Lung Cancer, Mesothelioma, Dose-response relationship, Silicate.

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