Ingested mineral fibers. Do they penetrate tissue or cause cancer?

Our plain-English summary of Ingested mineral fibers. Do they penetrate tissue or cause cancer?, by P. Gross, R.A. Harley, L.M. Swinburne, et al (Archives of Environmental Health, 1974).

The study

• Authors: P. Gross, R.A. Harley, L.M. Swinburne, J.M. Davis, W.B. Greene
• Journal: Archives of Environmental Health
• Published: 1974;29(6):341-7
• Cited by: 54 articles, as counted by NIH iCite
• Read it: PubMed · Publisher
• In J-M's library: Johns-Manville's company medical library held a copy.

Abstract

Three different laboratories independently investigated the ability of ingested mineral fibers to penetrate tissues in rats. All concluded that there was no evidence of tissue penetration by ingested mineral fibers. These experimental observations are supported by the findings in coal and hard-rock miners who swallow, during their lifetime, nearly 100 times the amount of dust that is stored in their lungs. The intestinal wall or mesenteric lymph nodes of these people show no evidence of storage of the ingested dust particles. Animals fed asbestos over much of their lifetime and allowed to live to the age of cancer production, failed to provide evidence of a cancerogenic effect

PubMed has no abstract for this study. This is the authors' abstract, copied from the article.

In plain English

This 1974 paper pooled rat experiments from three laboratories on whether swallowed mineral fibers cross the gut wall or cause cancer. In the first laboratory, ten rats ate chow containing 5% chrysotile for 21 months without gut lesions. Others got a single 400 mg dose of amosite or taconite tailings by stomach tube, or ate either dust in oleomargarine for six days. Some fibers appeared outside the gut, including nine in five of 37 dust-treated rats. Controls also had fibers. The authors judged these findings probably contamination, except in one gavaged rat whose torn esophagus they linked to fiber spread. In the second laboratory, rats were fed chrysotile or crocidolite in butter weekly for 16 or 18 weeks. Apart from a few killed within six months, they lived until illness or natural death. None developed gut or mesothelial tumors. Breast cancers and lymphoma occurred in an asbestos-fed group and in controls. An unfinished third-laboratory study saw no fiber penetration at one and three months. The authors concluded there was no evidence that ingested fibers penetrate tissue. They acknowledged possible false positives from contamination and possible false negatives.

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 experiments (rat feeding and gavage studies) from three laboratories, pooled into one joint report
• Who was studied: Rats. First laboratory: 10 male rats fed chrysotile, with 5 controls; 10 amosite-gavaged, 10 taconite-gavaged and 5 control rats; 10 rats per mineral fed in oleomargarine, with 10 controls. Second laboratory: SPF Wistar rats about 12 weeks old. A small preliminary group came first. Experiment 2 used groups of 31, 33 and 34 rats with 24 controls, and experiment 3 used groups of 35 and 28 rats with 24 controls. Third laboratory: two groups of 12 rats. The article does not say where each laboratory's experiments were done or when most of the work took place; part of it was begun nine years before publication.
• Exposure: First laboratory: ball-milled chrysotile at 5% by weight of chow for 21 months. Amosite or taconite mine tailings as a single 400 mg gavage dose. Amosite (10%) or taconite tailings (20%) in oleomargarine for six days, with consumption per rat of 2.2 to 5 gm amosite and 6.0 to 10 gm taconite. Second laboratory: crocidolite as a single 20 or 40 mg dose. Then, once weekly in butter, either 10 mg Rhodesian chrysotile or 5 or 10 mg crocidolite for 16 weeks, or 10 mg North West Cape or Transvaal crocidolite for 18 weeks. Third laboratory: chrysotile or crocidolite at 5 mg/gm of butter, fed ad libitum.
• Main results:
  • Rats fed chrysotile for 21 months (first laboratory) all survived and had no gross lesions in the GI tract. The authors summarized that this feeding failed to produce cancer or any other kind of lesion.
  • In the chrysotile-fed rats, electron microscopy of intestinal sections found no fibers. Digests of mesenteric tissue contained fibers in three of the ten rats (one chrysotile, two non-chrysotile). A mixed organ digest from a fourth rat contained two non-chrysotile fibers. Two control rats also had chrysotile fibers in GI tract digests.
  • After gavage, fibers outside the GI tract were found in one of eight amosite rats examined. That rat had a lacerated and perforated esophagus, and amosite had spread into the surrounding muscles. The authors likened the spread in that rat to an injection technique known to tear tissue.
  • Fibers outside the GI tract were also found in one of ten taconite-gavaged rats: three fibers in lung tissue. Two control rats in the gavage series also had fibers in mesentery or kidney digests.
  • Across rats fed or gavaged with amosite or taconite tailings, tissue digests from five of 37 rats contained nine fibers. The authors judged these very probably contaminants.
  • In the second laboratory's long-term butter-feeding experiments, one asbestos-fed group had two breast cancers and one lymphoma of axillary nodes. The control group had three breast cancers, one lymphoma and one sarcoma of the thigh. No mesothelioma or GI tumor occurred, and no lesions were seen that could be related to the ingested asbestos.
  • In the second laboratory, a few animals from each group were killed in the first six months. The rest were kept until they became ill or died a natural death, and no tumors arose in the GI tract or mesothelium.
  • In the third laboratory, light microscopy of rats killed one and three months after asbestos feeding began showed no fibers that had penetrated tissue.
• The authors' conclusion: The authors concluded that all three laboratories found no evidence of tissue penetration by ingested mineral fibers.
• Limitations the authors state:
  • In the chrysotile study, GI tract tissue was not kept apart from other tissues during storage, so other tissues could have been contaminated with fibers.
  • Some rats struggled during gavage, causing trauma that could introduce fibers into tissues.
  • The tissue-digestion method could give false-positive results from contamination.
  • Unfiltered water and reagents may have been a source of contamination.
  • False-negative results were also possible, for example from fibers lost during digestion or centrifugation.
  • The third laboratory's study was incomplete; electron microscopy for the three-month animals was not finished.

Topics

Medical terms in this study, explained in our Lexicon: Mineral Fiber.

Related studies

• Quantitative assessment of chrysotile fibrils in the bloodstream of rats which have ingested the mineral under different dietary conditions — M. Weinzweig, R.J. Richards (1983)
• Identification of a particular amphibole asbestos fiber in tissues of persons exposed to a high oral intake of the mineral — R.E. Carter, W.F. Taylor (1980)
• Accumulation of ingested asbestos fibers in rat tissues over time — K. Patel-Mandlik, J. Millette (1983)
• On the mechanism of cogenotoxic action between ingested amphibole asbestos fibres and benzo[a]pyrene: II. Tissue specificity studies using comet assay — C. Varga, G. Horváth, et al. (1999)
• Long crocidolite asbestos fibers cause polyploidy by sterically blocking cytokinesis — C.G. Jensen, L.C. Jensen, et al. (1996)
• Man-made mineral fiber effects on the expression of anti-oncogenes P53 and P16 and oncogenes C-JUN and C-FOS in the lung tissue of Wistar rats — Y. Cui, L. Huang, et al. (2019)

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