Heat and your heart: Researchers warn of hidden risks

CBC

19-07-2025

As Ottawa and the surrounding region emerges from an extended summer heat wave, heart researchers say all that sultry weather can cause cardiovascular damage you might not be aware of.
The latest heat wave lasted nearly a week with temperatures in the low 30s. With the humidity factored in, some days felt above 40.
"Heat makes your heart work really hard. It puts stress on it," said Katey Rayner, vice-president of research and chief scientific officer at the University of Ottawa Heart Institute. "If you already have a heart condition, that just means it's an added layer of stress."
She said exposure to high temperatures can make it difficult to maintain blood flow to certain organs, and can cause a person's blood pressure and heart rate to rise.
"It may put [your heart rate] into a dangerous level," Rayner warned. "But instead of being in a hospital where we're monitoring, people are at home and they're trying to stay healthy and trying to sort of stay active, but they might be inadvertently causing strain and stress on the heart."
Hot, sticky summers
Because of Ottawa's location in a major river valley, summers in and around the capital tend to be hot and sticky.
"And that just means humidity makes it really hard for your body to cool down," Rayner said. "Your body tries to sweat and get rid of its excess fluid and cool itself, and it does that through sweating — and it really can't do that effectively when there's basically a layer of fluid sitting on top of your body."
Staying in the shade provides little relief because "that humidity is just sitting on top of you," she said.
Researchers at the Heart Institute are working with climate scientists to better understand the link between hot weather and heart health. Rayner said it's a complex problem because not everyone has the resources to keep cool, putting them at higher risk.
"It's really easy to tell people to just stay in air conditioning, [but] that's not actually feasible for a lot of people, particularly in disadvantaged communities," she explained.
Effects can be cumulative
Researchers have also found that the health effects of extreme heat can be cumulative.
"When we are exposed to heat, there's a lot that's changing in terms of our body's physiology because we're always under stress," said Glen Kenny, director of the human environmental physiology research unit at the University of Ottawa.
"That strain on the body can be the result of gradual loss of body water, so dehydration that essentially is just going to strain the heart even more."
Kenny warned people who use fans instead of air conditioning to keep cool could be endangering their health, especially when the temperature tops 33 C.
"[A fan] makes you feel cool, but it does not reduce the temperature that you experience or the strain on the heart, and for that reason you may not recognize the danger you're experiencing when using an a fan in an overheated home," he said.
Along with such pre-existing health conditions as Type 2 diabetes, kidney or cardiovascular disease, one of the biggest risk factors is age. Kenny said our ability to sweat diminishes by about five per cent per decade, raising our core body temperature and increasing our risk of heat-related injury.
Kenny and Rayner both recommend avoiding exercise in extreme heat because it forces our core temperature up faster.