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Healthy worker effect in a longitudinal study of one-second forced expiratory volume (FEV1) and chronic exposure to
E A Eisen1, D H Wegman, T A Louis
1Department of Work Environment, University of Massachusetts, Lowell 01854, USA.
Background:
Low level effects of granite dust on one-second forced expiratory volume (FEV1) are estimated in 618 Vermont granite workers followed for 5 years with annual pulmonary function tests. Reduced pulmonary function has already been reported for the subset of subjects lost to follow-up (dropouts) suggesting possible bias in analyses based only on survivors.
Method:
Healthy worker selection bias is directly assessed by comparing the dose-response associations between survivors who remained in the study for the full 5-year observation period and the dropouts.
Results:
The 353 survivors had an FEV1 of 96% of predicted at baseline and were losing FEV1 at an average rate of 44 ml/yr. No association was found in this group between the rate of FEV1 decline and lifetime dust exposure. However, the 265 workers with incomplete follow-up, 'dropouts', had a lower FEV1 at baseline (94%) and were losing FEV1 at an average rate of 69 ml/yr. The dose-response parameter in this group was estimated to be 4 ml/yr loss per mg/m3-year and was statistically significant.
Conclusions:
These results provide an illustration of bias due to the healthy worker effect and an example of the failure to detect a true work-related health effect in a study based only on a 'survivor' population.