Healthy Worker Effect

Definition

HWE most commonly refers to the situation in which workers exposed to occupational hazards exhibit lower mortality rates than a reference population without the exposure of interest. Typically affecting occupational studies, the HWE arises when cohorts of workers are compared with reference groups that include the severely sick, terminally ill, and others incapable of working. The strength of the effect varies by occupation and may partially or completely mask any negative exposure-outcome association. In some circumstances, an exposure with negative health effects may appear to confer protection to the exposed.

History

The HWE was discovered in 1885 by William Ogle, who noticed that workers in more demanding occupations had lower mortality rates than those who were employed in less vigorous work or who were unemployed. More than 100 years later, in 1974, the term healthy worker effect was coined by McMichael. A few years later, in 1976, Fox and Collier first quantified the HWE by calculating the standardized mortality ratio using the general population as a reference.

Healthy Worker Effect Modifiers

Diverse factors, including personal characteristics (e.g., race, gender), study design (e.g., completeness and length of follow-up), worker screening (e.g., health/ability to perform at hire, age of hire), and disease traits (e.g., incubation period, clinical presentation), may affect the strength of the HWE. The duration of employment, age of hire, and presence of overt symptoms are examples of factors that may cause the study population to mistakenly appear [Page 488]healthier than the reference population and that therefore affect the HWE. Workers who have participated in the workforce for a longer duration are less likely to have terminated work prior to standard retirement age due to illness. Those with a higher age of hire are more highly influenced by selective processes and therefore experience a stronger healthy worker effect than workers hired at a younger age; those who are hired at an older age are likely to be in better health than similarly aged individuals in the general population. HWE is also increased in studies involving diseases with overt symptoms that interfere with work or that compel individuals to leave the workforce prior to retirement.

Reducing the Healthy Worker Effect

The choice of comparison group is critical in reducing the HWE, and various suggestions have been proposed. One option is to identify an external comparison group from an industry that lacks the hazard being studied but that applies similar worker screening processes. Another option is to choose an internal reference group from within the occupation. In an internal comparison, rates among those with high exposures are compared with those with low or no exposures. Ensuring complete follow-up of all individuals regardless of subsequent worker status will minimize bias, although complete follow-up cannot completely remove bias introduced by the initial selection of healthy workers into employment.

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