Environmental Public Health Tracking: Air Quality Data

Read frequently asked questions about the air quality data

Ozone

Annual days above standard

This measure is the annual number of days with maximum eight-hour average ozone concentration above the National Ambient Air Quality Standard. The measure includes both monitored and modeled data. The monitored data comes from the U.S. Environmental Protection Agency (EPA) Air Quality System (AQS). When AQS data are available from multiple monitors for a given county and day, the highest eight-hour maximum (daily) ozone concentration among all the monitors is selected for purposes of creating daily county-level data. EPA provides modeled estimates of ozone using Downscaler (DS) model, which uses a statistical approach to fuse monitored data in areas where monitors exist, and relies on Community Multi-scale Air Quality (CMAQ) modeled output in areas without monitors. DS modeled estimates are available by census tract centroid; the geographic center of the census tract. Daily county-level modeled estimates are obtained by selecting the maximum value observed among all the census tracts within each county. County-level ozone measures are created using monitor data when available and using modeled estimates for days and locations without such data.

Annual person-days above standard

This measure is the annual number of person-days with maximum eight-hour average ozone concentration above the National Ambient Air Quality Standard. The measure includes both monitored and modeled data. The monitored data comes from the U.S. Environmental Protection Agency (EPA) Air Quality System (AQS). When AQS data are available from multiple monitors for a given county and day, the highest eight-hour maximum (daily) ozone concentration among all the monitors is selected for purposes of creating daily county-level data. EPA provides modeled estimates of ozone using Downscaler (DS) model, which uses a statistical approach to fuse monitored data in areas where monitors exist, and relies on Community Multi-scale Air Quality (CMAQ) modeled output in areas without monitors. DS modeled estimates are available by census tract centroid; the geographic center of the census tract. Daily county-level modeled estimates are obtained by selecting the maximum value observed among all the census tracts within each county. County-level ozone measures are created using monitor data when available and using modeled estimates for days and locations without such data.

Particulate matter less than 2.5 micrometers (PM2.5)

Annual person-days above standard

This measure is the annual number of person-days with particulate matter less than 2.5 micrometers (PM2.5) levels above the National Ambient Air Quality Standard. The measure includes both monitored and modeled data. The monitored data comes from the U.S. Environmental Protection Agency (EPA) Air Quality System (AQS). When AQS data are available from multiple monitors for a given county and day, the highest 24-hour average (daily) PM2.5 concentration among all the monitors is selected for purposes of creating daily county-level data. EPA provides modeled estimates of PM2.5 using Downscaler (DS) model, which uses a statistical approach to fuse monitored data in areas where monitors exist, and relies on Community Multi-scale Air Quality (CMAQ) modeled output in areas without monitors. DS modeled estimates are available by census tract centroid; the geographic center of the census tract. Daily county-level modeled estimates are obtained by selecting the maximum value observed among all the census tracts within each county. County-level PM2.5 measures are created using monitor data when available and using modeled estimates for days and locations without such data.

Average annual concentration (μ/m3)

This measure is the annual average ambient concentration of particulate matter less than 2.5 micrometers (PM2.5) in micrograms per cubic meter (based on seasonal averages and daily measurements). The measure includes both monitored and modeled data. The monitored data comes from the U.S. Environmental Protection Agency’s (EPA) Air Quality System (AQS). When AQS data are available from multiple monitors for a given county and day, the highest 24-hour average (daily) PM2.5 concentration among all the monitors is selected for purposes of creating daily county-level data. EPA provides modeled estimates of PM2.5 using Downscaler (DS) model, which uses a statistical approach to fuse monitored data in areas where monitors exist, and relies on Community Multi-scale Air Quality (CMAQ) modeled output in areas without monitors. DS modeled estimates are available by census tract centroid the geographic center of the census tract. Daily county-level modeled estimates are obtained by selecting the maximum value observed among all the census tracts within each county. County-level PM2.5 measures are created using monitor data when available and using modeled estimates for days and locations without such data.