About the National Data
Data Source: Air Quality System (AQS), EPA
Baseline: 4,295,962,018 Air Quality Index (AQI)-weighted people days exceeded 100 on the AQI in 2016-18
Target: 3,866,365,816 AQI-weighted people days
Steps in the calculation of weighted days with an Air Quality Index (AQI) greater than 100:
- For each year in the base period (e.g., for 2008 use 2006, 2007 and 2008), determine the daily AQI values by county.
- For days with AQI values over 100, divide the actual AQI value by 100.
- Sum these AQI-weighted days by year for each county.
- For each county, multiply the sum of yearly AQI-weighted days by the county's population for a reference year (e.g., 2008).
- Average the population-weighted and AQI-weighted value for each county across the three years in the period.
- Sum the three-year average population-weighted and AQI-weighted values across all counties to determine the national metric value.
The mix of trend sites changes from one year to the next. To ensure this metric is tracking changes in ambient concentrations (and not changes in the monitoring network), a fixed set of sites are selected that meet minimum completeness criteria during the trend period.