Asbestos fiber length and its relation to disease risk

Our plain-English summary of Asbestos fiber length and its relation to disease risk, by C.A. Barlow, M. Grespin, E.A. Best (Inhalation Toxicology, 2017).

The study

• Authors: C.A. Barlow, M. Grespin, E.A. Best
• Journal: Inhalation Toxicology
• Published: 2017;29(12-14):541-554
• Type of article: Review
• Cited by: 45 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Differences in chemical and crystalline composition, fiber dimension, aerodynamic characteristics and biodurability are among the critical factors that define the toxicological and pathological consequences of asbestos exposure. Specifically, fiber dimension can impact whether the fiber is respired, whether and how deeply it is deposited in the lung, and how efficiently and rapidly it may be cleared. This paper provides a current, comprehensive evaluation of the weight of evidence regarding the relationship between asbestos fiber length and disease potency (for malignant and nonmalignant endpoints). In vitro studies, animal exposure studies and epidemiology data were reviewed. We found that the data reported over the last several decades consistently support the conclusions that exposure to fibers longer than 10 µm and perhaps 20 µm are required to significantly increase the risk of developing asbestos-related disease in humans and that there is very little, if any, risk associated with exposure to fibers shorter than 5 µm. Fiber length as a predictor of potency has been evaluated by several federal agencies in the U.S. and could significantly influence future regulatory decisions for elongated mineral particles (EMPs) and high-aspect ratio nanoparticles (HARNs).

In plain English

This article was a literature review, not a new study. The authors searched databases such as PubMed and ToxNet for studies that directly compared short and long asbestos fibers, in cells, animals and exposed workers. The outcomes were asbestosis, lung cancer and mesothelioma. In the animal studies they summarized, short fibers showed much less fibrosis-causing and cancer-causing activity than long fibers. One rat inhalation study of chrysotile they cited reported fibrosis in 12.6% of rats given long fibers and 2.4% given short fibers. For workers, they cited a 2016 analysis finding no increased disease risk in friction or cement product manufacturing groups, which used shorter chrysotile. Every textile study in that analysis reported increased risk at one or more exposure levels. The authors concluded that fibers longer than 10, and perhaps 20, micrometers are needed to significantly raise human disease risk, and that fibers shorter than 5 micrometers carry very little, if any, risk. They added that fiber length cannot be the only measure of risk. They noted that few, if any, known worker groups had high exposure, as in work with raw fiber, to short fibers only.

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 literature (in vitro, animal and epidemiology studies); no new data collected
• Who was studied: Published studies found through database searches (e.g. PubMed and ToxNet), limited to studies that directly compared short and long asbestos fiber preparations; the number of studies included is not stated
• Exposure: Asbestos fibers of differing lengths, with no restriction by fiber type (the studies reviewed covered chrysotile, amosite, crocidolite, tremolite and anthophyllite), examined for asbestosis, lung cancer and mesothelioma
• Main results:
  • Across the animal studies reviewed, short fibers showed much less fibrosis-causing and cancer-causing activity than long fibers.
  • In a 1988 rat inhalation study of short and long UICC chrysotile (Davis and Jones), as reported by the review, fibrosis occurred in 12.6% of rats given long fibers and 2.4% of rats given short fibers.
  • The authors judged that the cell-culture evidence suggested that toxicity, mutagenicity and proliferative potential increase with fiber length.
  • A 2016 analysis cited by the review (Pierce et al.) reported that no friction or cement product manufacturing cohorts, industries that used shorter chrysotile fibers, showed increased disease risk at any exposure level.
  • In that same 2016 analysis, all of the textile worker cohort studies reported increased risk of disease at one or more exposure levels.
• The authors' conclusion: The authors concluded that exposure to fibers longer than 10 micrometers, and perhaps 20 micrometers, is required to significantly increase the risk of asbestos-related disease in humans, and that there is very little, if any, risk from fibers shorter than 5 micrometers. They also stated that fiber length cannot be the only measure relied on in determining risk.
• Limitations the authors state:
  • Few, if any, human cohorts are known in which workers were exposed only to short fibers in high-exposure situations, such as work with raw fiber.
  • The animal study results are difficult to evaluate because the studies used different protocols (different concentrations, modes of delivery and species).
  • In vitro studies do not mimic human exposure.
  • Measuring fibers left in human lung tissue is a poor index of dose and fiber characteristics.

Topics

Medical terms in this study, explained in our Lexicon: Fiber Length, Crystalline, Fiber Dimensions, Asbestos Exposure, Epidemiology, Aspect Ratio (Length-to-diameter ratio).

Related studies

• The association among ferruginous body, uncoated fibers, asbestos and non-asbestos fibers in lung tissue in terms of length — T. Suzuki, Y. Sakakibara, et al. (2016)
• The importance of fiber biopersistence and lung dose in determining the chronic inhalation effects of X607, RCF1, and chrysotile asbestos in rats — T.W. Hesterberg, G.A. Hart, et al. (1998)
• Long and short fiber amosite asbestos alters at a different extent the redox metabolism in human lung epithelial cells — C. Riganti, E. Aldieri, et al. (2003)
• Standardized methods for preparation and bi-variate length & diameter counting/sizing of aerosol and tissue digestion fiber samples — D.M. Bernstein, P. Kunzendorf (2018)
• Man-made mineral fibers and interstitial lung diseases — E. Fireman (2014)
• Silica, asbestos, man-made mineral fibers, and cancer — K. Steenland, L. Stayner (1997)

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