Overnight low temperatures are forecast in the mid-teens, providing some relief from the heat of the weekend.
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. The record for warmest July 1 in Ottawa history is 36.7C set in 1963.
"If we take a look at the individual temperatures for the weekend from Friday to Monday, we were basically looking at the low 30s in terms of temperatures", says Environment Canada Meteorologist Gerald Cheng. That would set a record for the highest humidex ever recorded in Ottawa.
The health unit advises people to prepare for the heat by staying hydrated, staying cool, and to check air quality.
A section of Highway 3 in Ontario's Essex County was closed Sunday after extreme heat caused damage to two sections of roadway.
The move is part of the federal government's climate change adaptation in expectation of more frequent heat events.
Look for shade or a cool shelter in an air-conditioned location such as a shopping mall, local library or community centre.
The Salvation Army was out at Gore Park handing out bottles of water reminding people to heed the warnings and protect themselves, and police have already received a number of calls including one on Appleby Line and Upper Middle Road in Burlington where they rescued a dog from a hot auto where the temperature was 66 degrees. If you can not shower or bath easily, sponge often with cool wet towels. "If you're going to outside - it's going to be hot - so seek shade when possible to avoid overexposure".