Synergy between asbestos and smoking on lung cancer risks

We have examined data from 12 epidemiologic studies for quantitative eviden ce of biologic synergy between asbestos and smoking on lung cancer risks. E stimates of the effect associated with joint exposure to the two agents exc eeded the sum of their separate effects in each study. We explored the vari ations in the strength of the synergistic effect across the studies using t hree indices: the ratio of the combined effects to the sum of the separate effects of smoking and asbestos (S), the relative excess risk due to intera ction (RERI), and the attributable proportion of risk due to interaction (A P). The weighted average of S across all studies was 1.64 (95% confidence i nterval = 1.33-2.03). The attributable proportion associated with this aver age S was estimated as 33%, that suggests that one third of cancer cases am ong smokers who were exposed to asbestos can be attributed to the synergist ic behavior of the two carcinogens, as distinct from their separate effects and those attributable to other ("background") factors.