Toronto is facing a heat wave, with with hot and humid conditions expected to hang around the city until Friday, Environment Canada says.

The federal weather agency issued a heat warning Tuesday, advising residents in the Toronto area of temperatures to reach 30 C in the daytime on Wednesday.

While slightly cooler, Thursday's forecast to be very humid with humidex values expected to be in the upper 30s, Environment Canada says. Meanwhile, Friday, which is expected to be the hottest day of the week, will be "extremely hot," the agency says, with humidex values reaching 40 in many areas.

Night time temperatures will likely hover in the low 20s but will provide "little relief" from the heat, it said.

Cooler temperatures are expected to arrive Saturday.

Heat warnings are posted when conditions are expected to pose a risk of heat illnesses, such as heat stroke and heat exhaustion.

The hot conditions may lead air quality to deterioriate Environment Canada says, which means the Air Quality Health Index could reach the "high risk" category at various points.

The agency says risks are greater for young children, pregnant women, older adults, people with chronic illnesses and those working or exercising outdoors.

The weather agency says it's important to drink plenty of water before you feel thirsty and stay in a cool place if possible. If you need to be outside, be sure to take regularly scheduled breaks in a cool area.

Environment Canada is also reminding drivers to never leave people or pets inside of a parked vehicle.