Occupational epidemiology in the rubber industry: implications of exposure variability

The variability of exposure has important implications for the design of occupational epidemiologic studies. To assess the influence of this variability in the rubber industry, the efficiency of different schemes for classification of exposure to particulates, cyclohexane-soluble contaminants, and solvents was assessed. Groupings based on job title, plant, factors affecting exposure, published classifications, and the International Standard Classification of Occupations of the International Labor Organization (ISCO-ILO) were compared. Statistical parameters for contrast in average exposure between groups and precision of average exposure within groups were developed to enable comparison of the efficiency of different classification schemes. Grouping of exposure to particulates and dermal exposure appeared to be less efficient than grouping of exposure to solvents. Grouping of solvent exposure using occupational title groups, existing classification schemes, and schemes based on factors affecting exposure showed comparable high resolution in exposure levels. Even the most detailed grouping schemes based on the combination of plant and occupational title group showed relatively modest resolution in particulate and dermal exposure levels. Groupings based on factors affecting exposure showed similar resolution for these exposures, but had higher precision due to having fewer groups. Application of optimal exposure grouping strategies will benefit new research on cancer among rubber workers. Eventually, this might resolve the situation in which a complete industry was included on the list of proven human carcinogens.