What’s the science behind air quality readings? Health Canada explains

Wildfire smoke in downtown Winnipeg on July 11, 2025. (CTV News Winnipeg)

Amid Winnipeg’s smokiest year on record, the Air Quality Health Index (AQHI) may sound familiar to many—but what factors determine the rating on the scale developed by Environment and Climate Change Canada and Health Canada?

According to Jeff Eyamie, an air quality health specialist with Health Canada, fine particulate matter—or particles that have a diameter of 2.5 micrometres or less (PM2.5)—is one of the determining factors when evaluating air quality.

“You’re talking about a particle size that is so small that it can infiltrate the natural filtration of your respiratory system,” said Eyamie.

According to Environment and Climate Change Canada’s website, fine particles represent the main health risk from wildfire smoke.

Eyamie said at that size, particles can get through your lungs and into your bloodstream—posing a hazard to your health.

These particles can exhibit a diameter less than one-twentieth the width of a human hair or less than one-quarter the diameter of a pollen grain, according to the U.S. Environmental Protection Agency.

Though fine particulate matter is formed during wildfires, Eyamie said many factors contribute to the composition of each plume of smoke.

“There are all kinds of compounds that are constantly being formed and reformed as the smoke travels and is affected by temperature and atmospheric conditions,” he said.

“Every single plume of smoke is different.”

Eyamie said that each integer on the AQHI, which ranges from 1 to 10+, equates to roughly 10 micrograms per cubic metre of PM2.5.

“So when you get those AQHI 10+ days, we’re looking at PM2.5 that is forecasted to be measured at 100 micrograms and higher,” he said.

The AQHI also takes into consideration the relative risks of nitrogen dioxide and ground-level ozone—other air pollutants known to harm human health. However, Eyamie said the levels of those compounds remain relatively stable when smoke is in the area compared to fine particulate matter levels.

Four well-calibrated scientific instruments across the province, including two in Winnipeg, form the foundation of the AQHI measurements in Manitoba, according to Eyamie.

“Use the AQHI forecast to gauge your behaviour so that you’re reducing exposures and also plan to be active when the air is good,” he said.