Although the amounts of environmental tobacco smoke absorbed by passive smokers are small in comparison to those absorbed by active smokers, the fact that (almost) everybody in modern society is exposed makes it an important public health problem. The many published epidemiologic studies are all consistent with a 30% increase in the risk of lung cancer. This increase is plausible in relation to the exposure levels derived from various biological dose indicators. The risks of passive smokers are, of course, smaller than those of active smokers, but it is generally accepted that involuntary risks should be much smaller than those that are self-inflicted. Even a relative risk for lung cancer of 1.3 due to passive smoking would constitute an increase on the order of 1 in 1,000 in the lifetime risk. Normally, this level would be considered "unacceptable," and preventive measures should be taken.