Why Ottawa's air quality is among the worst in Canada

Smoke from wildfires in and around eastern Ontario and western Quebec have led to several air quality warnings issued in the surrounding cities.

Ottawa's air quality has been the worst among major Canadian cities at several points over the past two days, which a meteorologist says is thanks to a combination of the city's geography and wind patterns.

On Wednesday morning, Environment Canada's Air Quality Health Index was a very high 10+ in Ottawa, which the weather agency categorized as "very high risk." It stayed that way until Wednesday afternoon, and as of 4 p.m., was back to a moderate risk level.

Wind patterns are funnelling the wildfire smoke straight into Ottawa, said Steven Flisfeder, a warning preparedness meteorologist with Environment Canada.

With it, several contaminants that are polluting the air.

"In addition to that, there is the valley effect. So depending on the season, the valley kind of helps to trap those contaminants, especially in the overnight periods where we have the air cooling," he said.

As the air cools, the contaminants are brought further to the ground and trapped at the surface, leading to "very high" smoke concentrations, especially in the mornings.

"Typically we want a threshold of those concentrations to be below 51 micrograms, but over the past couple of days we've been seeing it pushed to a 300- or 400-microgram level, so a very elevated level of contaminants in the air," said Flisfeder.

As the weather warms throughout the day, the warmer air is able to push the polluted air upwards and the air quality improves.

'Unusually high' levels of harmful fine particles

Eric Lavigne, an adjunct professor at the University of Ottawa's School of Epidemiology and Public Health, says Environment Canada's air quality index indicates when there is a high level of harmful fine particles in the air.

In Ottawa, particles have been at an "unusually high level," he said.

"Usually in the Ottawa region we see on a daily basis probably around between five and 10 micrograms per cubic metre," said Lavigne.

With the smoke over the past few days, Ottawa is now having around 200 micrograms of particles per cubic metre of air, he said.

The inhalation of those small particles "can be infiltrated deep into the lungs and create inflammation," which can develop into symptoms including severe coughing and dizziness.

Ottawa's air quality conditions are expected to slightly improve Wednesday night, and remain at a more moderate risk level on the air quality index.

Winds will shift slightly eastwards, pushing the smoky air further west.

"There will be some improvements over the next 24 hours, but we're not completely out of the risk in the coming days," Flisfeder said, adding a deterioration in air quality is still expected.