Figure 1: U.S. counties with high ozone concentrations in 2009. This map depicts ozone concentrations by U.S. county for 2009, showing where high ambient ozone concentrations were found in the continental United States. The map's color key is based on the categories of the Air Quality Index (AQI) (see Patient Exposure and the Air Quality Index ). All orange, red, and purple areas exceeded the 8-hour ambient air quality standard for ozone during 2009. The map illustrates how likely it may be for a particular area to experience air quality advisories for ozone.