Poor air quality may persist in Toronto on Thursday, Friday due to wildfire smoke

Poor air quality may persist in some areas of Toronto on Thursday and Friday due to forest fire smoke, Environment Canada says.

The federal weather agency issued a special weather statement for Toronto on Wednesday, saying "very high levels of air pollution" were expected in the city throughout the day and into the night.

Smoke plumes from fires over northeastern Ontario and Quebec have moved into the area, "resulting in deteriorated air quality," Environment Canada said in the statement.

At one point on Wednesday afternoon, Toronto's air quality was ranked worst among major cities of the world due to the forest fire smoke, according to a ranking provided by air quality tracker IQAir.

Later in the evening, IQAir ranked Toronto as the sixth worst for air quality, behind Dubai in the United Arab Emirates, Chicago, Delhi, Washington, D.C., and Detroit. 

Conditions, however, are expected to improve for some areas on Thursday or Friday, according to Environment Canada.

"Air quality and visibility due to wildfire smoke can fluctuate over short distances and can vary considerably from hour to hour," the federal weather agency said.

The city was last under a special weather statement for poor air quality slightly over a week ago and it experienced days of smoky air earlier in the month. 

Consider wearing mask outdoors: Environment Canada

Different countries use different indexes to measure air quality.

The Air Quality Health Index (AQHI) used in Canada reflects current knowledge of the health effects associated with air pollution and measures on a scale of 10.

On Wednesday, Toronto's air quality at one point reached a 10 on the 10-point Air Quality Health Index (AQHI) scale, which carries with it a "high" risk. By 10:30 p.m., the air quality listing had fallen to 6, which signifies "moderate" risk.

The Air Quality Index (AQI) used in the U.S., meanwhile, is based on air quality standards and is measured on a scale of 0 to 500. The higher the value, the greater the level of air pollution.

Toronto's AQI was around 137 on Wednesday night, according to IQAir, which it lists as "unhealthy for sensitive groups."

People at risk include seniors, infants, children, pregnant women, those who work outside or who are engaging in strenuous outdoor activities, and people with chronic health problems such as cancer or diabetes. 

Those who are not considered high risk should also consider reducing or rescheduling strenuous outdoor activities if they experience symptoms such as coughing and throat irritation.

"If you must spend time outdoors, a well-fitted respirator type mask (such as a NIOSH certified N95 or equivalent respirator) that does not allow air to pass through small openings between the mask and face, can help reduce your exposure to the fine particles in smoke. These fine particles generally pose the greatest risk to health," Environment Canada said.

The agency added, however, that respirators do not reduce exposure to the gases in wildfire smoke.

City modifies some programming

Based on recommendations from Toronto Public Health, city-run outdoor recreation programs moved indoors, the city said in a news release on Wednesday. Those enrolled in the programs will be contacted if there are changes.

Outdoor activities were also suspended at city-run early learning and child care centres, according to the city. Parents were notified about those changes. 

Outdoor museum programs run by the city also moved indoors. 

Check the CBC News Climate Dashboard for information on air quality and live updates on active fires across the country. You can also set your location to find out how today's temperatures compare to historical trends.