An estimate of the rate at which crocidolite asbestos fibres are cleared from the lung

Our plain-English summary of An estimate of the rate at which crocidolite asbestos fibres are cleared from the lung, by R.S. Du Toit (The Annals of Occupational Hygiene, 1991).

The study

• Author: R.S. Du Toit
• Journal: The Annals of Occupational Hygiene
• Published: 1991;35(4):433-8
• Cited by: 10 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

The cumulative fibre exposure of 36 deceased persons who had been employed on crocidolite asbestos mines of the NW Cape was compared with their lung burden determined by fibre count by optical microscope. Allowance for the period which lapsed between date of last exposure and death revealed a lung clearance rate amounting to a half-life of about 6 years.

In plain English

This short communication estimated how quickly crocidolite asbestos fibres are cleared from the human lung. The author studied the lungs of 36 deceased people who had worked on NW Cape crocidolite asbestos mines; their lungs had been sent for examination under a Government requirement. Each person's cumulative exposure, in fibre-years, was estimated from employment records, job dustiness weightings and average recorded fibre levels for all Cape crocidolite mines. Lung samples were digested and ashed, and fibres in the resulting preparation were counted by light microscope, expressed per gram of dry lung. Each lung burden was divided by the person's estimated exposure and multiplied by 100 to give an index. This index fell as the time between last exposure and death increased (r2 = 0.42, P = 0.0001). The author interpreted this as clearance with a half-life of about 6 years. On average, 52.9% of fibres were sub-microscopic. The author noted that uncertainties and assumptions about past fibre levels gave wide limits to individual exposure estimates, especially for work before 1940. The author also said the main value lay in the trend rather than actual fibre numbers.

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: Short communication: estimated cumulative fibre exposure compared with light-microscope lung fibre counts in deceased former mine workers, analysed against time since last exposure
• Who was studied: 36 deceased persons who had worked only on NW Cape crocidolite asbestos mines (South Africa) and had a satisfactory employment record. Their lungs were examined at the National Centre for Occupational Health under a routine Government mandate. Exposure periods ran from 1914 to 1983, with a median start of about 1943 and a median end of about 1965.
• Exposure: Crocidolite asbestos mining. Cumulative fibre dose (fibre per ml times years) was computed from service records. Occupations were grouped into 17 surface and 16 underground categories, each weighted for dustiness. Each mine's mean dust level was assumed to equal the mean recorded for all Cape crocidolite mines in the same period.
• Main results:
  • In the 36 cases, the lung burden index decreased as the years between last exposure and death increased. The index is lung burden in millions of fibres per gram of dry lung, divided by fibre-years of exposure, multiplied by 100. On a log scale the decline was a straight line with a slope of minus 0.1194 per year; r2 = 0.42, P = 0.0001.
  • The decline corresponded to a lung clearance half-life of about 6 years.
  • The percentage of sub-microscopic fibres had an arithmetic mean of 52.9% (SD 19.6%).
  • From the fitted line, the author estimated that exposure to 10 fibres per ml for 1 year would give a lung burden of 4.3 million fibres per gram of dry lung. Exposure at that level for 20 years would give 86 million fibres per gram.
  • The author estimated that a lung burden would be expected to halve when 6 years had passed between last exposure and death.
• The authors' conclusion: The author concluded that the index decreases with longer time between end of exposure and death, meaning that fibre clearance takes place, at a rate amounting to a half-life of about 6 years.
• Limitations the authors state:
  • Numerous uncertainties and assumptions about the true fibre concentrations the cases were exposed to gave wide confidence limits to individual exposure estimates, especially for those exposed for substantial periods before 1940.
  • The author considered the main value to lie in the observed trend rather than in the actual number of fibres.
  • Each mine's dust concentration was assumed to equal the average for all Cape crocidolite mines in the same period, rather than being measured for that mine.

Topics

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

Medical terms in this study, explained in our Lexicon: Crocidolite.

Related studies

• Mass transfer rates of polycyclic aromatic hydrocarbons between micron-size particles and their environment--theoretical estimates — P. Gerde, P. Scholander (1989)
• Thickening of pulmonary interlobar fissures: exposure-response relationship in crocidolite and amosite miners — A. Solomon, L.M. Irwig, et al. (1979)
• Prevalence of radiographic asbestosis in crocidolite miners and millers at Wittenoom, Western Australia — W.O. Cookson, N.H. De Klerk, et al. (1986)
• Gallium lung scanning and bronchoalveolar lavage in crocidolite-exposed workers — A.A. Hayes, B. Mullan, et al. (1989)
• Response of mouse lung to crocidolite asbestos. 2. Pulmonary fibrosis after long fibres — I.Y. Adamson, D.H. Bowden (1987)
• Response of mouse lung to crocidolite asbestos. 1. Minimal fibrotic reaction to short fibres — I.Y. Adamson, D.H. Bowden (1987)

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