A technique to re-assess epidemiologic evidence in light of the healthy worker effect: The case of firefighting and heart disease

The healthy worker effect (HWE) is a bias that is believed to have strongly affected the validity of previous cohort mortality studies on the relation ship between firefighting and heart disease. There is a strong healthy hire d effect (a component of the HWE) among firefighters, owing particularly to the recruitment of nondiabetic candidates. This is shown in previous studi es in which the reported standardized mortality ratios for diabetes are muc h less than unity, generally around 0.3 to 0.5. Because diabetes is known t o increase the risk of heart disease, a deficit of diabetes among firefight ers is expected to lead to a deficit of heart injury and disease. This woul d make the cohort mortality studies incapable of detecting any increase in risk of heart injury and disease among firefighters. There is also a strong healthy worker survivor effect (another component of the HWE) among firefi ghters. In addition, heart disease is a classic example of the HWE because heart disease is chronic and its risk factors can be identified in the sele ction process. It is believed that (1) a major problem of previous studies on firefighting and heart disease is their failure to recognize the importa nce of the HWE when interpreting their results, and (2) a technique to re-a ssess results in light of the HWE is urgently needed. This article addresse s the generally accepted principles relating to the HWE, including its defi nition and sources, and proposes a technique for re-assessing the literatur e in light of the HWE. The technique was applied to carefully re-assess 23 studies that provided direct evidence for the relationship between firefigh ting and heart disease. Before the re-assessment, 7 of the 23 studies showe d positive evidence and 16 showed no evidence. After the re-assessment, 11 studies showed positive evidence and 12 showed no evidence. Based on the re sults of the re-assessment of the 23 studies, we concluded that (1) there i s strong evidence of an increased risk of death overall from heart disease among firefighters (2) there is insufficient evidence, even after consideri ng the HWE, that there is an increased risk of death from aortic aneurysm a mong firefighters; and (3) there is insufficient evidence, even after consi dering the HWE, for a relationship between firefighting and any heart disea se subtype, such as acute myocardial infarction.