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. The British Columbia AQIs are directly comparable to AQIs issued in all major Canadian cities, since B.C. follows the same federal guidelines.

The AQI numbers are interpreted thus:

The main purpose is to inform the public about the present state of air quality. Having a daily Air Quality Index available via the media builds an awareness in the public: people will, for the first time, 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.

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:

Particulate matter (PM₁₀) is matter containing airborne particles with a diameter less than 10 microns (micrometres). Such particles are able to penetrate deep into the respiratory tract and make breathing more difficult. The latest research indicates that for each increment of 10 that the AQI exceeds 20, there are the following health outcomes related to inhalable particulates (PM₁₀):

• a 0.8% increase in hospitalizations and a 1% increase in emergency room visits for respiratory illnesses;
• a 9.5% increase in days of restricted activity due to respiratory symptoms;
• a 4.1% increase in school absenteeism, and a 1.2% increase in reporting of cough.

(Source: Health Effects of Inhalable Particles: Implications for British Columbia).

The people who are most sensitive to the impacts of PM₁₀ include children, the elderly, and those with pre-existing lung and heart disease.

For more information about fine particulates, see Particulate Matter. Also, the Environmental Quality Branch site has several publications on fine particulates and smoke, including: Fine Particulates: What They Are and How They Affect Us.