What is the Air Quality Index?

The Air Quality Index (AQI) is an indicator of air quality, based on air pollutants that have adverse effects on human health and the environment. The pollutants are ozone, fine particulate matter, nitrogen dioxide, carbon monoxide, sulphur dioxide and total reduced sulphur compounds.

Several state-of-the-art air monitoring stations, operated by the Ministry across the province, form the Air Quality Index (AQI) network.

The ministry takes real-time air quality data from its AQI monitoring sites to produce AQI readings for each location. AQI readings are reported to the public and news media hourly. The public can access the index values by calling the ministry's automatic telephone answering device (English recording:1-800-387-7768 or in Toronto 416-246-0411. French recording: 1-800-221-8852). The AQI can also be obtained from the ministry's Web site: www.airqualityontario.com

Here's how an AQI is determined:

  • At the end of each hour, the concentration of each pollutant, measured at each site, is converted into a number ranging from zero upwards, using a common scale, or index. The calculated number for each pollutant is referred to as a sub-index.
  • At a given site, the highest sub-index for any given hour becomes the AQI reading for that hour. The index is a relative scale, in that, the lower the index, the better the air quality.

Here's what the readings mean:

  • If the AQI reading is below 16, the air quality is in the very good category.
  • If the AQI reading is in the range of 16 to 31, the air quality is in the good category.
  • If the AQI reading is in the range of 32 to 49, the air quality is in the moderate category, and there may be some adverse effects for very sensitive people.
  • If the AQI reading is in the range of 50 to 99, the air quality is in the poor category, and may have adverse effects for sensitive members of human and animal populations, and may cause significant damage to vegetation and property.
  • If the AQI reading is above 99, the air quality is in the very poor category, and may have adverse effects for a large proportion of those exposed.