Hot and increasingly humid weather has prompted Environment Canada to issue a special weather statement for Waterloo-Wellington and much southern Ontario.
It will be the first heat event of the season, the weather agency says.
Temperatures will reach near 30 degrees Celsius on Monday in many areas, before briefly dipping into the mid to upper teens Monday night.
Tuesday will be even hotter, with maximum temperatures in the low 30s and minimum temperatures in the low 20s.
Cooler air is expected to arrive Wednesday.
Environment Canada says these conditions pose a health risk, especially for older adults, infants, young children and people with chronic illnesses.
People are advised to drink plenty of cool liquids before feeling thirsty and keep cool by dressing for the weather and spending time in a cool place.