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A man skates with his dog as smog hangs over Toronto. ((Adrian Wyld/Canadian Press))

The Air Quality Health Index has been in the works since June 2001, when the federal government started consultations with a number of groups on how to replace the widely used Air Quality Index (AQI).

The goal was to come up with a tool that would give people a clearer idea of how day-to-day pollution levels could put their health at risk.

The AQI provides a general idea of air pollution in a specific place and time. It is expressed as a numerical scale that goes from one to 100 as well as a rating such as good, fair or poor. It is an index of air quality — but is not an expression of risk to your health.

The index looks at levels of pollutants such as sulphur dioxide, ozone, nitrogen dioxide, total reduced sulphur compounds, carbon monoxide and fine and coarse particulate matter. The readings are based on whichever pollutant is highest at the time of reading or predicted to be during the forecast period (usually the next day or two).

The following are the ratings under the index:

  • 1-15: very good.
  • 16-31: good.
  • 32-49: moderate.
  • 50-99: poor.
  • 100+: very poor.

The ratings and categories are set provincially and may vary slightly across the country. But, generally, a good rating means there are relatively low levels of air pollution in the atmosphere. Bump it up to moderate and people who are very sensitive to pollution may experience some adverse effects. A reading of poor will keep people with respiratory problems indoors and could expose most people to short-term adverse effects, such that they might want to refrain from exercising outdoors.

Environment Canada will issue an air quality advisory in a region when the index exceeds — or is expected to exceed — 50 for several days.

Health Canada notes there are inconsistencies in how air quality indices are reported across the country. For instance, the same reading might trigger a smog alert in Toronto but only an air quality advisory in Montreal.

"The thresholds used in determining AQI levels and categories are based on outdated health science and tend to reflect environmental regulatory imperatives rather than implications for human health," the agency notes on its website.

The Air Quality Health Index was launched in 14 communities in B.C. in the summer of 2006. It has been extended to communities in every province except Alberta. It has not been extended to Yukon, the Northwest Territories or Nunavut.

The index is a scale (from one to 10) that was designed to help people understand what the air quality means to their health. The higher the number, the greater the health risk for most people. People who are in "at risk" groups would be exposed to greater health risks at levels most people would find tolerable.

The AQHI is calculated based on the relative risks of a combination of air pollutants including:

  • Ozone at ground level.
  • Fine particulate matter.
  • Nitrogen oxide.
  • Sulphur dioxide

Each is known to be harmful.

The ratings are as follows:

AQHI risk level AQHI value  At-risk population  General population 
 Low  0-3 Enjoy usual outdoor activities. Follow doctor's advice on exercise.  Ideal conditions.
 Moderate  4-6 Consider reducing outdoor activities if you show symptoms of heart or breathing problems.  No need to modify your usual outdoor activities.
 High  7-10 Children, the elderly and people with heart or breathing problems should reduce or reschedule outdoor physical exertion until levels are lower. Anyone experiencing discomfort such as coughing or throat irritation should consider reducing or rescheduling strenuous outdoor activities to periods when the index is lower. 
 Very high  10+ Avoid outdoor physical exertion. Limit strenuous outdoor activities. 

The at-risk population includes people with respiratory or cardiovascular conditions, young children, the elderly and people who are frequently active outdoors (either by participating in sports or in doing strenuous work).

You may be sensitive to air pollution if you exhibit the following symptoms:

  • Irritated eyes.
  • Increased mucus production in the nose or throat.
  • Cough.
  • Difficulty breathing, especially during exercise.