Why Ottawa’s heat and humidity is more dangerous than it seems

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A heat event is expected to begin today in the Ottawa area and continue through Friday, and possibly into the weekend as a hot and humid airmass arrives, Environment Canada has warned.

Maximum daytime temperatures of 30 to 32 degrees Celsius are expected, with humidex values near 40. Overnight minimum temperatures will be 19 to 21 degrees Celsius. All of the ingredient will be there for thunderstorms. 

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“Saturday is a big question mark. There may be enough cloud cover to prevent high temperatures,” said Gerald Cheng, a warning preparedness meteorologist at Environment and Climate Change Canada.

That’s hot, but not too bad, right?

Wrong, says a University of Ottawa physiologist who studies how the human body regulates heat and has conducted extensive research on long-term heat events in LTC homes and workplaces. 

“The highest numbers of heat-related deaths don’t always happen at the highest temperatures. They happen with lower temperatures and high humidity,” said Glen Kenny, who has studied first responders, miners and other workers and their response to heat.

“It can equate to deadly heat. If you can’t evaporate sweat, you’re not cooling your body,” he said.

“A worker working today will not be the same person three or four days down the road. A 40-year-old worker will have the same capacity to lose heat as a 60-year-old.”

Young people sweat a lot. Older people don’t sweat as much, said Kenny. As people grow older, they can’t dissipate heat in the same way as younger people and the situation gets worse as a heat event continues, said Kenny, who has also studied heat responses in the elderly and people with chronic conditions such as diabetes. 

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Whether someone is old or young, everyone continues to generate heat as time goes on, although they may not recognize the fact. By the end of a working day, many workers are already severely dehydrated and are at risk of heat-related illness or death, he said.

After a single day of heat exposure, older adults will say they feel fine, but people who are dehydrated lose control of their blood pressure, resulting in the risk of falls and fractures.

There’s also a misguided belief that fans are effective, he said. But while a fan makes you feel better, it does not adjust behaviour. The same goes for wrapping a cold towel around your neck. It might feel good, but it has no benefits, he said. It’s particularly important that older adults, who have a limited ability to stay cool, have access to a cool place.

“When people feel better, they don’t adjust their behaviour and they put themselves at great risk of heat stress.” said Kenny.

One study found that while cooling centres can have a positive effect on “perceived thermal strain and mood state” in older adults, there still needs to be continued vigilance to mitigate the strain of over-heating.

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The “western heat dome” of 2021, was considered the deadliest weather event in Canada. The BC Coroners Service confirmed that there were 619 heat-related deaths from June 25 to July 1, 2021. 

But there’s not enough information on the number of heat deaths. Being overheated leads to confusion and falls, but fractures related to being overheated are often not reported, for example. Thousands have died in heat waves, said Kenny.  “Part of the problem is that it’s not reported very well.” 

Environment Canada warns that extreme heat can affect everyone’s health.

“The health risks are greater for older adults, infants and young children, pregnant people, people with physical and/or mental illnesses, and people with disabilities or mobility issues,” says the national weather office, which advises people to drink plenty of water regularly, even before you feel thirsty, to decrease the risk of dehydration.

“Thirst is not a good indicator of dehydration. Never leave people, particularly children, or pets inside a parked vehicle. 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.” 

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Ottawa Public Health urges residents to explore its interactive map of (literally) the coolest public places in Ottawa. Entering a location in the search box will generate a list of the 20 closest cool spaces. Clicking on any of the icons on the map will provide more information about the cool spaces around that location.

All city pools, indoor and outdoor, have removed some lanes to accommodate additional public swimmers during scheduled lane swims. 

As of Thursday morning, city beaches at Britannia, Mooney’s Bay. and Petrie River were safe to swim, but water at Petrie East Bay had exceeded the provincial bacteria standard and swimming is not recommended there. 

For information about overnight shelters and services for people experiencing homelessness, call 2-1-1 for locations and access information for shelters in Ottawa or call  3-1-1 to get assistance for someone who needs transportation to a shelter to get out of the heat.

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