The body of information presented in this paper is directed to policy makers and administrators involved in the evaluation and assessment of damages caused by oxidant air pollution on human health and welfare and of possible benefits of control. To provide a comparison of some of the benefits that can be obtained by reducing photochemical oxidant levels, estimated health costs were derived from data relating adverse health effects to hourly oxidant concentrations. Hourly oxidant or ozone concentrations were measured at approximately 400 monitoring stations scattered throughout the U.S. Most of these sites were located in major urban areas or in other areas where high oxidant concentrations prevailed. Estimates of populations at risk and per capita health costs were generated for those areas where oxidant data was available. During the period 1971-1973, nearly two-thirds of the U.S. population resided in areas where the hourly primary standard for oxidants of 160 micrograms/cu m was exceeded. The total annual health cost attributable to oxidants was estimated to range from $120 to over $240 million in the U.S.