An index of past asbestos exposure as applied to car and bus mechanics

Different ways to describe historic fibre exposure from asbestos-containing friction materials were studied and compared for a group of 103 car and bus mechanics with more than 20 years employment and 15 years of asbestos exposure. A model was constructed to calculate cumulative asbestos exposure from friction materials including duration, intensity and exposure last year. The model is a combination of an additive and a multiplicative model, where an asbestos index was constructed that takes both near field and far field exposure into consideration. The model was based upon data from the international literature and quantitative asbestos measurements performed 1976-1988 in Swedish car repair workshops. The fibres were counted by phase-contrast microscopy with fibre criteria of length > 5 microns and aspect ratio > or = 3:1. The mechanics' fibre exposure at 398 repair workshops during a period of 48 years were calculated using the model. The mean cumulative exposure was estimated to be 2.6 f ml-1 * year. The annual cumulative exposure was highest for truck mechanics in the early 1960s. The car mechanics had a time-weighted average fibre exposure range of 0.11-0.41 f ml-1 (mean 0.21 f ml-1) in 1965 compared to 0.003-0.08 f ml-1 (mean 0.021 f ml-1) in 1985. In order to validate the model, the mechanic's fibre exposure estimated using the model were compared with representative asbestos exposure measurements for car mechanics during the 1960s and the 1970s (correlation coefficient = 0.69). Five lung physiological variables (FVC, TLC, FEV1, TLco and CV%) were used to study exposure-response relationships. None of the exposure parameters suggested any significant relationship between exposure and decrease in lung function.