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.