How to watch for the hidden symptoms of extreme heat

Climate change is making summers in Canada hotter, with more days of extreme heat and fewer nights that let our bodies cool down. 

The consequences of extreme heat can be severe: last year, more than 600 people died during the heat dome in British Columbia and, in 2018, up to 70 deaths in Quebec were linked to a heat wave there. 

Research suggests Southwestern Ontario experienced a 22 per cent increase in emergency department visits during periods of extreme heat, according to data analyzed from 2002 to 2019.

What temperature threshold triggers a heat warning varies depending on your location, but the symptoms of heat-related illnesses remain the same — and doctors say people aren't fully aware what to watch for.

"Heat-related illnesses arise when an individual is exposed to environmental heat and their own body is not able to accommodate or acclimatize quickly," said Dr. Justin Yan, an emergency physician in London, Ont.

Yan has treated several patients experiencing heat-related illnesses in the emergency room. He says these symptoms are potentially life-threatening and should be taken seriously when they happen. But he cautioned that it's not just about heat stroke.

"There's a spectrum of heat-related illness symptoms." 

Heat-related illnesses can range from mild, requiring cooling and rehydration, to severe — requiring emergency medical treatment. 

Yellow zone: Mild heat-related illness

Orange zone: Moderate to severe heat-related illness

Yan says the most common symptom he sees is heat exhaustion. 

"People will feel lightheaded, they might have a bit of a headache, feel tired and weak, they may feel nauseated. They could pass out," he described. 

"What is actually happening is the body is sending a message to drink some water and get to a cool environment."

Red zone: Severe heat-related illness