Note that the data published in the 2002 State of the Nation’s Ecosystems Report as well as the 2003 and 2005 Web-Only Updates have been superseded by the 2008 Report and thus should be used with caution. For the most recent data, purchase the 2008 Report from Island Press.

What Is This Indicator, and Why Is It Important? This indicator reports the percentage of air pollution monitoring stations in urban and suburban areas with "high" ozone concentrations at least 1, 4, 10, and 25 days a year. Ground-level ozone is considered high when the 8-hour average concentration exceeds 0.08 parts per million (ppm). Violations of federal air quality standards are triggered by ozone concentrations exceeding this level, on average, for 4 or more days per year over three consecutive years. For this reason, the maps show monitoring stations with less than 4 days and 4 or more days of high concentrations in 2004.

Ground-level ozone is one of the most pervasive air quality problems in the United States. Children and adults who are active outdoors, and people with respiratory diseases, are most likely to be harmed. Ozone can inflame the lungs, make people more susceptible to respiratory infection, and aggravate respiratory diseases such as asthma; repeated exposure may lead to permanent lung damage. High concentrations can harm trees, other plants, wildlife, and pets, and can damage painted surfaces, plastics, and rubber materials. In contrast, ozone in the upper atmosphere absorbs harmful ultraviolet radiation.

What Do the Data Show? In 2004, 11% of monitoring stations in urban and suburban areas recorded high ozone levels on 4 or more days. The percentage of urban and suburban monitors with high ozone levels on 4 or more days per year year. Fluctuations in air quality conditions are generally believed to result from a combination of year-to-year variability in weather conditions (ozone forms more readily in sunny, stagnant conditions) and changes in emissions of ozone forming compounds. EPA believes the large decreases in 2003 and 2004 resulted from a combination of weather and reduced emissions of ozone forming compounds (see technical note). The percentage of monitors recording high levels 25 or more times per year declined over the same period, to about 1% in 2004, all of which were in Southern California.

Discussion Ground-level ozone forms when pollutants from vehicles, paints and solvents, unburned fuel, and industrial sources “bake” in hot, sunny, stagnant weather. Ground-level ozone is one of six common air pollutants considered harmful to human health and the environment (the others are lead, carbon monoxide, nitrogen dioxide, sulfur dioxide, and particulate matter). While high concentrations of other pollutants do occur in some urban and suburban areas, ozone is responsible for more than 95% of all days with violations of any air quality standard.