This statistic is measured at the level of the Metropolitan Statistical Area (MSA). The number of unhealthy air days for a given MSA is applied to each city in the study that is part of that MSA. The Pollutant Standards Index (PSI) integrates information on criteria pollutant concentrations (CO, O3, PM10, SO2) across an entire monitoring network into a single number that represents the worst daily air quality experienced in an urban area. PSI values used in this study are based on ozone alone.

"Table A-16. (Ozone only) Number of days with PSI Values Greater Than 100 at Trend Sites 1988-1997, and All Sites in 1997," National Air Quality and Emission Trends Report 1997, Office of Air Quality Planning and Standards, U.S. Environmental Protection Agency.