What is the AQI?
The Air Quality Index (AQI) is a number used by government agencies to communicate to the public how polluted the air currently is or how polluted it is forecast to become. The AQI focuses on health effects you may experience within a few hours or days after breathing polluted air.
The AQI Scale
The AQI runs from 0 to 500. The higher the AQI value, the greater the level of air pollution and the greater the health concern. An AQI value of 50 or below represents good air quality, while an AQI value over 300 represents hazardous air quality.
How is AQI Calculated?
The AQI is calculated for five major air pollutants regulated by the Clean Air Act:
- Ground-level ozone (O3)
- Particle pollution (PM2.5 and PM10)
- Carbon monoxide (CO)
- Sulfur dioxide (SO2)
- Nitrogen dioxide (NO2)
The AQI value reported is always for the pollutant with the highest concentration. For example, if PM2.5 has an AQI of 120 and ozone has an AQI of 85, the reported AQI would be 120, and PM2.5 would be the "dominant pollutant."
Who Should Pay Attention to the AQI?
Everyone should be aware of the AQI, but certain groups are more sensitive to air pollution:
- People with lung diseases such as asthma and COPD
- People with heart disease
- Children and teenagers
- Older adults
- People who work or exercise outdoors
Taking Action Based on the AQI
Use the AQI to plan your daily activities. If the AQI is forecast to be unhealthy, you may want to reschedule outdoor activities to avoid exposure. Here are some general guidelines:
- 0-50 (Good): No precautions needed
- 51-100 (Moderate): Unusually sensitive people should consider reducing prolonged or heavy outdoor exertion
- 101-150 (Unhealthy for Sensitive Groups): Sensitive groups should reduce prolonged or heavy outdoor exertion
- 151-200 (Unhealthy): Everyone should reduce prolonged or heavy outdoor exertion
- 201-300 (Very Unhealthy): Everyone should avoid prolonged or heavy outdoor exertion
- 301+ (Hazardous): Everyone should avoid all outdoor physical activity
Local vs. National AQI
The AQI is location-specific. Air quality can vary significantly from one area to another, even within the same city. That's why it is important to check the AQI for your specific location, not just a general regional forecast.
Quick Tip
Check the AQI forecast in the morning and again in the afternoon. Air quality can change throughout the day, especially ozone levels which tend to peak in the afternoon during warm, sunny weather.