Air Quality Index

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The U.S. Environmental Protection Agency (EPA) has established an overall air quality index known as the Pollutant Standards Index (PSI). PSI values are derived from measured pollutant concentrations, which are reported daily in all urban areas of the United States with populations exceeding 200,000. The PSI is reported as a value between zero and five hundred, or as a descriptive word (e.g., "unhealthy"), and is often featured on local television or radio news programs and in newspapers.

Based on the short-term National Ambient Air Quality Standards (NAAQS), Federal Episode Criteria, and Significant Harm Levels, the PSI is computed for particulate matter that can penetrate into the lungs (PM10), sulfur dioxide (SO2), carbon monoxide (CO), ozone (O3), and nitrogen dioxide (NO2). Since the PSI is a tool used to communicate pollution concerns to a wide audience, there are also colors linked to the general descriptors of air quality.

The PSI integrates information on pollutant concentrations across an entire monitoring network into a single number that represents the worst daily air quality experienced in an urban area. For each of the criteria pollutants, concentrations are converted into an index value between zero and five hundred. The pollutant with the highest index value is reported as the PSI for that day. Therefore, the PSI does not take into account the possible adverse effects associated with combinations of pollutants.

A PSI value of 100 corresponds to the NAAQS established under the Clean Air Act. A PSI value greater than 100 (yellow) indicates that at least one criteria pollutant (with the exception of NO2) exceeded the level of the NAAQS, therefore designating air quality to be in the unhealthful range on that day. Relatively high PSI values activate public health warnings. For example, a PSI above 200 (orange) initiates a First Stage Alert, at which time sensitive populations (e.g., the elderly and persons with respiratory illnesses) are advised to remain indoors and reduce physical activity. A PSI above 300 (red) initiates a Second Stage Alert, at which time the general public is advised to avoid outdoor activity.

Since a PSI value greater than 100 indicates that the level of the NAAQS for at least one criteria pollutant has been exceeded on a given day, the number of days with PSI values greater than 100 provides an indicator of air quality in urban areas. Between 1988 and 1997, the total number of days with PSI values greater than 100 decreased 56 percent in Southern California and 66 percent in the remaining major cities across the United States.

PSI estimates depend on the number of pollutants monitored as well as the number of monitoring sites where data are collected. The more pollutants measured and sites that are available in an area, the better the estimate of the PSI for a given day. Ozone accounts for the majority of days with PSI values above 100, and the number of days with a PSI above 100 are increasingly due to ozone. In fact, the percentage of days with a PSI above 100 due to ozone increased from 92 percent in 1988 to 97 percent in 1997. The increase is even more dramatic when the unusual meteorology experienced in 1988 is taken into account (the 1989 percentage was 82%).

Morton Lippmann

(see also: Airborne Particles; Ambient Air Quality [Air Pollution]; Atmosphere; Carbon Monoxide; Environmental Determinants of Health; Hazardous Air Pollutants )


Environmental Protection Agency (1998). National Air Quality and Emissions Trends Report, 1997. Washington, DC: Author.