Isocyanate exposure and occupational asthma: a case-referent study

Objective-To examine the quantitative relation between exposure to isocyana tes and occupational asthma, and to explore the role of atopy and smoking i n occurrence of the disease. Method-A case-referent study was undertaken of cases from two manufacturing companies (A and B) from which referents without disease could be selected and reliable exposure measurements were available. In company A, 27 cases mainly attributed to toluene diisocyanate (TDI) were matched to 51 referent s on work area, start and duration of employment, sex, and age. Exposures w ere estimated from existing measurements by job category. In company B ther e were seven eases attributed to 4,4'-diphenylmethane diisocyanate (MDI) in two areas of the plant; 12 non-cases from the same areas were used as refe rents. Personal exposure measurements were available for all cases and II r eferents. Results-No difference in peak exposures between cases and referents was fou nd in either plant; but in both, time weighted average (TWA) exposures at t he time of onset of asthma were higher for cases. In A, the mean TWA exposu re for cases was 1.5 (95% confidence interval (95% CI) 1.2 to 1.8) ppb comp ared with 1.2 (1.0 to 1.4) ppb for referents. From a matched analysis, the odds ratio (OR) associated with 8 hour TWA exposure to isocyanates greater than 1.125 ppb (the median concentration for the referent group) was 3.2 (9 5% CI 0.96 to 10.6; p=0.06). Occupational asthma was associated with a pre- employment history of atopic illness (OR 3.5, p=0.04) and, less strongly, w ith smoking (OR 2.1, p=0.14). In B, small numbers Limited analysis, but thr ee of seven cases had at least one TWA exposure measurement greater than 5 ppb compared with one of 11 referents (OR 7.5, p=0.09). Conclusion-Asthma can occur at low concentrations of isocyanates, but even at low concentrations, the higher the exposure the greater the risk. By con trast with other studies, smoking and atopy seemed to increase the odds of occupational asthma due to isocyanates, but did not affect the estimate of risk associated with exposure.