The air quality in parts of Ontario and Quebec impacted by smoke is improving.
The smoke is from forest fires burning several kilometres away or in other parts of the country.
Environment Canada says any improvement may only be temporary.
Meteorologist Gerald Cheng says a lot depends on weather patterns and the number and size of the fires burning.
"It really depends on the weather pattern of the day of the week and where that smoke is going. So we have to constantly monitor how the atmosphere is behaving and even if the jet stream is carrying smoke into northwestern Ontario and coming down to the ground," says Cheng.
Several air quality warnings were issued for southern Ontario, Quebec, and much of the eastern seaboard in the United States.
Cheng says the air quality has improved in several areas but warns it may only be temporary.
"There is a system on the way that could bring some rain, and the rain certainly will help because it washes the air and also because it will change the wind direction. So those two are key factors in improving the air quality, at least when we look at Monday. However, after that, because there are still wildfires burning, this season is something that we have to continue to monitor."
Cheng notes the air quality remains poor in parts of northern Ontario and western Quebec where many forest fires are burning.
Poor air quality warnings in themselves are not unusual, especially in southern Ontario, where the air is effected by smog from time to time.
Cheng says there have also been large areas of the country, such as BC, where warnings were issued because of smoke.
"When it comes to smoke effecting a large area, that has happened before, but smoke affecting major population centers in Canada and in the US, particularly the eastern seaboard, that is unusual."