A smog advisory has been extended for much of southern Ontario, and residents are urged to keep heavy outdoor activities to a minimum.
The Ontario Ministry of the Environment issued the first advisory of the year on Monday as temperatures reached 27.7C, breaking the old record of 27.2C set in 1939.
When an alert is issued, it means there is a strong possibility of poor air quality in the next 24 hours.
The mercury is expected to hit 26C Wednesday and the forecast calls for a mix of sun and clouds.
Residents are urged to take public transit or carpool and refrain from using gas-powered motors, aerosol sprays and solvents as the air quality index is expected to exceed the "poor" mark of 50 due to ground level ozone and fine particulate matter, the government said.
Pollutants from the U.S. and Ontario sources will accumulate and react with sunlight to form photochemical smog.
"This is a problem that is imported from the United States. We also create our own pollutants; we're not lily-white in this. We are also contributing to the bad air sheds here in eastern North America," David Phillips of Environment Canada told CTV Newsnet.
"These pollutants get caught up in the air currents, the jet stream (and) it carries them through an area and then with a high pressure area, which we've seen over eastern North America for the last several days, it's like putting eastern Canada under The Skydome. There's no circulation, there's no ventilation just fumigation."
Heavy outdoor activities may cause respiratory symptoms such as coughing or shortness of breath. Residents with heart or lung disease including asthma may experience a worsening of their condition.
The smog advisory is in effect from Windsor in the southwest to Muskoka in the north.
The Ontario government reports on the Air Quality Index every hour of each day, year-round. While smog can occur at any time of the year, it forms most frequently during the summer months.
