to some recent ETS, as indicated by their non-zero levels of urinary cotinine or nicotine. Solving Equation 4 gives:
Thus, the relative risk for a self-identified “unexposed” person compared with a truly unexposed person is:
and the relative risk for an “exposed” person compared with a truly unexposed person is:
To see what possible effect these relative risk estimates would have on the population-attributable risk, i.e., the fraction of lung cancer in nonsmoking individuals attributable to ETS, the proportion of the population that is exposed to ETS needs to be estimated. Wald and colleagues (1984) have reported that 17% of nonsmoking women and 12% of nonsmoking men fall into the category of “exposed,” i.e., nonsmoking spouses of smokers. By subtraction, this means that 83% of nonsmoking women and 88% of nonsmoking men would consider themselves “unexposed.” Given this, we can estimate the population-attributable risk, which is given in general form as:
where p1 is the proportion of people who call themselves “exposed,” RR1 is the relative risk of self-reported “exposed” persons, and RR2 is the relative risk of self-reported “unexposed” persons. Thus, for men:
and for women:
That is, about 21% of the lung cancers in nonsmoking women and 20% in nonsmoking men may be attributable to exposure to ETS.