A PSI value is an approximate indicator of overall air quality, because it takes into account all of the pollutants measured within a county. Many counties have monitoring stations for some, but not all, of the pollutants.
PSI values reflect all of the available measurements in each county.
Each row of the Monitor PSI Report lists summary values for a single county and a single year. Sorting You can use the SORT buttons in each report column to change the order of rows in the report. The default sort order is median PSI value, from highest to lowest. AIR Data automatically adds maximum PSI value as a secondary sort key for most report columns. If a report includes rows with identical values for the primary sort key, which you choose with the SORT buttons, those rows are arranged in descending order by PSI maximum value.