The short-term effects of chronic asbestos ingestion in rats

Our plain-English summary of The short-term effects of chronic asbestos ingestion in rats, by R.E. Bolton, J.M. Davis (The Annals of Occupational Hygiene, 1976).

The study

• Authors: R.E. Bolton, J.M. Davis
• Journal: The Annals of Occupational Hygiene
• Published: 1976;19(2):121-8
• Cited by: 16 articles, as counted by NIH iCite
• Read it: PubMed · Publisher
• In J-M's library: Johns-Manville's company medical library held a copy.

In plain English

In this animal experiment, the authors fed male rats margarine containing UICC standard chrysotile, crocidolite or amosite asbestos for up to 1 year. The rats ate on average 250-300 mg of asbestos weekly. Groups had two to four rats per fibre type in each experiment. In clearance tests, most fibres cleared the gut within 48 hours; none appeared in faeces 7 to 28 days after feeding stopped. After 1 year of feeding and 1 month without asbestos, the authors examined ashed guts by electron microscopy. Among 180 preparations they found one possible asbestos fibre, in an amosite-fed rat. With only 0.15% to 4% of residues examined, and assuming fibres were spread through them in a Poisson distribution, they estimated with 90% probability that fewer than 100 amosite, 550 crocidolite or 1500 chrysotile fibres had penetrated a rat's entire intestine. They said these figures reflect sample size, not relative risk. Microscopy showed no cell damage or fibre penetration. The authors concluded that, in rats at least, prolonged asbestos feeding caused no widespread penetration of the gut lining. They noted the single fibre might be contamination.

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 experiment (feeding study in rats) made up of three experiments: short-term gut clearance tests, ashing analysis of whole guts after 1 year of feeding, and microscopic examination of gut tissue at intervals over 1 year
• Who was studied: Male SPF Han rats, 10 weeks old at the start, at the Institute of Occupational Medicine, Edinburgh. The clearance tests used four rats per asbestos type plus two controls. The 1-year ashing analysis used two animals per asbestos type plus controls. The microscopic study used four animals per asbestos type, killed at 2 weeks, 3 months, 6 months and 1 year, with controls examined at each time.
• Exposure: UICC standard reference samples of chrysotile, crocidolite and amosite asbestos, mixed into margarine at 5 mg of asbestos per 1 g of margarine and given ad libitum as a supplement to the normal pelleted diet. Average consumption was 250-300 mg of asbestos per rat per week, which the authors put at 0.27% of the rats' diet. Feeding lasted up to 1 year.
• Main results:
  • Clearance tests, all asbestos types: the vast majority of fibres passed through the gut in less than 48 hours. Only trace levels were detectable at 72 hours, and no asbestos was found in the faeces from 7 to 28 days.
  • Clearance tests: analysis of the whole gut and its contents 28 days after asbestos feeding stopped found no asbestos.
  • In the 1-year ashing experiment, the rats spent 1 month on an asbestos-free diet before being killed and having their gastro-intestinal tracts removed for analysis.
  • Rats fed asbestos for 1 year: only one possible asbestos fibre was found across 180 preparations of ashed gut residue.
  • The single fibre came from one amosite-fed animal. It measured 11.6 by 0.5 micrometres, resembled known amosite, and was accepted as amosite. No fibres resembling crocidolite or chrysotile were found in the animals fed those types.
  • Fraction of residue examined: about 0.4% of the total ashed residue for crocidolite and 4% for amosite.
  • Fraction of residue examined for chrysotile: the 80 preparations made up only 0.15% of the total.
  • The statistical estimate of maximum fibre content rested on the assumption that fibres follow a Poisson distribution throughout the ashed residues.
  • Under that assumption, there was a 90% probability that, after 1 year of feeding, maximum fibre penetration of an entire rat intestine was less than 100 for amosite, 550 for crocidolite and 1500 for chrysotile. The figures for an even chance were 40, 170 and 500 respectively.
  • The authors stated that these estimates reflect differences in sample size, not the relative risks of the three fibre types.
  • Gut tissue examined from 2 weeks to 1 year: light microscopy and transmission electron microscopy showed no signs of cell damage or penetration of the gut lining by asbestos fibres.
  • Occasional breakdown of the microvilli in small-intestine cells was also seen in control animals.
  • The mesenteric lymph nodes showed age-related changes, which were also seen in controls. None of the macrophage inclusions examined contained anything resembling asbestos fibres.
• The authors' conclusion: The authors concluded that, in rats at least, prolonged asbestos feeding does not result in any widespread penetration of the gut lining by asbestos fibres of any size range.
• Limitations the authors state:
  • The authors could not examine all of the ashed residue, so they could not prove that there was no gut penetration at all.
  • No X-ray analysis equipment was available to confirm the identity of fibres seen by scanning electron microscopy.
  • Specimen contamination cannot be eliminated in a laboratory that uses asbestos, so even the single fibre found may not have been in the original gut sample.
  • The acid-washing steps were left out for the chrysotile samples because acid can alter chrysotile. This greatly increased the amount of residue to be examined.
  • The authors assumed that examining the guts of control animals would detect any fibres introduced during dissection.
  • The authors noted that asbestos fibres passing through the gut might still exert a carcinogenic effect without any tissue penetration.

Related studies

• Light and electron microscope studies of the rat digestive tract following prolonged and short-term ingestion of chrysotile asbestos — R. Jacobs, J. Humphrys, et al. (1978)
• Nucleic acid metabolism in the rat following short-term and prolonged ingestion of chrysotile asbestos or cigarette-smoke condensate — R. Jacobs, M. Weinzweig, et al. (1978)
• The pathological effects of prolonged asbestos ingestion in rats — R.E. Bolton, J.M. Davis, et al. (1982)
• Further observations on the short-term retention and clearance of asbestos by rats, using UICC reference samples — A.P. Middleton, S.T. Beckett, et al. (1979)
• A preliminary study of biochemical changes in the rat small intestine following long-term ingestion of chrysotile asbestos — R. Jacobs, K.S. Dodgson, et al. (1977)
• Changes in numbers and dimensions of chrysotile asbestos fibers in lungs of rats following short-term exposure — V.L. Roggli, A.R. Brody (1984)

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