Further observations on the short-term retention and clearance of asbestos by rats, using UICC reference samples

Our plain-English summary of Further observations on the short-term retention and clearance of asbestos by rats, using UICC reference samples, by A.P. Middleton, S.T. Beckett, J.M. Davis (The Annals of Occupational Hygiene, 1979).

The study

• Authors: A.P. Middleton, S.T. Beckett, J.M. Davis
• Journal: The Annals of Occupational Hygiene
• Published: 1979;22(2):141-52
• Cited by: 28 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Rats exposed to UICC chrysotile 'A', amosite and crocidolite at concentrations close to 1, 5 and 10 mg/m3 of respirable fibre for a total of 210 h over a 6-week period were killed in five groups at approximately monthly intervals. The asbestos retained in the lungs was estimated by infra-red spectrophotometry after ashing. The results indicated different initial fractional retentions but that the rate of clearance was independent of the type of asbestos and of the concentration during exposure. The rates of deposition and clearance could be described in terms of a model, modified from that published by MORGAN et al. (1977). In this model it is assumed that a fraction K, of the estimated aggregate dose is deposited in three 'compartments', from which asbestos is subsequently cleared with half-times of 0.38, 8 and 118 days. It is also assumed that asbestos enters the three 'compartments' in the proportions 86.5:5.3:8.2, respectively. The fractional deposition, K, of chrysotile was much less than for the two amphiboles but increased with diminishing concentration, from 0.17 at ca. 10 mg/m3 to 0.36 at ca. 1 mg/m3. The K-value for amosite, 0.65, was less than for crocidolite, 1.0, both being independent of concentration. In view of the nocturnal nature of rats it was thought that the normal practice of dust exposure during daylight, a period of reduced activity, might have led to misleading results. Two further experiments were therefore carried out in which rats were exposed to UICC chrysotile 'A' and crocidolite in a darkened chamber to simulate nocturnal conditions. The rates of clearance were closely similar to those observed after daylight exposure. Application of the model described above indicated that, although the fraction of crocidolite deposited was comparable with that observed after daylight exposure, the fraction of chrysotile deposited was approximately three times that deposited during the comparable daylight experiment. Possible reasons for the different retention pattern of chrysotile are discussed.

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In plain English

This animal experiment, from the Institute of Occupational Medicine in Edinburgh, measured how much inhaled asbestos stayed in rats' lungs and how fast it cleared. Rats breathed UICC reference samples of chrysotile 'A', amosite or crocidolite at target concentrations of 1, 5 and 10 mg/m3 of respirable fibre for 210 hours over 6 weeks. Groups of 7-10 rats were killed at roughly monthly intervals, starting 5 days after exposure ended, and the asbestos left in their lungs was measured. The rate of clearance did not appear to differ between the three asbestos types. Using a model adapted from earlier published work, the authors estimated the fraction of the dose deposited, called K. It was about 1 for crocidolite and 0.65 for amosite. For chrysotile it was about 0.17 to 0.36, with lower values at higher concentrations. When rats were exposed to chrysotile and crocidolite in the dark, when rats are more active, clearance rates were similar to daylight exposure. Chrysotile retention, however, was about three times higher. The authors suggested that the curly shape of chrysotile fibres, and filtering by the rats' fur, may explain the lower chrysotile retention.

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: Animal inhalation experiment in rats, with groups killed at intervals after exposure to measure lung retention and clearance
• Who was studied: Rats at the Institute of Occupational Medicine, Edinburgh. For each experiment, rats were killed in five groups of 7-10 animals at approximately monthly intervals, the first group 5 days after exposure ended. The total number of rats, the strain and the study dates are not given.
• Exposure: UICC standard reference samples of chrysotile 'A', amosite and crocidolite, each at target concentrations of 1, 5 and 10 mg/m3 of respirable fibre. Exposure lasted 210 hours: 7 hours a day, 5 days a week, for 6 weeks. In a separate 'nocturnal-exposure' experiment, chrysotile 'A' and crocidolite were given at a target of 10 mg/m3 in a chamber blacked out during the day. Asbestos in the lungs was measured by infra-red spectrophotometry after ashing.
• Main results:
  • The rate of clearance of asbestos from the rats' lungs was virtually the same for chrysotile, amosite and crocidolite.
  • Retention of chrysotile in the lungs was markedly lower than retention of amosite or crocidolite, particularly at the two higher concentrations. This difference held throughout the observed clearance period.
  • Crocidolite lung burdens matched the model with full deposition (K = 1). Amosite was better predicted with K = 0.65.
  • For chrysotile in daylight exposure, K was about 0.17 at the highest concentration, about 0.19 at the intermediate concentration and about 0.36 at the lowest concentration.
  • In the nocturnal-exposure experiment, exposure in the dark had little influence on clearance rates of either chrysotile or crocidolite. Crocidolite still fitted K = 1. Chrysotile retention was approximately three times that seen with daylight exposure, requiring K = 0.5, compared with 0.17 in the corresponding daylight experiment.
  • The authors fitted the slowest clearance phase using the last three killing times, pooling all three asbestos types because no difference was apparent. This gave a value of 172 days, close to the 170 days used in the earlier model by Morgan and colleagues.
  • Gas analyses suggested that rats in darkness consumed 1.25 times as much oxygen as during inactive daylight periods.
• The authors' conclusion: The authors concluded that the rate of clearance of asbestos from rats' lungs was largely independent of fibre type and dust concentration. The apparent deposition of chrysotile was approximately one quarter that of amosite and crocidolite, and was relatively lower at higher airborne concentrations. They suggested this may be peculiar to chrysotile because its fibres are curly. They also suggested that during daylight exposure, filtration by the rats' own and each other's fur may reduce the chrysotile deposited.
• Limitations the authors state:
  • The modified model did not account for each day's exposure lasting 7 hours, or for day-to-day changes in chamber concentration. Estimates soon after exposure are therefore at best approximate.
  • No measurements were made while much dust remained in the fast-clearance compartment. The study therefore could not directly distinguish lower initial deposition of chrysotile from differences in fast-phase deposition, and the fast clearance phase of the earlier model could not be checked.
  • The target exposure concentrations were not achieved exactly.
  • Muffle-ashing broke down some crocidolite. A correction was applied on the assumption that all amorphous silica in the residue came from crocidolite.
  • The nocturnal-exposure experiment was described as limited.
  • The authors stated that the clearance model from these short-term experiments does not apply to long-term (1 year) inhalation experiments.

Topics

Medical terms in this study, explained in our Lexicon: UICC, Chrysotile, Amosite, Crocidolite, Deposition, Amphibole Group.

Related studies

• An overload hypothesis for pulmonary clearance of UICC amosite fibres inhaled by rats — R.E. Bolton, J.H. Vincent, et al. (1983)
• Clearance and dimensional changes of crocidolite asbestos fibers isolated from lungs of rats following short-term exposure — V.L. Roggli, M.H. George, et al. (1987)
• UICC standard reference samples of asbestos — V. Timbrell, J.C. Gibson, et al. (1968)
• Development of the Regular Interlaboratory Counting Exchanges (RICE) scheme to include visual reference counts and samples from asbestos clearance operations — P.W. Brown, N.P. Crawford, et al. (1994)
• The short-term effects of chronic asbestos ingestion in rats — R.E. Bolton, J.M. Davis (1976)
• Rapid short-term clearance of chrysotile compared with amosite asbestos in the guinea pig — A. Churg, J.L. Wright, et al. (1989)

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