Most of southern Ontario, including the GTA is under a heat warning.
Environment Canada says a hot and increasingly humid air mass will last through at least Sunday.
Slightly cooler and less humid conditions may arrive on Monday.
Daytime high temperatures are excepted to be between 31 and 34 degrees with overnight lows in the 20 to 24 degree range.
Humidex values are forcecast to be in the high 30s Saturday, increasing to low 40s Sunday.
The UV index for Saturday is expected to be 10 (very high).
Under sun that strong, unprotected skin can get sunburn in less than 20 minutes.
Hot and humid air can also bring deteriorating air quality and can result in the Air Quality Health Index approaching the high risk category.
The agency says never leave people or pets inside a parked vehicle and keep any eye out for the effects of heat illness: swelling, rash, cramps, fainting, heat exhaustion, heat stroke and the worsening of some health conditions.
Heat warnings are issued when very high temperature or humidity conditions are expected to pose an elevated risk of heat illnesses, such as heat stroke or heat exhaustion.