Healthy-worker effect

A mortality study of a cohort of 3,827 licensed male pesticide applicators was conducted in Florida. This cohort did not exhibit the healthy worker effect, as the overall SMR was close to expected (Blair et al. 1983). Increased SMRs, although not statistically significant, were seen for leukemia, cancers of the brain, and lung cancer. Follow-up was achieved for over 95% of the identified cohort members, but no information was available for smoking history. 

Death. Occupational mortality studies of pesticide workers exposed to heptachlor have not revealed an excess number of deaths in these cohorts compared to the general U.S. population. This may possibly be explained as a healthy worker effect. The ERA has described human case reports in which convulsions and death were reported following suicidal ingestion of technical-grade chlordane, which typically contains 6-30% heptachlor, but these effects cannot be attributed to heptachlor or heptachlor epoxide. There are no controlled, quantitative human data for any route of exposure. Acute lethality data were located for animals exposed via the oral and dermal routes. Both heptachlor and heptachlor epoxide may be considered very toxic via the oral route on the basis of acute animal data in rats and mice. Intermediate oral exposure to these compounds also caused up to 40% and 100% mortality in rats and mice, respectively. There appear to be differences in sensitivity in males and females in some species with the males being most sensitive. Heptachlor epoxide is more toxic than heptachlor. Heptachlor may be considered very toxic to extremely toxic via the dermal route on the basis of acute lethality data in rats and mice. The severity of acute effects may possibly depend upon the extent of formation of heptachlor epoxide and the species tested. 

Studies of workers often find that workers are healthier than the general population. This "healthy-worker effect" Is generally ascribed to the fact that the working population Is in better overall health than the general population, which Includes the very sick, the elderly, and the hypersusceptible. 

Interpretation of cohort studies is not always straightforward there are a number of selection effects and biases that must be considered. Cohort studies routinely report that the mortality of active workers is less than that of the population as a whole. It is not an unexpected finding since workers usually have to undergo some sort of selection process to become or remain workers. Nevertheless, this selection effect, known as the healthy worker effect, can lead to considerable arguments over the interpretation of study results, particularly if the cancer mortality is as expected but the all-cause mortality is much lower than expected. However, even an experimental science such as toxicology is not without a similar problem of interpretation, namely, the problem of distinguishing between the effects of age and treatment on tumor incidence. 

Data from the Blair et al. (1986) study were reanalyzed by Sterling and Weinkam (1989, 1995) to account for the healthy worker effect. The cohort was composed of all workers employed in 10 selected... 

Analyses of mortality among persons chronically exposed to plutonium in the workplace have been conducted. In the three occupational cohorts studied (Los Alamos Laboratory, Rocky Flats facility, and Hanford Plant), there were consistently fewer deaths than expected based on data for United States white males (Gilbert and Marks 1979 Voelz et al. 1983a, 1983b Wilkinson et al. 1987). This phenomenon is generally attributed to the "healthy worker effect," which holds that individuals in the work force are healthier than those in the general population. However, in a refined cohort from the Rocky Flats facility, the mortality of plutonium-exposed workers was compared to that of unexposed workers from the same plant. It was reported that death from all causes was elevated in exposed individuals but the increase was not statistically significant (Wilkinson et al. 1987). 

A cohort of white male chemical workers in Louisiana exposed to a variety of chemicals including tetrachloroethylene did not have an increased risk for total mortality or cancer (Olsen et al. 1989). This nonpositive study was confounded by the healthy worker effect, defined as the observation that employed persons have lower mortality rates than the general population. Blacks and women were not analyzed in this study because of the small numbers of representatives from these groups. 

Individuals exposed to colophony in pine wood, sawdust and wood wool may develop hand dermatitis and dermatitis due to airborne exposure (Burry 1976 Daly and Stevenson 1984 Meding et al. 1996 Watsky 1997) The prevalence of contact allergy to colophony among the employees in a factory for production of tail-oil rosin was found to be in accordance with that of dermatitis patients and, thus, was higher than in the general population. However, the clinical symptoms were rare. A healthy-worker effect can not be excluded (Farm et al. 1994). 

A number of complications and empirical conundrums also arise in assessing lead worker cohorts with reference to endpoints such as morbidity and mortality associated with Pb nephropathy. There is the weU-recognized and general healthy worker effect, where the healthiest and/or least adversely responsive workers exposed to toxics are most likely to remain at the job over time, potentially biasing the overall dose—toxic response relationships for various endpoints in humans to a less robust relationship. Survivor bias is related to this phenomenon in mortality studies. That is, those workers leaving employment within shorter time periods are not likely included in the statistical analyses of survival rates. 

Retrospective studies evaluated the mortality experience in two cohorts of chemical workers in an Alabama and a Louisiana manufacturing plant (Delzell and Sathiakumar 1992 Sathiakumar et al. 1992). Data from the two studies were pooled to increase the database and thus enhance the informativeness of the results. The 2683 workers with definite or probable exposure to triazines, especially the long-term employees, had a lower mortality rate than the general population (healthy worker effect). The mortality rate in 2234 possibly exposed workers was similar to that of the general population. 

Healthy worker effect. Sociologists and occupational health specialists have known for some time about the strong correlation between steady employment in a large organization and good health. This phenomenon has been termed the healthy worker effect. ... 

---