Ozone remains an important phytotoxic air pollutant and is also recognized as a significant greenhouse gas. In North America

Europe, and Asia, incidence of high concentrations is decreasing, but background levels are steadily rising. There is a need to develop a biologically signif-icant and usable standard for ozone. We compare the strengths and weaknesses of concentration-based, exposure-based and threshold-based in-dices, such as SUM60 and AOT40, and examine the O3 flux concept. We also present major challenges to the development of an air quality standard for ozone that has both biological significance and practicality in usage