Ozone accumulates about 15 to 50 kilometers above the surface of the Earth in a protective layer that reflects ultraviolet radiation. Ground-level accumulations of ozone, however, are dangerous to living organisms. Ozone can corrosively damage plant surfaces and irritate animal tissues. Plants can also directly absorb ozone through their pores, which can severely inhibit their functioning and growth. Thus ozone has the potential to degrade overall ecosystem health and reduce crop productivity.
The ecological ozone metric seeks to specifically assess the impact of ozone on ecosystems. Ozone’s human health effects are measured separately in the environmental health category.
Our ecological ozone indicator measures the extent to which very high ozone concentrations are present during the vegetative growing season. Because ozone acutely affects plant growth and development, the growing season and daylight intensity are important factors in this metric. For the 2008 EPI we determined ozone exposure during summer daylight hours. Ozone’s negative effects on plants are most acute at particularly high levels or prolonged exposures. The parameter that we chose for assessing the critical level of ozone exposure for vegetation is the Accumulated Ozone Threshold of 40 parts per billion (ppb). Our target comes from the International Cooperative Programme on Effects of Air Pollution on Natural Vegetation and Crops and stipulates that long-term ozone exposure should not exceed 3000 ppb-hours over the three-month summer period (Mauzerall and Wong 2001). The 3000 ppb-hour figure is calculated by summing the ppb exposures for all hours that exceed the minimal 40 ppb threshold. For example, an hour of 50 ppb exposure and another hour of 40 ppb exposure sum to 90 “ppb-hours.”
