Lung Cancer Risk Associated with Regulated and Unregulated Chrysotile Asbestos Fibers

Our plain-English summary of Lung Cancer Risk Associated with Regulated and Unregulated Chrysotile Asbestos Fibers, by G.B. Hamra, D.B. Richardson, J. Dement, et al (Epidemiology, 2017).

The study

• Authors: G.B. Hamra, D.B. Richardson, J. Dement, D. Loomis
• Journal: Epidemiology
• Published: 2017;28(2):275-280
• Cited by: 13 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Background: Regulation of asbestos fibers in the workplace is partly determined by which fibers can be visually counted. However, a majority of fibers are too short and thin to count this way and are, consequently, not subject to regulation.

Methods: We estimate lung cancer risk associated with asbestos fibers of varying length and width. We apply an order-constrained prior both to leverage external information from toxicological studies of asbestos health effects. This prior assumes that risk from asbestos fibers increases with increasing length and decreases with increasing width.

Results: When we apply a shared mean for the effect of all asbestos fiber exposure groups, the rate ratios for each fiber group per unit exposure appear mostly equal. Rate ratio estimates for fibers of diameter <0.25 μm and length <1.5 and 1.5-5.0 μm are the most precise. When applying an order-constrained prior, we find that estimates of lung cancer rate ratio per unit of exposure to unregulated fibers 20-40 and >40 μm in the thinnest fiber group are similar in magnitude to estimates of risk associated with long fibers in the regulated fraction of airborne asbestos fibers. Rate ratio estimates for longer fibers are larger than those for shorter fibers, but thicker and thinner fibers do not differ as the toxicologically derived prior had expected.

Conclusion: Credible intervals for fiber size-specific risk estimates overlap; thus, we cannot conclude that there are substantial differences in effect by fiber size. Nonetheless, our results suggest that some unregulated asbestos fibers may be associated with increased incidence of lung cancer.

In plain English

The authors studied whether the size of asbestos fibers affected lung cancer risk in 6,136 textile workers from one South Carolina plant and three North Carolina plants. The vast majority of asbestos used in these plants was chrysotile. There were 361 lung cancer deaths. Using electron microscope analysis of historical dust samples, the authors estimated each worker's cumulative exposure to fibers in 21 width and length groups. These groups included short or thin fibers that standard counting misses, which are unregulated. Fibers under 0.25 micrometers wide and under 5.0 micrometers long made up about 79% of all fibers. In a model letting the size groups share a common average, estimates were mostly equal. In a model assuming, from animal studies, that longer and thinner fibers are more harmful, risk rose with length but not with thinness. In that model, per 100 fiber-years/ml, the rate ratio for the thinnest fibers over 40 micrometers long was 1.15 (credible interval 1.04 to 1.45). Because the intervals overlapped, the authors could not conclude that effects differed substantially by size. They did conclude that some unregulated fibers are likely carcinogenic. They noted that the fiber size groups were highly correlated.

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: Nested case-control study within a pooled cohort of asbestos textile workers, with 10 age-matched controls per lung cancer case, analysed with two Bayesian regression models: one with a shared mean across fiber size groups, and one with order constraints based on toxicology
• Who was studied: 6,136 asbestos textile workers (3,717 male). South Carolina, one plant: 1,791 men and 1,263 women. North Carolina, three plants: 1,926 men and 1,156 women. Vital status was followed through 2001 in South Carolina and through 2003 in North Carolina. 361 lung cancer deaths (202 in South Carolina, 159 in North Carolina) over 218,631 person-years.
• Exposure: Mainly raw chrysotile. Small amounts of crocidolite yarn were used in the South Carolina plant (1950–1975), and some amosite was used in one North Carolina plant (1963–1976). Exposure by fiber size came from transmission electron microscopy of 160 historical dust samples from 1964–1971, applied to standard fiber-count estimates. This gave cumulative exposure in fiber-years/ml, lagged 10 years, for 21 groups formed from four width and six length categories.
• Main results:
  • Short, thin fibers (under 0.25 micrometers in diameter and under 5.0 micrometers in length), which are unregulated, made up about 79% of all fibers workers were exposed to. The smallest group (under 0.25 micrometers wide and under 1.5 micrometers long) alone made up 54%.
  • In the shared-mean model, the rate ratios for all fiber size groups appeared equal in magnitude. The shared mean rate ratio estimated from the data was 1.02.
  • In the order-constrained model, lung cancer risk increased with increasing fiber length but did not increase with decreasing diameter, contrary to the prior assumption.
  • In both models, the most precise estimates were for fibers under 0.25 micrometers in diameter and under 1.5 or 1.5 to 5.0 micrometers in length. These estimates were at or near the null.
  • In the order-constrained model, per 100 f-yr/ml, the rate ratio for the thinnest fibers (under 0.25 micrometers) was 1.08 (credible interval 1.02, 1.24) at 20–40 micrometers long. It was 1.15 (1.04, 1.45) at over 40 micrometers long.
  • The largest estimates were for the thickest fibers (over 3.0 micrometers). For example, the order-constrained rate ratio was 1.40 (1.14, 1.93) for those over 40 micrometers long. The largest estimates were also the least precise.
• The authors' conclusion: The authors concluded that, because the credible intervals overlapped, they could not firmly conclude that effects differ substantially by fiber size. They also concluded that their results suggest some unregulated asbestos fibers are likely carcinogenic.
• Limitations the authors state:
  • The fiber size groups were highly correlated, some nearly perfectly. The authors believe this limits the ability to consider the effects of any one size group outside a controlled experimental setting.
  • The order-constrained model relies on animal studies, whose findings are uncertain when applied to humans.
  • Workers' overall lung cancer risk results from all inhaled fibers acting together, so the size-specific estimates are an attempt to disaggregate a combined effect.
  • Fibers over 3.0 micrometers in diameter, which had the higher estimates in the constrained model, were rare in the airborne dust. They would also have a lower probability of being deposited at critical sites in the lung.

Topics

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

Medical terms in this study, explained in our Lexicon: Lung Cancer, Chrysotile, rate ratio.

Related studies

• Lung cancer mortality in North Carolina and South Carolina chrysotile asbestos textile workers — L. Elliott, D. Loomis, et al. (2012)
• Asbestos standards: Impact of currently uncounted chrysotile asbestos fibers on lifetime lung cancer risk — D.B. Richardson, A.P. Keil, et al. (2018)
• Increased lung cancer mortality among chrysotile asbestos textile workers is more strongly associated with exposure to long thin fibres — D. Loomis, J.M. Dement, et al. (2012)
• An epidemiological study of the role of chrysotile asbestos fibre dimensions in determining respiratory disease risk in exposed workers — L. Stayner, E. Kuempel, et al. (2008)
• Lung cancer mortality and fibre exposures among North Carolina asbestos textile workers — D. Loomis, J.M. Dement, et al. (2009)
• Increased phosphorylated extracellular signal-regulated kinase immunoreactivity associated with proliferative and morphologic lung alterations after chrysotile asbestos inhalation in mice — R.F. Robledo, S.A. Buder-Hoffmann, et al. (2000)

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