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

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.