Air Quality Index (AQI) Basics

The U.S. Air Quality Index (AQI) is EPA's tool for communicating about outdoor air quality and health. The AQI includes six color-coded categories, each corresponding to a range of index values. The higher the AQI value, the greater the level of air pollution and the greater the health concern. AQI values 50 and below are considered good while those above 100 are unhealthy for one or more groups.

The DNR uses weather and air monitoring data to forecast air quality throughout the state. When the AQI exceeds, or is expected to exceed, a value of 100 for ozone or fine particles (PM_2.5), the DNR issues an air quality advisory to protect public health. These advisories are issued at a county level. The table below displays AQI values with their associated pollutant concentrations.

Color coded chart showing ozone and PM_2.5 concentration ranges for each AQI level
AQI Levels of Health Concern	Numerical Value	8-hr Ozone (ppb)	24-hr PM_2.5 (ug/m³)
Good	0-50	0-54	0.0 to 9.0
Moderate	51-100	55-70	9.1 to 35.4
Unhealthy for Sensitive Groups	101-150	71-85	35.5 to 55.4
Unhealthy	151-200	86-105	55.5 to 125.4
Very Unhealthy	201-300	106-200	125.5 to 225.4
Hazardous	> 300	> 201	> 225.4

The ozone AQI is calculated from 8-hour rolling average concentrations. The PM_2.5 AQI is calculated using the 24-hour rolling average concentrations.

Ozone and particulate (PM_2,5 and PM₁₀) AQI displayed on the maps and reports for the current day 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, ozone and particulate AQI values for the previous day are recalculated using the official AQI calculation; therefore, values for previous days correspond to regulatory values.

To help users better understand current air quality conditions, this website shows timestamps for ozone AQI and 8-hour average ozone concentrations using an "hour ending" convention rather than the “running forward” convention used for regulatory purposes. Therefore an 08:00 timestamp on this website includes data for from 1 a.m. to 8 a.m. while regulatory data with an 08:00 timestamp would include data from 8 a.m. to 3 p.m.

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