Why the air quality index needs an upgrade to better measure wildfire smoke

In a summer of historic wildfires, Canadians have undoubtedly found themselves checking the air quality more than usual.

But the way that number is calculated varies by province, and experts say it will need to be changed to better reflect the risks and prevalence of wildfire smoke.

For millions of Canadians, the particulate matter turning their skies grey or orange in the summer — and leading to health impacts over time — isn't fully showing up in the metrics they see.

Here's a closer look at the issue.

Canada's Air Quality Health Index (AQHI) is based on three different pollutants: ozone, nitrogen oxide and particulate matter (PM2.5).

All three are found in wildfire smoke, though there are especially high levels of PM2.5.

Wildfires release tiny, health-harming particles 2.5 microns or smaller in size — smaller than a human hair — that can penetrate your lungs and enter your bloodstream and negatively impact your health.

The AQHI is on a scale between one and 10+, and is designed to provide the public with a sense of the risk involved in exposure.

The idea is simple — the higher the number, the higher the health risks, particularly for seniors, children and people with pre-existing health conditions such as cardiovascular disease.

The original metric, still in use in Ontario, Quebec and Alberta, essentially averages the risk of the three pollutants.

But experts say AQHI doesn't work as effectively in instances where there is a lot of wildfire smoke and not other pollutants, since the average is lower than the risk presented by PM2.5.

For instance, on the morning of Aug. 16, Toronto had an AQHI measuring four (moderate risk), even though the concentration of PM2.5 was 42.6 micrograms per cubic metre, which is considered "very poor."

Note: An 11 on this chart corresponds to a 10+ on the AQHI scale.

British Columbia, which is no stranger to wildfire smoke, pioneered a slightly modified version of the index, known as AQHI-plus.