As Toronto stares down another scorching week, some experts are warning about the dangers of prolonged heat exposure indoors — even if it’s just a couple degrees above room temperature.
According to recent guidance from Health Canada, too much time spent inside in temperatures above 26°C could lead to heat-related illness and even death, especially among those over 60.
That’s why there is momentum to introduce legislation limiting indoor temperatures. Toronto City Council recently passed a motion to draft a bylaw that would cap the indoor temperature of certain rental units to 26°C.
“It’s become clear that indoor temperatures at 26 degrees or below are health protective,” said Dr. Samantha Green, a family doctor at St. Michael’s Hospital. “Whenever indoor temperatures climb above 26 degrees, there is a risk to human health — especially for people who are older and people with chronic conditions.”
Here’s what that means.
Why 26°C is a threshold for indoor temperature
The problem with indoor heat is that we’re exposed to it constantly, often for prolonged periods, like when we’re sleeping. That puts a strain on the body that can add up, experts say.
Health Canada advises avoiding exposure to indoor temperatures between 26°C and 31°C “whenever possible” — especially overnight or for multiple consecutive days.
Evidence suggests 31°C represents a “critical threshold where health impacts escalate dramatically,” according to the health agency.
Its guidance was influenced by the work of Glen Kenny, a University of Ottawa professor whose research has redefined our understanding of how extreme heat affects the body.
In 2024, Kenny and his team exposed a group of older Ontarians to temperatures between 22 and 36°C for eight hours. They found that anything higher than 26°C led to increased core temperature and cardiac strain.
This refers to prolonged, constant exposure. “You can certainly go for short periods of time at 31 or 36°C,” Kenny told the Star. “The most important thing to recognize is that, when you are at elevated temperatures above 26, you’re accumulating strain.”
The 26°C figure is a “conservative threshold to keep as many people safe as possible,” said Stephen Cheung, a distinguished professor of kinesiology who researches heat stress at Brock University. Some people are more sensitive to or tolerant of heat than others. “There is no magic number that fits for everyone.”
How prolonged heat affects our bodies
Think of extreme heat exposure as a tug of war between the body and the environment. The ambient temperature works to increase our own core temperature, and our body fights back in two main ways: through sweating, which cools the skin through evaporation; and by sending more blood toward the skin, allowing heat to dissipate into the environment, which causes skin to redden.
“In response to extreme heat, the (brain) tells the heart to start beating faster, the lungs to start breathing faster and the sweat glands to activate,” Green explained. All of these processes put strain on the body, which can accumulate over days spent at an elevated temperature.
Given enough time, typically within a few days, the strain can lead to cardiovascular and respiratory distress, reduced physical and cognitive performance and higher risk of death, especially in vulnerable populations, according to Public Health Ontario.
That’s why, when there’s a prolonged heat wave, we typically don’t see increased hospital admissions for heat-related illness until the second or third day in, Cheung said.
If the ambient temperature overwhelms our defences and raises our core temperature to 40°C or above, our cooling systems are no longer able to compensate, resulting in heat stroke, a potentially fatal condition, Green said.
Who is most vulnerable to heat-related illness?
People 60 and older are among the most vulnerable to heat, because our natural cooling systems deteriorate with age.
During the 2021 British Columbia heat dome, which resulted in more than 600 confirmed heat-related deaths, 90 per cent of the fatalities were in people over 60. The vast majority of the deaths, 98 per cent, occurred indoors.
Young children are also at risk as their bodies aren’t able to cool as efficiently as adults’ are. Many schools across Ontario and Canada lack air conditioning, Kenny said. “And sometimes, they don’t know when to stop. If they’re outdoors playing in the heat, by the time their bodies tell them to stop, it’s sometimes too late.”
Extreme heat is known to exacerbate pre-existing conditions, especially cardiovascular issues due to its strain on the heart, Cheung said. People with diabetes and hypertension are at increased risk, according to Kenny’s research.
Heat exposure is also a socioeconomic issue. People experiencing homelessness or who can’t afford air conditioning are exposed to much greater heat for longer periods of time, and may not have regular access to water.
Toronto has more than 500 cooling spaces across the city; while a visit to these locations can offer relief, research suggests the benefits tend to wear off after eight or nine hours.
As climate change progresses, extreme heat events will only become more common. “We can expect that each summer from here on will get even hotter until we eliminate the use of fossil fuels and fossil fuel pollution,” Green said.
As a society, he added, we will need to adapt to our new reality. “This may feel hot, but this is just the beginning.”
