Understanding the air quality index (AQI)
The Air Quality Index (AQI) was developed by the U.S. Environmental Protection Agency (EPA) to provide a simple, uniform way to report daily air quality conditions. Minnesota AQI numbers are determined by hourly measurements of five pollutants:
- fine particles
- ground-level ozone
- sulfur dioxide
- nitrogen dioxide
- carbon monoxide
The pollutant with the highest AQI value determines the overall AQI for that hour. The MPCA produces daily air quality forecasts for levels of fine particles and of ground-level ozone (from March through October) in eighteen locations in Minnesota. When the forecast is for AQI values to be near or to exceed 101, an air quality alert is issued.
AQI forecasts heavily depend on temperatures, precipitation, wind, and cloud cover. Weather affects pollution creation and transport from other areas. The AQI forecasts enhance human health by helping residents avoid risks from air pollution.
Fine-particle pollution is measured year round in all reporting areas. Ozone pollution is monitored from April through October in all reporting areas except Grand Portage and Virginia. Carbon monoxide, sulfur dioxide, and nitrogen dioxide rarely influence the AQI and are only measured at a few sites. The AQI does not account for temperature or pollen levels, which may increase sensitivity to air pollutants.
Calculating the Air Quality Index
The AQI is calculated by converting measured pollutant concentrations to a uniform index which is based on the health effects associated with a pollutant. The health benchmarks used for calculating the AQI are pollutant specific and are established by the EPA through the National Ambient Air Quality Standards. The Clean Air Act requires the EPA to review these standards every five years.
Air quality conditions in areas without air pollution monitors are estimated based upon the nearest air monitor, including those in surrounding states. The estimates may not reflect localized air quality events, such as fires. Current conditions are not estimated for northwestern Minnesota because there are not enough monitors.
Air quality alerts
An air quality alert is issued when measured or forecasted air quality conditions are greater than 101 on the index. An air quality alert means current or forecasted conditions can be harmful to those who are sensitive to air pollution, including:
- People with lung or heart disease
- Older people
- Children
- People exerting themselves in prolonged, heavy activities
Most poor air quality days are not caused by a short-term increase in emissions from industries, cars, or other sources (notable exceptions include wildfire smoke or fireworks). Instead, air quality alerts are most often driven by weather conditions that increase the rate at which air pollutants are formed or accumulate in the air.
For example, ozone is formed by a chemical reaction between nitrogen oxides and volatile organic compounds in the air, and hot, sunny weather speeds up the process. Similarly, high humidity or pressure, strong overnight temperature inversions, or low wind speeds can cause fine particle pollution to increase. Fine particles are emitted from cars or other pollution sources but are also created in reactions between sulfur oxides, nitrogen oxides, and ammonia in the air.
Wildfire smoke
In recent years, more wildfires are burning bigger and hotter in the western U.S. and central and western Canada, and are expected o become more frequent, with longer burning seasons. “Mega fires” like those that have been devastating California are expected to continue. Climate change, past fire-suppression practices, and populations expanding into wildfire regions are all playing a role in the increasing intensity and number of wildfires.
In Minnesota, higher AQIs in the summer are increasingly attributed to smoke from the Canadian wildfires and those in the northern Rockies. The MPCA continually monitors wildfire activity to track how they will affect Minnesota’s air quality.
