Anatomy of the healthy worker effect: a critical review.

The healthy worker effect is the composite result from factors such as (1) selection of the work force, (2) changes in lifestyle accompanying employment, and (3) methodological characteristics of the standardized mortality ratio (SMR). While the choice of the general population as the comparison population is the underlying reason for this effect, no more feasible, widely accepted or "better" alternative is yet available. Much of the current understanding of the healthy worker effect has been limited to an examination of selection of the work force. The purpose of this report is to illustrate, using examples from a large cohort study, the importance of many factors other than selection. They include employment-associated benefits such as economic gain, medical insurance and lifestyle changes, the proportion of active workers, the calculation method, data completeness, length of follow-up and certain characteristics of SMR methodology. It is shown that some of the healthy worker effect is characteristic of SMR methodology and that the strength of the healthy worker effect depends on the proportion of active workers in the cohort. Furthermore, the disappearance of the healthy worker effect may be due to factors such as aging of the cohort and can be totally unrelated to the true increase in the mortality risk. In other words, its disappearance, in many instances, may be an artifact of SMR methodology.