What Is An Air Quality Index (AQI)?

The Air Quality Index is a rating scale for outdoor air in Ontario. The AQI is a numerical scale ranging from 0 - 100+. The following scale is used to explain the quality of outdoor air:

AQI Level

Air Quality

Health Effects

0 - 15


No known health effects.

16 - 31


No known health effects for the majority of the population.

32 - 49


Some adverse health effects for very sensitive people.

50 - 99


The most sensitive people and animals may have adverse health effects. Vegetation and property may also be damaged.



May affect the health of a large number of people and animals as well as plant life and property.

How is the Air Quality Index calculated?

The Ministry of the Environment measures the following six key pollutants as part of the AQI:

These pollutants were selected because of their potential for adverse human health effect and environmental impact when levels are high. Monitoring data is collected from 33 air monitoring stations across Ontario.

On an hourly basis, the Ministry of the Environment collects concentration data for each of the six key pollutants. This data is compared to the Ambient Air Quality Criterion (AAQC), the maximum safe level for each pollutant. If any one of the six pollutants exceed the Ambient Air Quality Criterion, the pollutant will begin to have an undesirable impact on people and the environment. Using the AAQC, the monitoring data are converted into a number from zero upward using a common scale/index known as the Air Quality Index (AQI).

Of the six pollutants measured, the pollutant with the highest AQI number becomes the "overall" AQI for a particular location (e.g. York-Durham). For example, if the AQI for ozone is 20 and this happens to be the highest number out of the six pollutants measured, then this number would be reported as the overall AQI for a particular location. More specifically, the Ministry of the Environment would report this as an "AQI of 20, Reason: Ozone."

Where can I obtain current Air Quality Index readings and smog alert information?