Asbestos and colon cancer: lack of association in a large case-control study

Our plain-English summary of Asbestos and colon cancer: lack of association in a large case-control study, by D.H. Garabrant, R.K. Peters, D.M. Homa (American Journal of Epidemiology, 1992).

The study

• Authors: D.H. Garabrant, R.K. Peters, D.M. Homa
• Journal: American Journal of Epidemiology
• Published: 1992;135(8):843-53
• Cited by: 24 articles, as counted by NIH iCite
• Read it: PubMed · Publisher
• In J-M's library: Johns-Manville's company medical library held a copy (J-M no. 004130).

Abstract

Previous studies linking exposure to asbestos with human colon cancer have used mortality rather than incidence as their endpoint and have neither assessed nor controlled for confounding by diet, genetic factors, or other risk factors for colon cancer. A case-control study of 746 histologically confirmed cases of colon cancer and 746 matched neighborhood controls was conducted in Los Angeles County, California. In univariate analyses of the 419 male pairs, a weak association was found between asbestos exposure and colon cancer (odds ratio (OR) = 1.16, 95% confidence interval (CI) 0.80-1.69). When confounding by family history of large bowel cancer, diet, body weight, and physical activity was controlled, there was no association between colon cancer and exposure to asbestos among males (OR = 0.99, 95% CI 0.66-1.50). When asbestos exposure was restricted to occurrences preceding diagnosis by more than 15 years, there was no clear association between such exposure and colon cancer, either before (OR = 1.14, 95% CI 0.76-1.70) or after confounding was controlled (OR = 0.93, 95% CI 0.60-1.44). Further analyses by frequency and duration of exposure failed to show any association between asbestos and risk of colon cancer, but did show a consistent pattern of confounding by nonoccupational factors that, when controlled, invariably produced a weak protective effect of asbestos exposure. Among the 327 female pairs, only 6 cases and 11 controls reported asbestos exposure (OR = 0.55, 95% CI 0.20-1.48), and there was no evidence of risk increasing as the frequency or duration of exposure increased. This study suggests not only that occupational exposure to asbestos is not a risk factor for colon cancer in the general population of Los Angeles, but also that observed associations between asbestos and colon cancer should not be interpreted as causal unless confounding by nonoccupational factors has been evaluated and controlled.

In plain English

This case-control study took place in Los Angeles County, California. Cases were people aged 45 to 70 first diagnosed with colon adenocarcinoma between November 1983 and June 1986. Each was matched with a neighborhood control and interviewed about work with asbestos, diet, weight, physical activity and family history. The report focused on 419 male pairs, in which 66 cases and 58 controls reported asbestos exposure. For men, the unadjusted odds ratio for any exposure was 1.16 (95 percent CI 0.80-1.69). After adjustment for family history of large bowel cancer, diet, weight and physical activity, it was 0.99 (0.66-1.50). For exposure more than 15 years before diagnosis, the adjusted odds ratio was 0.93 (0.60-1.44), and none of these odds ratios was statistically significant. There were no clear trends with frequency or duration of exposure. Only six female cases and 11 female controls reported exposure (unadjusted odds ratio 0.55, 0.20-1.48; adjusted 0.39, 0.12-1.25). The authors concluded that asbestos-colon cancer associations should not be read as causal unless nonoccupational confounders are controlled. They noted low power to detect small risks.

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: Population-based, individually matched case-control study of incident (newly diagnosed) colon cancer, using in-person structured interviews
• Who was studied: White, English-speaking men and women aged 45 to 70, born in the United States, Canada or Western Europe, with histologically confirmed adenocarcinoma of the colon first diagnosed November 1983 to June 1986. Cases came from the Los Angeles County Cancer Surveillance Program. Each was matched to a neighborhood control on sex and date of birth. There were 419 male pairs (70.3 percent of 596 potentially eligible male cases) and 327 female pairs.
• Exposure: Self-reported work with or around asbestos, at work or at home, counted only if it happened at least once a week for 6 months or more, or was very heavy for a briefer period. Example jobs in the question were brake lining repairs, insulation work, steam fitting, pipe fitting, shipyard work and building demolition. Subjects were also asked about frequency, years of exposure, asbestos on hands or clothes, and mask use. No fiber type was assessed.
• Main results:
  • Among the 419 male pairs, 66 cases and 58 controls reported asbestos exposure.
  • Men, any asbestos exposure: unadjusted odds ratio 1.16 (95 percent CI 0.80-1.69).
  • Men, any asbestos exposure: after adjustment for nonoccupational risk factors (family history of large bowel cancer, calories, carbohydrates, calcium, weight, physical activity), the odds ratio was 0.99 (95 percent CI 0.66-1.50).
  • Men, exposure restricted to more than 15 years before diagnosis: unadjusted odds ratio 1.14 (0.76-1.70) and adjusted 0.93 (0.60-1.44). None of the odds ratios in this analysis was statistically significant.
  • Men: no clear trends with increasing frequency or duration of exposure.
  • Men with the highest cumulative exposure score: adjusting for other risk factors reduced the odds ratio from 1.05 to 0.65.
  • Men: results by asbestos on hands or clothes and by mask use were inconsistent. Men whose hands and clothes were not contaminated had an adjusted odds ratio of 2.32 (95 percent CI 0.87-6.23).
  • Men, by part of the colon (unadjusted): right colon odds ratio 0.90 (95 percent CI 0.47-1.72); left colon 1.32 (95 percent CI 0.83-2.11). Adjustment reduced both.
  • Men: family history of large bowel cancer was an important confounder for all colon cancers combined. For right-colon cancer, weight 10 years before diagnosis was a slightly stronger confounder.
  • Women: only 6 of 327 cases and 11 of 327 controls reported asbestos exposure.
  • Women: unadjusted odds ratio for any asbestos exposure 0.55 (95 percent CI 0.20-1.48). After adjustment it was 0.39 (95 percent CI 0.12-1.25).
• The authors' conclusion: The authors concluded that, after nonoccupational risk factors were controlled, no positive association between asbestos exposure and colon cancer was found. They said observed associations between asbestos and colon cancer should not be interpreted as causal unless confounding by nonoccupational factors has been evaluated and controlled.
• Limitations the authors state:
  • The study had 80% power to detect an odds ratio of 1.7 or more.
  • Power was too low to detect very small risks: 13% for an odds ratio of 1.2 and 55% for 1.5.
  • Exposure may have been lower than in earlier cohort studies, or exposure may have been misclassified.
  • The pattern of confounding found may be specific to this population.
  • The findings cannot disprove the idea that very heavy asbestos exposure creates a risk for colon cancer.

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, Case-Control Study, Confounding, Asbestos Exposure, Odds Ratio, confidence interval, Occupational Exposure.

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