Environment Canada has issued a two-day heat warning for most of eastern Ontario and western Quebec this weekend.
Temperatures are expected to be in the high twenties or low thirties with the humidity making it feel above 40 C.
Evening lows are projected to be in the low twenties, bringing little relief from the heat.
Environment Canada says hot and humid air can bring with it lower air quality. Young children, pregnant women, older adults, people with chronic illnesses and people working or exercising outdoors are at the most risk from poor air quality.
People are advised to drink plenty of water, even before feeling thirsty, and to stay in a cool place, according to the weather agency.
It recommends people check in with older family, friends and neighbours to make sure they are staying cool and hydrated.
Those working outdoors should take regular breaks in cool places, Environment Canada says.
Kingston, Frontenac, Lennox & Addington Public Health (KFL&A) expanded on Environment Canada's advice, recommending that people reschedule strenuous activity for cooler parts of the day, close blinds and curtains during the day to block sun and wearing light-coloured, loose fitting clothing.
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.
In southern Ontario, including the Ottawa region, that means when there are either two days in a row with forecasted highs of at least 31 C and lows of at least 20 C, or two days in a row with humidity expecting to make it feel like 40.
The main sign of heat illness are dizziness or nausea, a headache and behaviour changes, according to KFL&A Public Health. It suggests moving to a cool place immediately and drinking water.