INTRODUCTION Air is the most vital and immediate need of man. In some places@ at some times@ concentrations of certain chemicals discharged into the air produce undesirable effects on the human@ the biota@ and the physical environment. Acute episodes such as Donora@ the Meuse Valley@ and the New York and London incidents (so called London smog) excite interest in and provide dramatic highlights to air pollution but represent abnormal and infrequent conditions. They were caused by burning fossil fuel with high sulfur content under unusual weather conditions and must be considered special cases@ natural disasters if you will. They are not identical with the problem posed by oxidants@ the major ingredients of photochemical air pollution@ a form which has never been implicated in an acute disaster. The oxidants cannot be discussed without reviewing the entire complex of photochemical air pollution??hereinafter for convenience called smog*--the phenomenon which produces reduced visibility@ plant damage and rapid and visual responses in humans@ namely tearing and smarting of the eyes. These symptoms which occur in 200 or more days of a year affect more than 80% of the population in Los Angeles. (1) In spite of almost no evidence in humans at this time that chronic disease results from these almost daily exposures@ there is considerable anxiety that in millions of people respiratory disease is being initiated or aggravated. Since there is no end in sight to the increasing population density of our urban areas and the use of gasoline-powered automobiles@ control of air contamination becomes increasingly urgent. The almost daily reminder of eye irritation and the gnawing fear that more subtle damage is taking place constitute the most consistent and continuous driving forces for control of air pollution. The pollutants which collectively constitute smog present a particularly difficult problem in that they do not follow the classis patterns of toxicology where the etiological agent remains unchanged@ except in concentration@ from source to host. Rather@ with these chemicals@ between source and host@ a vast number of interrelated photochemical and other reactions take lace. Only one of the three major pollutants@ nitrogen dioxide@ is measured directly as a discrete chemical compound. It is a reaction product derived principally from nitric oxide discharged into the atmosphere from the exhausts of motor vehicles and other combustion processes. The oxidants are principally ozone but include other materials that will oxidize the test reagent. The sources of the oxidants are the complex atmospheric photochemical reactions involving sunlight@ nitric oxide and hydrocarbons. A necessary ingredient@ the hydrocarbons@ are derived mainly from auto exhaust@ but also occur naturally from plant life. These hydrocarbons are essentially non-toxic at smog concentrations. The exact effect of control at the source of one or the other constituents@ as a consequence of application of standards@ is still uncertain. Reduction of hydrocarbons may increase the duration of nitrogen dioxide episodes in the atmosphere; restriction of reactive hydrocarbons only may increase the dose of non-reactive hydrocarbons; restriction of other materials such as sulfur dioxide may have profound effects on aerosol formation. These complicated reactions must be understood before reliable health criteria can be formulated for the control of all constituents of smog. This paper examines only the evidence of adverse health effects on humans and animals (there are no known beneficial effects) of smog and of its individual pollutants; i.e.@ nitrogen dioxide@ oxidants@ and the hydrocarbons. The criteria for standards which we will suggest are based on review of the literature@ an examination of California's 25-year experience with air pollution@ and on the doctrine that man must preserve his environment to preserve himself. *Smog@ a coined word for a special form of air pollution@ smoke plus fog@ has become the popular word describing air pollution in Los Angeles.