What is the Air Quality Index?
An Air Quality Index (AQI) is a way of transforming complex air quality measurements into a single number or descriptive term. It describes both the measured air quality and the publicly perceived air quality at any given time.

How is an AQI interpreted?
The AQI numbers are interpreted thus:
0 to 25 Good
26 to 50 Fair
51 to 100 Poor
100+ Very Poor

Why issue an Air Quality Index?
The main purpose is to inform the public about the present state of air quality. Having a daily Air Quality Index available builds an awareness in the public: people will be able to draw comparisons between the measured air quality and what they directly sense.

For asthmatics and other concerned residents, it may even influence their behaviour. For example, in a situation where the AQI is high (above 50), people with respiratory problems may choose to refrain from strenuous exercise or temporarily avoid the polluted region.

How is an AQI calculated?
Common air pollutants are measured continuously at state-of-the-art monitoring stations throughout British Columbia. This information is gathered by a central computer in Victoria and an hourly AQI value is then calculated.

The AQI is not an amalgamation of all measured pollutants, but the value of the pollutant with the highest AQI number. Every hour, each pollutant's concentration is converted into an AQI number according to a methodology developed by Environment Canada and the provincial environment ministries. The highest AQI number becomes that hour's overall AQI value. When issuing the AQI it is common practice to note which pollutant is "driving" the AQI.

The AQI for a given concentration of each pollutant is determined from a set of straight line interpolations that start at zero and go through break-points at AQI values of 25, 50 and 100. These break-points represent the limits of good, fair and poor air quality. The ambient (outdoor surrounding air) values of the break-points for the different pollutants measured over a set of averaging periods are as follows:

Parameter Sulphur Dioxide Carbon Monoxide Nitrogen Dioxide Ozone Particulates
<10 micrometres
(µg)
Averaging Time 1 Hour 24 Hour 1 Hour 8 Hour 1 Hour 1 Hour 24 Hour
Unit of Measure ppm ppm ppm ppm ppm ppm µg/m3
Break-point:              
AQI = 25 0.17 0.06 13 5.0 0.105 0.05 25
AQI = 50 0.34 0.11 30 11.0 0.210 0.08 50
AQI = 100 2.00 0.30 64 17.4 0.530 0.15 100


What is the Odour Index?
The Odour Index is calculated separately from the Air Quality Index, and is used to indicate the presence of pollutants that, aside from their odour, do not at low concentrations pose a concern to health or the environment. Pollutants that create an odour problem are generally comprised of compounds that contain sulphur in its reduced chemical state.

An Odour Index of 26 or greater indicates that reduced sulphur compounds are present in a total concentration greater than the "Desirable" Provincial Ambient Level. An Index of 51 or greater indicates concentrations in excess of the "Interim" Provincial Ambient Level.

The Odour Index for a given Reduced Sulphur concentration is determined from a set of linear interpolations starting at zero and having break-points at values of 25, 50 and 100. The ambient values of the break-point concentrations measured over 1-hour and 24-hour averaging periods are as follows:

Parameter Total Reduced Sulphur
Averaging Time 1 Hour 24 Hour
Unit of Measure ppm ppm
Break-point:    
Odour Index = 25 0.005 0.002
Odour Index = 50 0.020 0.004
Odour Index = 100 0.050 0.006