Smoking and degree of occupational exposure: Are internal analyses in cohort studies likely to be confounded by smoking status?

Abstract

Occupational cohort studies are usually carried out without the benefit of information on smoking habits of cohort members. One common approach to avoid confounding bias related to smoking habits is to carry out an internal analysis, comparing workers with different degrees of occupational exposure. The premise behind this approach is that within a cohort there is unlikely to be correlation between degree of exposure and smoking habits. If this were untrue, smoking could confound the disease-exposure relationships. Our purpose was to verify the premise. The study sample consisted of 857 French-Canadian men born between 1910 and 1930, with 11 or fewer years of education, and interviewed around 1980 in the context of an occupational cancer case-control study. For each man we had information on smoking habits, job history, and a history of the chemicals he was exposed to in each of his jobs. We computed two indices of the dirtiness of workers' job histories: one based on the job titles held by the man and a second based on the degree of exposures to workplace substances. There was no correlation between these indices of job dirtiness and smoking history. We also examined the smoking-exposure relationship among the subsets of men who had been occupationally exposed to ten especially noticeable substances. Within the subsets, there was no indication of a consistent difference among the smoking subgroups in level or duration of exposure to these index substances. These findings do not support the view that nonsmokers sought out cleaner job environments than smokers; they imply that internal analyses of "dose-response" in cohort studies are unlikely to be seriously confounded by smoking habits.

Mortality studies of machining fluid exposure in the automobile industry IV; a case-control study of lung cancer

“…Such a mechanism has been proposed for cotton industry workers as well [Hodgson and Jones, 1990]. Chance variations in smoking patterns are unlikely given the large overall size of the study, the use of an internal analysis, and the use of a matched analysis to control for year of birth sex, race, and plant [Siemiatycki et al, 1988].…”

Radiation and mortality of workers at Oak Ridge National Laboratory: positive associations for doses received at older ages.

“…These findings, which provide little suggestion of confounding by cigarette smoking, conform to expectations from the literature. General discussions about occupational epidemiology studies that use internal rather than external referent groups (29,30), empirical investigations in other worker studies (31)(32)(33), and simulations (34) suggest that confounding by cigarette smoking would be "relatively modest in most situations" (34). In addition, if smoking is the reason for the observed associations, smoking patterns would have to be positively associated with cumulative doses received at older ages, but not associated with cumulative doses received at younger ages.…”

General mortality and respiratory cancer among a cohort of male chemical workers in california

“…However, statistically significant dose-response relationships were not demonstrated using duration of employment in these categories or assuming a 15-year latency period. Although these relationships could have been slightly underestimated since smoking was not controlled (due to small numbers in some strata), Siemiatycki et al [1988] demonstrated that it was unlikely such dose-response analyses which used duration of exposure were seriously confounded by smoking.…”