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Noise monitoring program

Air Quality Index (AQI)

The DNR uses weather forecasts and data from air monitoring sites to determine air quality throughout the state, and notifies residents in real time when pollutants reach unhealthy levels. The DNR uses the U.S. EPA's Air Quality Index (AQI) to inform the public of current pollutant levels. The AQI uses colors and corresponding numbers to communicate the current air quality for air pollutants regulated by the Clean Air Act. To protect public health, DNR issues an air quality advisory for a county when the AQI exceeds, or is expected to exceed, the ozone or fine particle (PM2.5) standard.

How AQI ranges correspond to ozone and PM2.5 concentrations


Color coded chart showing ozone and PM2.5 concentration ranges for each AQI level
AQI Levels
of Health Concern
Numerical Value8-hr Ozone (ppb)24-hr PM2.5 (ug/m3)
Good0-500-540.0 to 12.0
Moderate51-10055-7012.1 to 35.4
Unhealthy for Sensitive Groups101-15071-8535.5 to 55.4
Unhealthy151-20086-10555.5 to 140.4
Very Unhealthy201-300106-200140.5 to 210.4
Hazardous> 300> 201> 210.4

The ozone AQI is calculated from 8-hour rolling average concentrations. The PM2.5 AQI is calculated using the 24-hour rolling average concentrations from the continuous monitors.

Air quality advisories are issued when the AQI exceeds (or is expected to exceed) a value of 100.

  • AQI calculations for the current day, as shown on the map and reports, use NowCast AQI calculations.*
    • NowCast AQI is a predictive model that shortens the averaging period and weights the most recent hours more heavily.
    • NowCast AQI is not used for regulatory purposes, including comparison to the National Ambient Air Quality Standards.
  • When a new day begins, AQI values for the previous day are recalculated using the official AQI calculation; therefore, values for previous days will correspond to regulatory values.
  • To help users better understand current air quality conditions, 8-hour Ozone and Ozone AQI timestamps use an "hour ending" convention (e.g., the "08" hour timestamp includes the hours of 01:00 – 08:00). Conversely, for regulatory purposes, a "running forward" convention (e.g., the "08" hour timestamp includes the hours of 08:00 – 15:00) is used.