Air quality monitors measure concentrations of ozone throughout the country. EPA, state, tribal and local agencies use that data to ensure that ozone is at levels that protect public health and the environment. Nationally, average ozone levels declined in the 1980's, leveled off in the 1990's, and showed a notable decline after 2002. The charts below show national and regional trends in ozone concentrations. For information on ozone standards, sources, health effects, and programs to reduce ozone, please see our Ground-level Ozone Pollution page.

EPA uses a statistical model to account for the weather-related variability of seasonal ozone concentrations to provide a more accurate assessment of the underlying trend in the precursor emissions that cause ozone. See Trends in Ozone Adjusted for Weather Conditions.