Electron spin resonance studies of the effects of nitrogen dioxide and cigarette smoke upon blood and lung tissue components were performed. In vivo exposure of small animals to NO2 and cigarette smoke has been interpreted in terms of pollutant reaction with the heme moiety of hemoglobin. Results imply that the pollutants have migrated through the erythrocyte membrane. So as to understand the effects of these pollutants upon the membranes themselves, various model systems have been studied, particularly the interaction of nitrogen dioxide with unsaturated lipids. Three stable free radicals have been characterized as products of these reactions; and a short-lived initial free radical was observed, the structure of which provides evidence as to the initial reaction site and mechanism.