Mortality of petroleum refinery workers

We would like to comment on the paper by Satin and colleagues,¹ which reports an update of a mortality investigation on two cohorts of petroleum refinery workers. The authors claim that one of the major aims of their study was the assessment of “health risks relative to more contemporary levels of exposure and work environments”. Nevertheless, they explicitly admit that a previous investigation in such cohorts,² using the population of California as referent, found a strong “healthy worker effect” (that is, a significantly lower than expected mortality risk from cardiovascular disease and lung cancer). In our opinion, this observation, along with some other drawbacks, only in part expressly acknowledged in the paper, might have biased most of the results obtained, leading the authors to draw unreliable conclusions. We shall discuss this issue in detail, illustrating the main possible biases and how we believe they should have been interpreted.

Comparison bias

Exposure effects should be assessed in cohort studies by comparing the exposed cohorts with at least an unexposed one, as similar as possible in all relevant aspects.³

The new results by Satin et al have confirmed the occurrence of the“healthy worker effect” observed in the previous follow up. Such a finding may indicate a comparison bias concealing the associations, if any, between exposure and health risks.² In fact, occupational cohorts may differ from the general population in many features that have been associated with various risk factors, including socioeconomic status and personal habits.³ The presence of a comparison bias, at least in the Richmond refinery cohort, seems to be suggested by the risk for leukaemia in the subgroup with the shortest duration of employment (⩽5 years), which is more than four times lower than the referent population (and nearly seven times lower than those of workers who …