Lung fiber burden in the Nottingham gas mask cohort

Our plain-English summary of Lung fiber burden in the Nottingham gas mask cohort, by G. Berry, F. Pooley, A. Gibbs, et al (Inhalation Toxicology, 2009).

The study

• Authors: G. Berry, F. Pooley, A. Gibbs, J.M. Harris, J.C. McDonald
• Journal: Inhalation Toxicology
• Published: 2009;21(2):168-72
• Cited by: 12 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

A cohort of 1,154 employees, mainly women, who had worked 1940-1945 on the manufacture of military gas masks using filter pads containing 20% crocidolite, was traced through 2003, by which time 65 were known to have died from mesothelioma. The last known death with mesothelioma was in 1994, whereas a further 5 cases would have been expected in those with known duration of exposure. Lung tissue samples, from 50 deaths from mesothelioma and 20 other causes, had been analyzed for mineral fiber content. For ten of the mesothelioma cases data on fiber size were collected. Crocidolite fiber concentrations were analyzed in relation to exposure by time and duration. Fiber concentrations overall fell fairly steadily by decade of death, and increased with length of exposure up to 36 months and then fell sharply. The annual rate of elimination estimated by regression was 7.5% corresponding to a half life of 9.2 years. The proportion of fibers longer than 6 mum increased over time implying that the shorter fibers were eliminated more rapidly than the longer ones. The decline in concentrations with time confirms the hypothesis that crocidolite and, by inference, other amphibole fibers are slowly removed from the lung, but since the longer more carcinogenic fibers were cleared more slowly it is unclear to what extent this clearance explains the slowing down of the increase in mesothelioma mortality from about 40 years from the most recent exposure. The exact biostatistical models which most closely conform with the data remain open to question.

In plain English

This study measured asbestos fibers in lung tissue from members of a cohort of 1,154 employees, mainly women, who assembled military gas masks at a Nottingham factory in 1940–1945 using filter pads containing 20% crocidolite. An earlier report had described 65 mesothelioma deaths in the cohort by the end of 2003, the last in 1994. The authors analysed post-mortem lung samples from 50 mesothelioma deaths (40 pleural, 10 peritoneal) and 20 deaths from other causes, and measured fiber sizes in ten mesothelioma cases. Crocidolite counts ranged from 0 to 1,949 fibers per microgram of dried lung. Counts differed by length of exposure (P = 0.047) but did not follow a systematic trend. Counts declined with time since exposure by an estimated 7.5% per year (P = 0.052), a half-life of 9.2 years. The share of fibers longer than 6 micrometres rose with later year of death, which the authors took to mean shorter fibers were removed faster. The authors concluded that it was unclear how far this clearance explains the absence of mesotheliomas after 1994, because the longer, more carcinogenic fibers were cleared more slowly.

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: Analysis of fiber content in post-mortem lung tissue from deaths within an occupational cohort, by electron microscopy, related by regression to length of exposure and time since exposure
• Who was studied: 70 deceased members of a cohort of 1,154 employees, mainly women, of a Nottingham gas mask factory (1940–1945), traced through 2003. The 70 were 50 of the 65 mesothelioma deaths (40 pleural, 10 peritoneal) and 20 deaths from other causes, with deaths from before 1980 up to 1994. Fiber size was measured in 10 mesothelioma cases who died between 1965 and 1988.
• Exposure: Crocidolite, from assembling military gas masks with filter pads containing 20% crocidolite. The period of possible exposure was taken as September 1940 to March 1945.
• Main results:
  • Across all 70 deaths, crocidolite counts ranged from 0 to 1,949 fibers per microgram of dried lung (mean 234, median 47).
  • By length of exposure (51 deaths with known duration), median (geometric mean) counts were: under 12 months (n=17) 14 (22); 12–23 months (n=14) 185 (108); 24–35 months (n=11) 186 (180); 36 months or more (n=9) 66 (61). The differences were statistically significant, but the values did not fit a systematic trend.
  • Median counts for all causes fell by decade of death: 93 before 1980 (n=27), 39 in 1980–89 (n=31) and 15 in 1990–94 (n=12). Neither the differences between decades of death nor those between diagnostic groups were statistically significant.
  • Counts were high for peritoneal mesothelioma (median 154, geometric mean 71, n=10) compared with pleural mesothelioma (median 39, geometric mean 34, n=40), although differences between diagnostic groups were not statistically significant.
  • In 69 deaths, regression on time from the mid-point of exposure to death gave a coefficient of −0.0752 (SE 0.0380, P = 0.052). This corresponds to an annual elimination rate of 7.5% and a half-life of 9.2 years, which the authors took as their best estimate.
  • In the 50 deaths with known start date and length of exposure, lung content was roughly proportional to length of exposure (b = 0.86, SE 0.24, P = 0.0008). After allowing for exposure length, the estimated elimination rate was 4.3% per year (P = 0.25), which was not statistically significant.
  • In the 10 mesothelioma cases with size data (4,537 fibers), 20.5% of fibers were longer than 6 micrometres overall. The figures were 8.5% and 12.1% in the two 1965 deaths and 34.9% and 39.6% in the 1987 and 1988 deaths. The proportion of fibers longer than 6 micrometres increased with year of death (logistic regression coefficient 0.100, SE 0.0051, P < 0.0001).
• The authors' conclusion: The authors stated that the lung results support the view that crocidolite fiber burden decreases over time. They concluded it was unclear how far this explains the absence of mesotheliomas after 1994, because the longer fibers, which are the more carcinogenic, were cleared more slowly.
• Limitations the authors state:
  • Duration of employment was known for only 51 of the 70 deaths.
  • For 19 deaths there was no information on either length of exposure or start date.
  • Interpretation is complicated because fibers break up in the lung, so removal is not a simple matter of fibers going in and coming out.
  • The data come from a relatively small workforce collected over about 70 years, and the authors did not expect them to fit any simple statistical model exactly.

Topics

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

Medical terms in this study, explained in our Lexicon: Fiber Burden, Filter pad, Crocidolite, Mesothelioma, Mineral Fiber, Amphibole Group, Carcinogenic.

Related studies

• Sixty years on: the price of assembling military gas masks in 1940 — J.C. McDonald, J.M. Harris, et al. (2006)
• Cumulative Retrospective Exposure Assessment (REA) as a predictor of amphibole asbestos lung burden: validation procedures and results for industrial hygiene and pathology estimates — J.O. Rasmuson, V.L. Roggli, et al. (2014)
• Chronic inhalation of short asbestos: lung fiber burdens and histopathology for monkeys maintained for 11.5 years after exposure — L.E. Stettler, D.D. Sharpnack, et al. (2008)
• The analysis of asbestos count data with "nondetects": the example of asbestos fiber concentrations in the lungs of brake workers — M.M. Finkelstein (2013)
• Asbestos fiber burden in lung tissues of occupationally exposed workers — D.K. Verma, N.E. Clark (2012)
• Evaluation of the dose-response and fate in the lung and pleura of chrysotile-containing brake dust compared to chrysotile or crocidolite asbestos in a 28-day quantitative inhalation toxicology study — D.M. Bernstein, B. Toth, et al. (2018)

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