Air Quality Health Index – Calculation

National AQHI

The formula developed to calculate the national Air Quality Health Index (AQHI) is based on research conducted by Health Canada using health and air quality data collected in major cities across Canada, including Calgary and Edmonton.

The AQHI represents the relative risk of a mixture of common air pollutants which are known to harm human health. Three pollutants were chosen as indicators of the overall outdoor air mixture.

Pollutants

Ground-level Ozone (O₃): Ground-level ozone is formed by photochemical reactions in the atmosphere. It mainly forms from vehicle and industrial emissions in urban centres. It can be a major component of smog during the summer, especially during hot sunny weather, and is generally low in the winter. Ozone can be transported long distances and can be responsible for large regional air pollution episodes.

Fine Particulate Matter (PM_2.5): Fine particulate matter is a mixture of tiny airborne particles that can be inhaled deep into the lungs. These particles can either be emitted directly by vehicles, industrial facilities, natural sources like wildfires, or formed indirectly as a result of chemical reactions among other pollutants. Particulate matter can reflect local air pollution sources and widespread air pollution.

Nitrogen Dioxide (NO₂): Nitrogen dioxide is released by motor vehicle emissions and industrial processes that rely on fossil fuels. Nitrogen dioxide contributes to the formation of the other two pollutants. It is often elevated in the vicinity of high traffic roadways and local industrial sources.

These three pollutants can threaten human health even at low levels of exposure and especially among those with pre-existing health problems.

In the development of the AQHI, a formula that combined these three pollutants was found to be the best indicator of health risk.

For more information on how the national AQHI formulation was developed see:

• A New Multi-pollutant, No-Threshold Air Quality Health Index Based on Short-Term Associations Observed in Daily Time-Series Analyses

AQHI in Alberta

In Alberta, the national Air Quality Health Index (AQHI) formulation is augmented to better account for rapidly changing air quality and to include additional pollutants.

The AQHI value is overridden when appropriate criteria are met to better reflect air quality conditions experienced by Albertans. The following pollutants are considered in Alberta’s AQHI:

• Carbon monoxide
• Fine particulate matter
• Hydrogen sulphide and total reduced sulphur
• Nitrogen dioxide
• Ozone
• Sulphur dioxide

Hourly concentrations of each pollutant are compared against Alberta’s Ambient Air Quality Objectives (AAAQOs) or informed by Occupational Health and Safety Standards. For fine particulate matter, the override formula for hourly concentrations is consistent with bordering provinces and territories for harmonized reporting of air quality conditions, especially during wildfire smoke events.

For more information about the fine particulate matter override, see:

• Assessment of the Air Quality Health Index (AQHI) and four alternate AQHI-Plus amendments for wildfire seasons in British Columbia

Alberta also includes special messaging for odour or visibility events. Special community level messaging is provided in cases when concentrations of specific pollutants are higher than specified odour or visibility thresholds and the AQHI is rated as Low or Moderate risk. 

The air pollutants and their odour or visibility thresholds are:

• 31 micrograms per cubic metre for fine particulate matter (based on visibility)
• 100 parts per billion for sulphur dioxide (based on odour)
• 10 parts per billion for hydrogen sulphide or total reduced sulphur (based on odour)

An example of this special messaging provided on the Alberta Air Quality Health Index map is indicated below: