How Long Will Ontario’s Dangerous Air Quality Last?

People look toward the downtown skyline from the Toronto Islands as forest fires in Ontario cause poor air quality over the city, in Toronto, on July 15, 2026. Cole Burston / AFP via Getty Images

Wildfire smoke that has blanketed vast regions of Ontario this week will begin to dissipate overnight for many of the hardest-hit cities, as changes in wind patterns improve air quality.

Air quality will improve across southern Ontario late on July 16 and overnight into July 17, according to forecasts from Environment Canada and IQAir, a Swiss company that operates the world’s largest real-time air quality information platform. However, the extent of this change will vary by region.

University of Waterloo environmental engineering professor Dr. Rebecca Saari said wind patterns determine how far smoke travels and in which direction. Wildfires emit smoke particles and other harmful pollutants that can create haze or smog more than 1,000 kilometres away, she said in a press release.

“Particles in the air settle to the ground depending on their size, with large particles settling quickly and small, submicron particles remaining suspended in the air for several weeks unless they are removed by other processes, such as rain,” Saari said. “This means fires can affect air quality both locally and across continents. These hot, sunny, dry days are conducive to fires, haze and smog.”

Here’s a look at what some of Ontario’s most severely affected cities can expect in the coming hours and days.

Toronto

Environment Canada is forecasting Toronto’s air will begin to clear around midnight and IQAir is predicting the city’s air quality will drop out of the unhealthy zone by late this afternoon.

IQAir uses the US EPA scale, which categorizes any reading above 300 as “hazardous” and indicates an emergency-level air pollution. Toronto had an air quality score of 400 at 9 a.m. but that had dipped to 153 by 3 p.m., dropping it to “unhealthy” only for those with breathing issues and allergies.

The forecast suggests that the air quality will reach a “moderate” rating, which is deemed acceptable for the majority of individuals, around midnight. It is expected to remain at this level throughout the weekend.

Windsor, Sarnia, and London

Windsor, Sarnia, and London will not see clear skies nearly as soon. Environment Canada forecasts show the southwestern Ontario cities will be contending with smoke well into the day on July 17 but a changing weather pattern will bring rain and clearer skies overnight.

All three cities had an air quality score of more than 200 at 3 p.m., placing them in the “very unhealthy” zone. They are expected to remain in that zone until this evening when it is forecast to drop to the less severe “unhealthy” rating, according to IQAir.

The cities are expected to reach a moderate rating the following morning.

Kitchener-Waterloo and Hamilton

Environment Canada is predicting widespread smoke dispersal overnight in Kitchener-Waterloo and Hamilton, and residents will wake to clear morning skies on July 17.

Conditions are expected to worsen once more that evening, leading to smoke-laden skies and a possibility of showers, before clearing up again for the weekend.

The air quality in the Kitchener region remained in the “very unhealthy” zone and Hamilton was in the “unhealthy” zone at 3 p.m. but both are expected to drop down to moderate during the early morning hours of July 17, IQAir predicts.

Peterborough and Belleville

Residents further east in Peterborough and Belleville can expect some relief from the smoky air come nightfall. Environment Canada said the widespread smoke residents grappled with all day would disperse early this evening, bringing clear skies and sun on July 17. The air quality in both cities is expected to drop from “very unhealthy” at mid-afternoon to “moderate” before midnight, according to IQAir. The air quality will improve to “good” by morning.

Central Ontario

The forecast was similar for much of central Ontario from Barrie and Orillia to Collingwood and Owen Sound.  Environment Canada is forecasting widespread smoke through tonight before clearing in the morning.

The air quality remained “hazardous” in Owen Sound and “very unhealthy” in Collingwood by mid-afternoon, but IQAir forecasts both areas will improve to a “moderate” rating during the early morning hours of July 17. Barrie and Orillia are forecast to hit “moderate” by midnight at the latest.

Northeastern Ontario

The poor air quality will continue throughout the afternoon in Sudbury, North Bay, and Timmins before dissipating in the evening, bringing clear skies overnight, according to Environment Canada. All three cities can expect smoky skies again late in the day on July 18.

The cities were all predicted to have a “good” air quality rating this evening but will dip to “moderate” when the smoke makes a slight reappearance the following evening, IQAir is predicting.

Smoke will linger overnight in Saul Ste. Marie before clearing slightly but the poor air quality will return again on July 17 and is not expected to clear until July 19.

Air quality in the Sault remained “hazardous” throughout the afternoon, but IQAir forecasts that it will change to “moderate” during the early morning hours.

Thunder Bay

Thunder Bay, the largest city near the wildfires raging in the northwestern part of the province, is expected to have widespread smoke in the air until at least July 19, according to Environment Canada.

The air quality remained at “hazardous” levels by late afternoon and  will fluctuate in the coming days, but IQAir is not predicting it will hit “moderate” until July 19 for a brief period before smoke pollution pushes it back up to “unhealthy” by mid-morning. The air is expected to improve again to “moderate” by the early morning hours of July 20.