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The U.S. Environmental Protection Agency (EPA) promulgates the National Ambient Air Quality Standards (NAAQS) on the basis of scientific information contained in air quality criteria documents. The previous ozone (O3) criteria document, Air Quality Criteria for Ozone and Other Photochemical Oxidants, was released in August 1986 and a supplement, Summary of Selected New Information on Effects of Ozone on Health and Vegetation, was released in January 1992. These documents were used as the basis for a March 1993 decision by EPA that revision of the existing 1-h NAAQS for O3 was not appropriate at that time. That decision, however, did not take into account some of the newer scientific data that became available after completion of the 1986 criteria document. The purpose of this revised air quality criteria document for O3 and related photochemical oxidants is to critically evaluate and assess the latest scientific data associated with exposure to the concentrations of these pollutants found in ambient air. Emphasis is placed on the presentation of health and environmental effects data; however, other scientific data are presented and evaluated in order to provide a better understanding of the nature, sources, distribution, measurement, and concentrations of O3 and related photochemical oxidants and their precursors in the environment. Although the document is not intended to be an exhaustive literature review, it is intended to cover all pertinent literature available through the end of 1993.