Environment Canada says a heat wave descending on parts of Ontario this week is expected to bring dangerously high temperatures.

The weather agency has issued a heat warning starting Monday for an area that spans London to Ottawa, stretching up to Sault Ste. Marie and as far north as Fort Albany.

Temperatures in the Greater Toronto Area and Ottawa could feel as hot as 45 C with the humidity.

Regions further north are also forecast to see humidex values that could feel like the low to mid 40s.

Environment Canada says the hot air will move eastward later in the week, and has issued a special weather statement starting Wednesday for much of southern Quebec.

The agency notes the health risks associated with heat illness are greater for older adults, infants and young children and other vulnerable groups.

It warns hot, humid air can also worsen air quality.