A comparative study of the clearance of respirable para-aramid, chrysotile and glass fibres from rat lungs

Our plain-English summary of A comparative study of the clearance of respirable para-aramid, chrysotile and glass fibres from rat lungs, by A. Searl (The Annals of Occupational Hygiene, 1997).

The study

• Author: A. Searl
• Journal: The Annals of Occupational Hygiene
• Published: 1997;41(2):217-33
• Cited by: 13 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

A number of studies have indicated that the relative toxicity of different fibre types after inhalation is related to their durability within lung tissue. The aim of this study was to assess the relative biopersistence of respirable para-aramid fibrils, chrysotile asbestos and code 100/475 glass fibres in rat lungs. The biopersistence of all three test fibres was measured by quantifying the changes in retained lung burden through time following 10 days inhalation exposure to the same target concentration (700 fibres ml-1) for each fibre type. The lung burden analyses for all three fibre types show large reductions in number and volume of retained fibres during the 16 months following exposure. Most of this reduction in lung fibre burden occurred during the first 3 months following exposure but the pattern of clearance of different size classes varies with fibre type. The para-aramid data show rapid clearance of the longest fibrils during the first months following exposure, combined with an initial increase in the numbers of shorter fibrils. This is consistent with the disintegration of para-aramid fibrils into shorter fragments that subsequently are more readily cleared by macrophages. The code 100/475 glass fibre data also show rapid clearance of the longer fibres combined with an increase in the numbers of very short fibres, consistent with the removal of long fibres through disintegration. In contrast, the chrysotile data show a more rapid reduction in the numbers of retained short fibres than of long fibres, which is consistent with preferential clearance of short fibres by macrophages and minimal transverse breakage of fibres. The biopersistence of all three fibre types, in terms of overall percentage of fibres retained after 16 months is similar, but the durability of long (> 15 microns) chrysotile fibres is very much greater than that of long fibres of code 100/475 glass or para-aramid. The clearance of the three fibre types cannot be adequately described by the first order kinetic model that is often applied in studies of lung clearance. The apparent half time of clearance of each of the fibre types depends on the size fraction and time interval examined.

In plain English

This animal experiment compared how quickly three fibre types were cleared from rat lungs: para-aramid fibrils, chrysotile asbestos, and a special-purpose glass microfibre called code 100/475. Groups of 40 male rats each breathed one fibre type for two weeks at the same target concentration of 700 fibres per millilitre. Groups of five rats were killed at set times up to 16 months later, and the fibres in their lungs were counted and measured by electron microscopy. For all three types, the total number of retained fibres fell sharply, with little of any type remaining after 12 months. The pattern differed by fibre length. Long para-aramid and glass fibres (longer than 15 micrometres) disappeared faster than shorter ones, which the author said was consistent with the fibres breaking into fragments. Long chrysotile fibres were markedly retained, with little further clearance after the first 3 months, while short chrysotile fibres cleared fastest. The author concluded that long chrysotile fibres were considerably more durable than long para-aramid or glass fibres, which appeared similar to each other.

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 measuring retained lung fibre burden over time (biopersistence study)
• Who was studied: Male AF/HAN Wistar laboratory rats, 40 per fibre type, at the Institute of Occupational Medicine, Edinburgh, U.K. Groups of five were killed at 0 and 3 days, 1, 4 and 12 weeks, and 6, 12 and 16 months after exposure.
• Exposure: Whole-body inhalation for 7 hours a day over two working weeks (2 x 5 days), with a target of 700 fibres per ml (fibres longer than 5 micrometres, counted by optical microscopy). The fibres were: para-aramid fine fibrils supplied by DuPont; a long-fibre chrysotile preparation of Canadian origin; and Manville code 100/475 glass microfibre. Measured mean concentrations were 650 (para-aramid), 680 (chrysotile) and 600 (code 100/475) fibres per ml.
• Main results:
  • For all three fibre types, the relative reduction in total lung burden over 16 months was similar, with little of any type remaining after 12 months.
  • Long (longer than 15 micrometres) chrysotile fibres were markedly retained, while long para-aramid and glass fibres disappeared more rapidly than the shorter fibres of those types.
  • Chrysotile: short fibres cleared rapidly until the end of the experiment, but there was little further clearance of long fibres (longer than 15 micrometres) after the first 3 months.
  • Chrysotile, all fibres longer than 0.4 micrometres: the apparent clearance half time was about 10 days in the first few weeks, about 5.5 weeks over the first 6 months, and about 9.2 weeks after 6 months.
  • Para-aramid: in all length categories shorter than 15 micrometres, fibril numbers at first rose above time-zero levels, while the longest fibrils were removed relatively rapidly.
  • Para-aramid, all fibrils longer than 0.4 micrometres: the apparent clearance half time depended on the period examined. It was 4.5 weeks for the first month, 7 weeks between 1 and 6 months, and 9.1 weeks for the remainder of the experiment.
  • Code 100/475 glass: fibres longer than 15 micrometres cleared much faster than shorter fibres in the first 3 months, although only 78 of 9537 glass fibres sized were that long.
  • By the end of the experiment, overall retention of short para-aramid fibres was greater than that of short chrysotile and glass fibres.
• The authors' conclusion: The author concluded that long para-aramid and code 100/475 glass fibres appear to have similar durabilities in lung fluids and appear to be removed through disintegration, whereas long chrysotile fibres are considerably more durable. The author also concluded that clearance of the three fibre types is not adequately described by a first order kinetic model.
• Limitations the authors state:
  • Clearance was poorly described by a first order model, so any half-life estimate is heavily influenced by the choice of time points. The author called for a more sophisticated mathematical description of clearance.
  • Most chrysotile fibres were only 0.1 micrometres thick, so a shift towards thinner fibres from splitting could not be reliably detected. Split fibres too thin to resolve by SEM may have contributed to the apparent early clearance of long chrysotile fibres.
  • The dust clouds were less well matched for short fibres (under 5 micrometres) and long fibres (over 20 micrometres) than for optically counted fibres. The author judged these differences not sufficient to obscure the comparison.
  • Long para-aramid fibres remaining at 12 and 16 months were at the detection limit of the method, so the apparent increase in mean length at 16 months is unlikely to be significant.
  • Further experiments are needed to determine whether fibre type has an important influence on how efficiently macrophages clear fibres.

Topics

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

Medical terms in this study, explained in our Lexicon: Chrysotile, biopersistence, Fibril, Macrophage.

Related studies

• Lung proliferative and clearance responses to inhaled para-aramid RFP in exposed hamsters and rats: comparisons with chrysotile asbestos fibers — D.B. Warheit, S.I. Snajdr, et al. (1997)
• Pulmonary effects in rats inhaling size-separated chrysotile asbestos fibres or p-aramid fibrils: differences in cellular proliferative responses — D.B. Warheit, M.A. Hartsky, et al. (1996)
• A review of inhalation toxicology studies with para-aramid fibrils — D.B. Warheit (1995)
• Cytotoxic effects of aramid fibres on rat pulmonary macrophages: comparison with chrysotile asbestos — J. Dunnigan, D. Nadeau, et al. (1984)
• Comparative hazards of chrysotile asbestos and its substitutes: A European perspective — P.T. Harrison, L.S. Levy, et al. (1999)
• Pathogenicity of a special-purpose glass microfiber (E glass) relative to another glass microfiber and amosite asbestos — R.T. Cullen, A. Searl, et al. (2000)

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