Asbestos and colon cancer: a weight-of-the-evidence review

Our plain-English summary of Asbestos and colon cancer: a weight-of-the-evidence review, by J.F. Gamble (Environmental Health Perspectives, 1994).

The study

• Author: J.F. Gamble
• Journal: Environmental Health Perspectives
• Published: 1994;102(12):1038-50
• Type of article: Meta-analysis
• Cited by: 17 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

What is the evidence that exposure to asbestos causes colon cancer? This weight-of-evidence review considers epidemiologic evidence from cohort studies of asbestos-exposed workers, case-control studies of colon cancer, animal bioassays, and other corroborative evidence. The major evidence for a causal association at high exposure is a combined colorectal standardized mortality ratio (SMR) of 1.5 for asbestos cohorts where the lung cancer SMR was greater than twofold. However, misdiagnosis may spuriously elevate the SMR. The strongest evidence against a causal association between colon cancer and asbestos exposure is the lack of an exposure-response gradient in asbestos cohorts where trends for lung cancer are observed. Population-based case-control studies of colon cancer do not show any consistent risk associated with asbestos exposure. Long-term ingestion studies show no evidence of an increased incidence of colon cancer in animals by this route of exposure and do not provide biological plausibility for a causal association between asbestos exposure and colon cancer.

In plain English

This article was a weight-of-the-evidence review by one author of cohort studies of asbestos-exposed workers, case-control studies of colon cancer, and animal feeding experiments. Cohorts were grouped by lung cancer standardized mortality ratio (SMR: observed relative to expected deaths), an imperfect stand-in for exposure because smoking was not controlled for. In seven cohorts with lung cancer SMRs above 2, the combined colorectal cancer SMR was 1.48 (95% CI 1.21 to 1.78). In 12 lower-exposure cohorts it was 0.95 (0.84 to 1.05). The author considered the higher-exposure result indeterminate because misdiagnosis could explain it. Colorectal cancer showed no apparent exposure-response trend in six of seven studies. In one Swedish asbestos cement cohort, the colorectal cancer SMR was 3.4 (95% CI 1.2 to 9.5) at 40 or more fiber-years per ml, with no increase at lower exposures. The case-control studies gave little consistent support, and the animal studies showed no increase in colon tumors. The author concluded that asbestos exposure does not appear to increase the risk of colon cancer.

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: Weight-of-the-evidence review of published cohort studies, case-control studies, animal bioassays and other corroborative evidence. Observed and expected deaths from the cohorts were summed into overall ratios.
• Who was studied: At least 19 published cohort studies of asbestos workers (or workers exposed to asbestos) with 10 to 20 or more years of latency, plus population-based case-control studies of colon cancer and animal ingestion studies.
• Exposure: Occupational asbestos exposure in the reviewed cohorts, which involved chrysotile, amosite and crocidolite. Cohorts were classed as high exposure (lung cancer SMR above 2) or low exposure (below 2), using lung cancer risk as a surrogate for exposure.
• Main results:
  • In the seven cohorts with lung cancer SMRs above 2, the overall SMR for colorectal cancer was 1.48 (95% CI 1.21, 1.78).
  • In the 12 cohorts with lung cancer SMRs below 2, the overall SMR for colorectal cancer was 0.95 (95% CI 0.84, 1.05).
  • The author judged the weak association at higher exposures indeterminate because misdiagnosis might explain the increase.
  • In cohorts that considered latency, the combined SMR was 0.83 for colorectal cancer and 0.60 for colon cancer alone.
  • Within studies that grouped workers by exposure, there was no apparent exposure-response trend for colorectal cancer in six of seven studies.
  • In the Swedish asbestos cement cohort (Albin et al.), colorectal cancer had an SMR of 3.4 in the highest cumulative exposure category (40 or more fiber-years/ml), with no increased SMR in the lower exposure categories.
  • The 95% confidence interval for that colorectal cancer SMR of 3.4 was 1.2 to 9.5.
  • Among the population-based case-control studies, the two with nearly twofold excess risk either did not control for confounders or involved other correlated occupational risk factors.
  • The animal studies showed no increase in colon tumors in at least two species.
  • The only experimental observation supporting the hypothesis was increased cell proliferation, measured by DNA synthesis.
• The authors' conclusion: The author concluded that asbestos exposure does not appear to increase the risk of colon cancer.
• Limitations the authors state:
  • Lung cancer SMR is only a surrogate for asbestos exposure, because smoking was not controlled for.
  • Few studies assessed exposure-response.
  • Misdiagnosis is likely to have inflated the colon cancer SMRs.
  • Exposure levels could not be compared between case-control studies, or between individuals within the same study.
  • The cohorts lacked information on colon cancer risk factors such as diet, obesity and physical activity.

Topics

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

Medical terms in this study, explained in our Lexicon: Colorectal Cancer, standardized mortality ratio (SMR), Lung Cancer, Misdiagnosis, Asbestos Exposure, Biological Plausibility.

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