How are the number of person-days exceeding the NAAQS calculated?
The person-days air quality measure takes into account both the number of days that the maximum 8-hour ozone concentration exceeds the NAAQS and the population within the county where monitors are located. The maximum 8-hour daily concentration for ozone is calculated based on the methodology presented above. The number of persons is derived from the Census Bureau population estimates. The Census Bureau typically updates the county-level population estimates for the years between censuses on an annual basis. The census county-level population is adjusted annually by considering births, deaths, immigration and other information sources. Documentation on the methodology used by the Census Bureau in estimating county-level can be found at http://www.census.gov/popest/topics/methodology/2009-st-co-meth.pdf.
