The present secondary National Ambient Air Quality Standard (NAAQS) for ozone, established in 1979, was based on limited information from a few field studies on vegetation and may not be protective of crop damage due to ozone, alone or in combination with other pollutants. Considerable information from field studies conducted in the past decade indicate that there is poor functional correspondence between agricultural production and the index of the current secondary NAAQS for ozone. Because the current ozone NAAQS is based on data for extreme values of air quality, the index of the NAAQS ignores key features of hourly exposure data that are desirable and important to minimizing the risk of crop damage. Improved federal ozone standards, sufficient to protect vegetation, require modifications to the form and averaging period of the current NAAQS with minimum increase to the number of nonattainment and crossover areas. Air quality indicators based on exposure indices developed in work involving ozone effects on crops show considerable improvement over modifications to the current NAAQS that lower the standard level to achieve a comparable level of protection, with respect to changes in nonattainment status.