Air Quality Index (AQI)
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, the 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 ConcernNumerical Value8-hr Ozone (ppb)24-hr PM2.5 (ug/m3)Good0-500-540.0 to 12.0Moderate51-10055-7012.1 to 35.4Unhealthy for Sensitive Groups101-15071-8535.5 to 55.4Unhealthy151-20086-10555.5 to 140.4Very Unhealthy201-300106-200140.5 to 210.4Hazardous> 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.
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.
* On August 1, 2019, the EPA completed a technical change in the background of the AirNow website and updated the algorithm associated with Nowcast, which is used to estimate the AQI in real time. The EPA anticipates that the revised algorithm will improve the accuracy of predictions about ozone air quality at the local level, specifically reducing the number of days when the Nowcast ozone value reported on AirNow appears to be higher than the day’s AQI forecast and/or the daily AQI. This change requires programming to apply the revised Nowcast algorithm to the DNR Air Quality web map. Until this change is made there will be differences between the DNR web map and the AirNow web map.
