OTTAWA -- Environment Canada has issued another air quality statement as smoke from wildfires in northwestern Ontario returns to the region.

A hazy sky blanketed Ottawa early last week until a cold front and thunderstorm pushed some of the smoky air away from the region. A statement about air quality due to wildfire smoke was first issued last Monday evening. Environment Canada lifted it at around 6:30 p.m. Tuesday.

New plumes of smoke have since blown in from the northwest.

"Smoke plumes from active forest fires in northwestern Ontario might cause deteriorated air quality for portions of eastern Ontario tonight. Conditions may persist into Monday," Environment Canada said.

"Individuals may experience symptoms such as increased coughing, throat irritation, headaches or shortness of breath. Children, seniors, and those with cardiovascular or lung disease, such as asthma, are especially at risk."

The alerts are in place for much of southern Ontario, including Toronto and Hamilton.

A smog warning is in effect for Gatineau, Que.

"Smog especially affects asthmatic children and people with respiratory ailments or heart disease. It is therefore recommended that these individuals avoid intense physical activity outdoors until the smog warning is lifted," Environment Canada said.

More than 500 people were flown to Cornwall, Ont. to find shelter after a full evacuation order was issued for Deer Lake First Nation, about 300 km north of Kenora. The residents are being sheltered at Cornwall's Nav Centre.

Severe thunderstorm warning issued

Environment Canada has also issued a severe thunderstorm warning for northern Ottawa, Kanata and Orléans.

A severe thunderstorm watch is in effect for Ottawa and for the Prescott and Russell area.

The storm could bring rain, possible hail, and strong winds, but should move out of the region Sunday night.

"Thunderstorms over Quebec are expected to intensify as they move southeastward towards extreme eastern Ontario. Quarter size hail and wind gusts to 90 km/h are possible. The severe weather threat should end after 9 p.m. EDT," Environment Canada said.