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To investigate the process of deciding on compensation claims by lung cancer patients exposed occupationally to coal tar pitch volatiles.For each case of lung cancer the probability that it was caused (probability of causation (PC)) by coal tar pitch volatiles was expressed as an increasing function of cumulative exposure to benzo-a-pyrene-years. This was assessed from several exposure-response models fitted to data from a large epidemiological study of aluminium production workers. For some models, PC depended also on the smoking habit of the cancer patient.Estimation of relative risk by exposure group indicated that over 50% of lung cancers were attributable to coal tar pitch volatiless (PC > 50%) at exposures above 100 micro gram/m3 -years benzo(a)pyrene. A linear relative risk model indicated that 50% PC was first achieved at 342.2 micro gram/m3 -years benzo(a)pyrene, or 190.1 micro gram/m3 -years benzo(a)pyrene according to the upper 95% confidence limit for risk increment. Corresponding figures for a power curve model were 210.3 and 45.9. With these five figures as compensation criteria compensation would have resulted in 31.4%, 2.7%, 19.2%, 15.7%, and 39.2% of cancers studied, compared with an estimated total proportion of cancers studied attributable to coal tar pitch volatiles of 15%-26%. If risks due to coal tar pitch volatiles and smoking multiply, PC does not depend on the amount smoked. If the two risks are additive, however, PC depends on the amount smoked according to a formula, with the figures mentioned applying to an average smoking history (24.4 pack-years).Because of its simplicity and because it falls within the range of criteria based on several more sophisticated approaches, we prefer the criterion of 100 micro gram/m3 -years, based on the relative risks by exposure group. However, the compensation board of the Canadian province of Quebec, on consideration of these alternatives, has proposed as a criterion that the upper 95% confidence limit of PC for the patient be at least 50%, assuming an additive relative risk model and allowing for their smoking habit.(Occup Environ Med 1996;53:160-167).