This document consolidates and assesses current knowledge regarding the origin of ozone and other photochemical oxidants and discusses their effect on health, vegetation, certain ecosystems, and materials. This document summarizes current data on the effects of oxidant/ozone in the ambient air on man, vegetation, and ecosystems. The effects that have been observed will form the scientific basis for supporting the present National Ambient Air Quality Standard of 160 micrograms/cu m (0.08 ppm) or a revised standard. Although nitrogen dioxide is considered one of the photochemical oxidants, oxides of nitrogen are the subject of a separate report and are therefore discussed in this document only as they participate in the formation and reactions of other photochemical oxidants. Hydrocarbons and other organics are important air pollutants because they too are precursors of other compounds formed in the atmospheric photochemical system. In this document, toxic organics are considered only with respect to eye irritation. The studies and data cited constitute the best available basis for specific standards aimed at protecting human health and the environment from photochemical oxidants in ambient air.