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Summary

As Environment Canada issues severe heat warnings, heat-related illness poses a significant but preventable risk to thousands of Canadians. This guide breaks down the four stages of heat illness, culminating in life-threatening heat stroke, and offers practical strategies for staying cool, protecting vulnerable neighbours, and identifying true medical emergencies.

If it feels like Canadian summers are getting hotter, that’s not just a feeling. Environment Canada has issued an extended heat warning across much of southern Ontario and Quebec, running through Canada Day and into the weekend, with daytime highs reaching 30 to 37°C and humidex values climbing as high as 45. It’s shaping up to be one of the most significant heat events of the summer so far and meteorologists say 2026 is on track to rank among the hottest years on record in Canada.

Heat-related illness sends thousands of Canadians to emergency rooms every summer and it’s almost entirely preventable. Here’s what you need to know to stay safe, recognize the warning signs, and know exactly when it’s time to call for help.

How Heat Affects Your Body

Your body normally maintains a temperature of about 37°C by sweating and increasing blood flow to your skin. When you’re exposed to heat for too long, your body becomes overworked trying to keep that temperature stable and that’s when things start to go wrong.

Humidity makes everything worse. Sweating is far less effective at cooling you down when the air is already heavy with moisture, which is why a humid 30°C day can feel more dangerous than a dry 35°C one. This is also why you’ll often hear weather reports use the “humidex” , a number that combines temperature and humidity into a single “feels like” value.

The Four Stages of Heat Illness

Heat-related illness isn’t a single condition, it’s a spectrum, from mild to life-threatening. The Government of Nova Scotia outlines four main types, from mildest to most severe:

1. Heat cramps

The mildest form. Causes painful muscle spasms, usually in the legs or abdomen, often after physical activity in the heat. What to do: Move to a cool place, drink water, and gently stretch or massage the affected muscles.

2. Heat exhaustion

A step up in severity. According to Health Canada, symptoms can include headache, nausea, vomiting, dizziness, fainting, rapid breathing, a fast heartbeat, extreme thirst, and unusually dark urine. In children, watch for unusual sleepiness or irritability. What to do: Move to a cool place immediately and drink water. If symptoms don’t improve within an hour, or get worse, contact a healthcare provider.

3. Heat syncope (fainting)

Less commonly discussed, but real, heat can cause blood pressure to drop suddenly, leading to fainting, especially when standing for long periods in the heat.

4. Heat stroke

The most severe and dangerous form, and a true medical emergency.

Heat Stroke: Know the Signs

This is the one that can kill, and it can escalate quickly. Environment Canada says to watch for red and hot skin, dizziness, nausea, confusion, and a change in consciousness. Other warning signs include a loss of coordination and stopping sweating altogether.  Counterintuitively, the skin can become hot and dry rather than sweaty as the body’s cooling system fails.

During heat stroke, core body temperature rises above 40°C. The longer the body stays at that temperature, the greater the risk of permanent organ damage or death.

If you suspect heat stroke:

  1. Call 911 immediately.
  2. While waiting for help, move the person to a cool, shaded place.
  3. Cool them down with cold water, sponge their skin, apply cold packs, or use a hose if available.
  4. Fan them to increase evaporative cooling.
  5. Keep trying to cool them until paramedics arrive. Don’t wait to see if they improve on their own.

Heat stroke does not resolve itself. It requires emergency medical treatment, full stop.

Who’s Most at Risk

Heat can affect anyone, but some Canadians face significantly higher risk: older adults, infants and young children, people with chronic health conditions, outdoor workers, pregnant people, and those without reliable access to air conditioning, including people experiencing homelessness. Certain medications, including some diuretics, antihistamines, and psychiatric medications, can also increase vulnerability to heat, so it’s worth checking with a pharmacist if you’re unsure.

Substance use is another overlooked risk factor, alcohol, cannabis, and stimulant drugs can all impair the body’s ability to regulate temperature, increasing danger during a heat wave.

How to Stay Cool and Hydrated

A few simple habits make a meaningful difference:

  • Drink water before you’re thirsty. Thirst is a lagging indicator, by the time you feel it, you’re already somewhat dehydrated. Avoid alcohol and excess caffeine, both of which contribute to dehydration.
  • Time your outdoor activities. Schedule anything strenuous for the cooler parts of the day, early morning or evening, and limit time outside during peak afternoon heat.
  • Dress for the heat. Lightweight, loose-fitting, light-coloured clothing and a wide-brimmed hat go a long way.
  • Know your indoor risk. Many heat-related deaths in Canada happen indoors, in homes that can’t cool down properly,  especially upper floors and units with poor ventilation. For heat-vulnerable people, risk increases at indoor temperatures above 26°C, and becomes very high above 31°C.
  • Use cooling strategies at home. Close blinds and windows during the day to block heat; open them at night once it cools down. Fans help with comfort but may not be enough during extreme heat, if you don’t have air conditioning, consider a cooling centre, library, or shopping mall during the worst of it.
  • Never leave anyone in a parked car. Temperatures inside a vehicle can reach 52°C within 20 minutes when it’s 34°C outside and cracking the windows doesn’t meaningfully help. This applies to children, pets, and dependent adults.

Check On Others

One of the simplest, most effective things you can do during a heat wave is check on the people around you, especially older neighbours, people living alone, and anyone without air conditioning. The Canadian Red Cross recommends finding a “heat buddy”: someone you check in on, and who checks in on you, throughout a heat event. A short visit or phone call can catch early warning signs before they become a 911 call.

Stay Informed

Environment Canada’s colour-coded heat warning system runs from yellow (lowest) to orange to red (most severe). You can get alerts through the WeatherCAN app, local news, or your province’s emergency alert system. It’s also worth checking the Air Quality Health Index, since heat waves often bring poor air quality alongside the heat itself.

Heat-related illness is almost entirely preventable with a bit of preparation and awareness. Drink water, know the signs, and don’t hesitate to call 911 if something feels seriously wrong, heat stroke is not something to wait out

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The information provided on TheHealthInsider.ca is for educational purposes only and does not substitute for professional medical advice. TheHealthInsider.ca advises consulting a medical professional or healthcare provider when seeking medical advice, diagnoses, or treatment. To read about our editorial review process click here.

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