To the Editor.— Epidemiologic studies have indicated an association of environmental smoke exposure with tobacco-related disease.1-4 The composition of tobacco sidestream smoke differs significantly from that of mainstream smoke. N-nitrosamines, benzo[a]pyrene, carbon monoxide, nicotine, and heavy metals are more abundant in sidestream than in mainstream smoke.5 Exposure to such harmful chemical compounds in heavily polluted environments may constitute a health risk to passively exposed individuals. By measuring cotinine in plasma and in urine, we have estimated the rate of elimination of this end product of nicotine metabolism in both smokers and passively exposed nonsmokers. Ten smokers reported to the clinic each day at 9:30 AM for blood sampling. At noon on day 5, the subjects quit smoking. Blood samples were taken daily from day 5 through day 12. All urine voided was collected daily throughout the study period. Four nonsmokers, aged 25 to 35 years,