TORONTO -- A smog advisory has been issued for much of southwestern Ontario as the region sweats out a wave of hot weather.

The warning comes as the thermostat hit the low-30s in many areas.

It affects communities from Niagara Falls to Windsor and up to Hamilton and Kitchener-Waterloo.

The Halton-Peel region and Elgin County are also hit by the smog notice.

Ontario's environment ministry says the elevated smog levels are the result of hot and sunny weather and light winds.

Smog is harmful to the lungs and heart and can reduce lung function even in healthy people.

Environment Canada forecasts weather just above 30 degrees before humidity for much of southern Ontario on Saturday.