Health Impacts of Extreme Heat

Extreme Heat

Many Canadians welcome the arrival of hot summer days as respite from our long, cold winters. Understandably, we tend to think of more summer heat as a good thing.

But too much heat can be dangerous.

Kim Perrotta, Senior Director of Health and Policy for the Canadian Association of Physicians for the Environment (CAPE), notes that high temperatures “can increase premature deaths, cause heat stroke, and can aggravate heart disease and respiratory diseases.” She warns that “this is actually a real concern for human health that’s affecting a large portion of the country.”

Extreme heat can also worsen mental health issues such as depression and anxiety. And it increases the risk of droughts and forest fires, which in turn have serious implications for our personal and community health.

Communities across Canada know the dangers of heat all too well. Heat waves and extreme heat warnings have happened frequently in recent summers, including in Winnipeg, Ottawa, Halifax, Toronto, and—most dramatically—Montreal. In the summer of 2018, temperatures in Montreal soared for eight days straight, reaching a searing high of over 40°C with the humidex. Sixty-six people died from the heat. [1]

Other areas of southern Quebec were also hit by heat waves, bringing the total death toll across the region to eighty-six. [2]

Read more: Anatomy of a Heat Wave

In general, a heat wave is an extended period of unusually hot weather. There isn’t a simple, commonly accepted scientific definition of a heat wave. We define it as a period of at least three days in a row that reach 30 °C or higher. We use this because 30 °C is experienced as a ‘hot’ day anywhere in Canada, and a string of these hot days will increase the likelihood of heat impacts that matter to Canadians.

Heat waves are projected to become longer, hotter, and more frequent. Check out our Special Report on Heat Waves and Health to see how heat waves are expected to increase and impact different parts of the country.

Rising temperatures are a problem especially for people in larger cities. Gord Perks, a city councillor in Toronto, worries about rising temperatures becoming hazardous to the people in his city. “We’ve been lucky so far that we haven’t had a deadly heat wave,” he said in 2018. “We haven’t had that, but we’re likely to. All the models say we’re going to.”

There’s no doubt that with climate change we’re going to see more heat and heat waves. Even temperate coastal cities such as Vancouver are preparing for extreme heat impacts. “I don’t think any of us ever thought we’d use the words ‘heat wave’ and ‘Vancouver’ in the same sentence,” said former Vancouver city councillor Andrea Reimer, “but now it’s something we not only have to expect, but that we’re experiencing.”

Why does heat cause so many health problems?

Extreme heat can do a lot of harm to your body if you’re unable to cool down. Health practitioners warn against a number of heat-related health problems. Heat impacts can range from cramps and rashes to more severe illnesses that are caused by prolonged exposure, such as fainting, heat exhaustion, and heat stroke. In the most extreme circumstances, being unable to lower your body temperature could result in death. [3]

Less directly, rising temperatures can impact our health by producing more air pollution. High temperatures “bake” vehicle exhaust, turning it into harmful surface-level ozone and smog. Smog is often concentrated in big cities, but air-quality problems can be just as bad in rural and suburban areas, especially as Canada experiences more wildfires. Air pollution can irritate your eyes, nose, and throat, worsen pre-existing heart and lung problems, or in some cases, cause long-term health issues. [4]

Importantly, heat can affect mental health and community well-being too. Occurrences of mood disorders, anxiety disorders, dementia, and psychological distress have all been shown to increase when the mercury goes up. [5] Heat can have an impact on interpersonal interactions, with increases in irritability and aggression. [6,7] Domestic violence and violent crimes have also been shown to spike during heat events. [8]

Who is Vulnerable to Heat?

Toronto City Councilor Gord Perks notes that when extended heat waves happen, “a large part of the population suddenly becomes vulnerable.” We saw the tragic reality of these risks in the summer of 2018, as serious heat waves struck many Canadian cities. In Montreal, temperatures remained high for eight days straight and 66 people died from the heat. Most of the victims were older men living alone. [9]

All of us experience increased health risks from hot weather, but as the 2018 Montreal heat wave shows, some people are more vulnerable than others. Managing the effects of heat is largely dictated by a person’s ability to access resources. [10] For example, people without adequate housing, air-conditioning, or sufficient supplies of drinking water will have difficulty dealing with the heat.

People who are socially isolated or those with mobility issues may have challenges getting help and moving to cooler locations. Individuals who have difficulty accessing or understanding public health information such as heat warnings may be at greater risk as well. Dr. Jean Zigby, a palliative care physician in Montreal, says that people experiencing social isolation, and especially people with mental health issues, are at higher risk: “they seem to be a disproportionate percentage of the population that are found, unfortunately, dead due to heat exhaustion.”

Other factors such as pre-existing medical conditions and being particularly sensitive to heat (especially seniors and children) affect vulnerability. Dr. Zigby says that “we see a lot of patients deteriorate after smog episodes and after heat waves,” and notes that when the heat rises it’s a “huge risk” for his patients “every time they take a trip outside.”

It’s also important to remember that anyone who is active outside, by participating in outdoor activities or working outdoors, is at increased risk.

Read more: What should you do during a heat wave?

1. Be aware when extreme heat warnings are issued in your area. Prepare yourself and those around you for the increased temperatures.

2. When it is hot out, limit the amount of time you spend outdoors, especially when participating in strenuous activities. Staying indoors in air-conditioned and well-circulated spaces is the best way to keep cool. If you don’t have access to air conditioning or a cool basement at home, public buildings such as libraries or cooling shelters offer spaces that can be used to cool down.

3. When outdoor activities are unavoidable, try to schedule them for earlier or later in the day when the temperature has cooled off. While outside, take frequent breaks in order to not overexert yourself.

4. Drink plenty of water to keep hydrated. Limit alcohol intake, and swap alcoholic drinks for non-alcoholic alternatives to reduce the risk of dehydration.

5. Wearing lightweight and lightly coloured clothing along with a hat (and sunscreen to prevent sunburns) can make all the difference by reducing your exposure to the sun’s rays and making sure your body can breathe. Stick to shaded areas away from the direct heat of the sun.

6. Reducing the risk from heat waves is a team effort. Look out for neighbours and people in your community who may need help protecting themselves. Checking in and making sure everyone is safe and doing well is an effective way to reduce risk.

Read more at Health Canada: “Extreme Heat: Heat Waves”[3]

Taking action to prevent health impacts

Communities across Canada are making an effort to manage the reality of increased heat under climate change. Both Montreal and Toronto have extreme heat response and adaptation plans, and are putting in measures to reduce risk.

Dr. Zigby says “we’re seeing an increased sensitivity on the part of our public health officials and our city officials in order to make sure that infrastructure has been adapted for heat waves.” He reports that over the past 15 years, Montreal has implemented “plans to actually contact people who are socially isolated in order to make sure that they get access to cool areas”. He also notes that there has been “a massive investment in parks where there is a component of water included” that allows children and other vulnerable people to cool themselves off.

Acting on their heat response plan, the City of Montreal took steps to reduce risk during the 2018 heat wave. Firefighters and police officers went door-to-door in vulnerable neighbourhoods to check on residents and offer advice on how to stay safe. The city also increased available paramedics and ambulances, sent out alert bulletins, extended public pool hours, and opened emergency cooling centres. [11] The mortality rate during the 2018 heat wave was much reduced compared to another serious heat wave in 2010, very likely thanks to better forecasting and public health interventions.[2]

Read More: Toronto’s Resilience Strategy

Toronto’s 2019 Resilience Strategy uses the Climate Atlas of Canada to consider what type of future climate the city will likely need to respond to. Based on these data, the Strategy includes a section on approaches to mitigating the effects of extreme heat. Some of their proposed measures include:

  • increased access to cooling centres;
  • promotion of a “neighbour checking program”;
  • addition of shade structures;
  • retrofits to apartment buildings;
  • installation of green infrastructure.

In Toronto, some community groups are taking the response to heat waves into their own hands. A group of faith organizations have banded together to open their doors during extreme heat events, creating a network of community-organized cooling centres and strengthening social resilience to the threat of extreme heat. [12]

Developing a support network within your community can be an effective way to look out for people who may be more vulnerable. Regular check-ins when weather warnings are issued can be an easy way to make sure everyone is safe, informed, and receiving help if needed.

Read more: Is air conditioning the solution?

In Canada, many of us rely on air conditioning during the summer heat. Unfortunately, spikes in the use of air conditioners during heat waves place a huge load on electrical systems.

“If everybody uses air conditioning, it's a huge strain on the power grids, and the likeliness of having a blackout or a brownout is going to increase,” explains Dr. Georgia Chaseling, a postdoctoral research fellow at the Montreal Heart Institute. Chaseling sites recent examples of blackouts that have caused up to 30% increase in mortality during heat waves.

At the same time, if the electricity that powers air conditioners is generated by high-carbon fossil fuels, then using them to deal with heat will worsen the global warming that created the heat in the first place. [13]

Many strategies promoting energy efficiency and cold-weather comfort are also excellent for dealing with high heat. Well-insulated buildings keep heat out in the summer, just as they keep the cold out in the winter. This increases comfort and reduces the need for air conditioning to begin with.

But it’s important to remember that simply buying an air-conditioner or retrofitting your home isn’t affordable or viable for many people in Canada. “Not everybody can afford air conditioning... We're trying to look at economical and ecological ways that we can keep people cool during heat waves,” says Chaseling. Their research team is looking at measures like “just using a fan, or wetting the skin with cold water with a fan as well, to try and really help people keep cool and alleviate the cardiovascular strain.”

To reduce illness and death from high heat events, cities need to take large-scale measures that reduce the vulnerability of high-risk citizens. The City of Toronto, for instance, provides public access to certain air-conditioned facilities during heat waves. [14] Increased access to pools, splash pads, and drinking water can offer city dwellers much needed relief.

Furthermore, enacting good policies that help reduce poverty and increase access to safe, affordable housing will reduce the risk of some of the most vulnerable people in our communities.

Individuals and communities can rise to the challenges of heat waves with these sorts of actions, but social and political change is needed as well to reduce the risks more generally.

For example, Montreal city planner Chakib Benramdane notes that the built environment of cities makes warming worse, but “where there is lots of greenery, lots of nature, this nature absorbs these effects. Nature is well-made, it balances itself.” He argues that cultivating more extensive vegetation, trees, water, and natural spaces is the best solution to the urban intensification of heat risks.

Developing, improving, or expanding green space within city centres can greatly decrease temperatures on the ground and reduce the impacts of extreme heat events. [15] And in addition to reducing temperatures, the presence of green spaces such as parks, trees, and natural vegetation has been linked to a number of additional health benefits. These include better mental and physical health, and overall well-being. [16,17] Studies also show green spaces can provide a boost to social connections and build community. [18]

By choosing to green our cities, promote transportation, and shift towards the use of renewables for our energy needs, we can create healthier communities with fewer air pollutants, which are also more resilient to extreme heat. See our “Take Action” topic for more ideas and stories about climate action.


  1. Santé Montréal. “Epidemiological Investigation: Heat Wave Summer 2018 in Montréal - Summary.”
  2. Climate Data Canada. “Extreme heat waves in Québec.”
  3. Health Canada. “Extreme heat: heat waves.”
  4. Jacob, Daniel J., and Winner, Darrell A. “Effect of climate change on air quality.” Atmospheric Environment 43.1: 51-63.
  5. Lõhmus, Mare. “Possible Biological Mechanisms Linking Mental Health and Heat—A Contemplative Review.” International Journal of Environmental Research and Public Health.
  6. WGBH News. “Heat and aggression: How hot weather makes it easy for us to offend."
  7. Anderson, Craig. 2001. “Heat and violence.” Current Directions in Psychological Science. Volume 10 Issue 1. pgs:33-38
  8. BBC News. “Heatwave: Is there more crime in hot weather?”
  9. Direction régionale de santé publique, CIUSSS du Centre-Sud-de-l’Île-de-Montréal. “Canicule : Juillet 2018 – Montréal – Bilan Préliminaire.”
  10. Banks, Nick et al. “Climate change and social justice: an evidence review.”
  11. City of Montréal. “Heat wave in Montréal: The city activates the intervention level of its extreme heat response plan.”
  12. Climate Atlas of Canada. “Heat Waves and Hope.”
  13. "Before, during, and after a summer power outage," Stephanie Fereiro, August 8, 2018.
  14. "Toronto gets brief relief but no long-term shelter from extreme heat," CBC News, July 6, 2018.
  15. University of Leeds. “A Brief Guide to the Benefits of Urban Green Spaces.”
  16. CBC News. “Green space improves mental health, well-being.”
  17. Terton, Anika. “Building a Climate Resilient City: Urban ecosystems.”
  18. Globe and Mail. “Growing cities struggle to stay green.”
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