The objective of the study is to develop an ambient ozone air quality indicator for examining O3 and its effects on vegetation and forests. The scientific and technical information on O3 and its effects on vegetation and natural ecosystems indicate that a secondary ambient air quality standard is needed that correlates well with both short- and long-term exposures. Air quality analysis of hourly ozone monitoring data from the National Air Data Branch for 82 non-urban sites in the United States was performed to identify the relationships of various ozone exposure indices. A three-month maximum period between April and September is adequate for capturing about 70% of the hourly concentrations of 0.06 ppm or higher. The cumulative censored indices that adequately predict crop loss are strongly correlated with a long-term single peak index. These indices that integrate hourly concentrations of 0.06 (or 0.7) ppm or higher have potential for setting a standard that minimizes the risk of adverse effects.