Hematologic effects of benzene. Job-specific trends during the first year of employment among a cohort of benzene-exposed rubber workers

Hematologic surveillance data from 1940 to 1975 were analyzed for a benzene-exposed cohort of 459 rubber workers. The present analyses are restricted to 161 workers with "preemployment" counts done before exposure and rely on their subsequent counts from the first 12 months of employment. While blood cell counts declined approximately 1000 cells/mm3 over the first 4 months of exposure. Using repeated-measures analysis of variance, workers exposed above the median benzene exposure at the plant had significantly lower average white and red blood cell counts at each month during the first year of work when compared with workers exposed below the median. These decreased counts suggest that clinically detectable bone marrow depression accompanied the onset of work in this plant during the 1940s and support exposure assessments that favor higher benzene levels in the 1940s when compared with subsequent decades. The general utility of repeated-measures analytic techniques for medical surveillance data is also demonstrated by this analysis.