Evaluation of potential gastrointestinal carcinogenicity associated with the ingestion of asbestos

Our plain-English summary of Evaluation of potential gastrointestinal carcinogenicity associated with the ingestion of asbestos, by T.J. Cheng, S.L. More, M.A. Maddaloni, et al (Reviews on Environmental Health, 2021).

The study

• Authors: T.J. Cheng, S.L. More, M.A. Maddaloni, E.S. Fung
• Journal: Reviews on Environmental Health
• Published: 2021;36(1):15-26
• Type of article: Review
• Cited by: 7 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

The inhalation of asbestos, depending on the fiber type and dose, may be associated with the development of mesothelioma and other asbestos-related diseases. However, little is known about the potential adverse effects associated with the ingestion of asbestos. Evidence of asbestos fibers released from asbestos-cement pipes used in water distribution systems has led to concerns of potentially contaminated drinking water. The purpose of this study is to determine whether ingestion of asbestos fibers may lead to cancerous effects on the gastrointestinal (GI) tract. Data from animal and human studies were analyzed using a weight-of-evidence approach to evaluate the potential risk of GI cancers associated with asbestos ingestion. Seventeen human and 23 animal studies were identified and evaluated in this study. Animal studies were conducted in multiple species with inconsistent dosing protocols. Overall, animal studies reported that the asbestos fibers, irrespective of fiber type and dose, failed to produce any definitive GI carcinogenic effect. The 17 identified human epidemiological studies reported the ingestion of asbestos-contaminated water with concentrations from 1 to 71,350 million fibers per liter (MFL). A majority of the epidemiology studies reported statistically significant increases in multiple GI-specific cancers. However, these findings are confounded due to several critical study limitations including flawed study design, small sample size, selection bias, lack of individual exposure history, lack of adequate latency, and the inability to account for confounders including occupational history, diet, and smoking history. Based on our weight-of-evidence assessment, there is insufficient evidence of causality between the ingestion of asbestos and an increased incidence of GI cancers.

© 2020 Walter de Gruyter GmbH, Berlin/Boston.

In plain English

This mini review asked whether swallowing asbestos, mainly in drinking water, is linked to cancers of the gastrointestinal (GI) tract, such as stomach, esophageal and colorectal cancer. The authors searched PubMed and Google Scholar up to May 2020. They assessed 40 articles, 23 animal studies and 17 human epidemiology studies, using the Bradford Hill criteria for judging causation. The animal studies gave chrysotile, amosite or crocidolite by mouth to rats, hamsters, mice and baboons. The authors reported that none produced a definitive GI cancer effect. The human studies covered water contaminated by mining waste in Duluth, Minnesota, by asbestos-cement pipes, by natural mineral deposits, and by asbestos roof tiles used by Norwegian lighthouse keepers. A majority reported statistically significant increases in some GI cancers. The authors judged these studies weakened by flawed design, small size, missing individual exposure histories and unmeasured confounders such as diet and smoking. They found no consistent dose response. The authors concluded there was insufficient evidence of a causal relationship between ingested asbestos and GI cancers.

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: Mini review of published animal and human studies, using a weight-of-evidence approach and the Bradford Hill criteria
• Who was studied: 40 published articles, 23 animal studies and 17 human epidemiology studies, found through PubMed and Google Scholar searches up to May 2020. The human studies were from the United States, Canada, Norway and the Netherlands.
• Exposure: Ingestion of asbestos. In animals, crocidolite, amosite and chrysotile were given by gavage, in the diet or in drinking water. In humans, the exposure was drinking water contaminated from asbestos mining waste (amphibole, Duluth), asbestos-cement pipes (chrysotile and/or crocidolite), asbestos mineral deposits (chrysotile) and asbestos roof tiles (Norway).
• Main results:
  • Eighteen rat studies and six studies in other animals (hamsters, mice, baboons) were reviewed. The authors reported that ingestion of various fiber types failed to produce any definitive GI-specific cancer effect.
  • The only animal study with a significant cancer finding (McConnell et al., 1% amosite in rat diet) found thyroid C-cell carcinoma and monocytic leukemia, which are not GI cancers.
  • A majority of the human epidemiology studies reported statistically significant increases in GI-specific cancers.
  • Duluth, Minnesota: two studies (Masson et al., Levy et al.) did not report a significant increase in cancers. A third (Sigurdson et al.) reported a significant increase in GI cancers compared with other cities.
  • Asbestos-cement pipes: no significant increase in GI cancer incidence was reported in Connecticut and Utah. A significant increase in stomach, digestive and mouth cancer was reported in Canada and Woodstock, New York.
  • Mineral deposits (chrysotile): all but one study reported significant increases in GI cancers, including stomach, large intestine, pharynx, esophagus and digestive tract.
  • Norwegian lighthouse keepers drank rainwater from asbestos roof tiles containing 1,760 to 71,350 million fibers per liter. The studies of this group reported a significant increase in stomach and colon cancers.
  • Under the Bradford Hill criteria, the authors found no consistent dose response in the human studies.
  • A case report of peritoneal mesothelioma in a 76-year-old woman attributed to asbestos-contaminated water was left out of the assessment. The authors gave lack of exposure information as the reason.
• The authors' conclusion: The authors concluded there was insufficient evidence of a causal relationship between ingesting asbestos of various fiber types, at up to 71,350 million fibers per liter, and an increased incidence of GI-specific cancers. They called for further research.
• Limitations the authors state:
  • The authors state that their own analysis was limited by the confounders in the underlying studies.
  • The animal studies varied significantly in dosing protocol.
  • Animal doses (mass of fiber per day or per body weight) were hard to compare with human doses (fiber concentrations in water).
  • The authors state that the human studies suffer from flawed design, small sample size, selection bias, lack of individual exposure history and lack of adequate latency.
  • The authors note that the human studies did not account for confounders such as diet and smoking.

Topics

Medical terms in this study, explained in our Lexicon: Mesothelioma, Asbestos-Cement Pipe, Carcinogenic, Epidemiology, Selection Bias, Exposure History, Latency, Occupational History.

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