Heat Illness: Symptoms, First Aid & When to Act Canada

Heat illness happens when your body cannot cool itself down fast enough in hot or humid weather, and it affects thousands of Canadians every summer. It ranges from painful muscle cramps to life-threatening heat stroke, and every year people across Canada end up in emergency rooms — or worse — because they did not recognize the warning signs in time.

What Is Heat Illness and Why Is It Dangerous in Canada?

Understanding heat exhaustion symptoms and the other stages of heat-related illness is critical for staying safe during increasingly intense Canadian summers. Children, older adults, and people with chronic health conditions face the greatest risk, but heat illness can strike anyone. This guide explains each type, what to do for each one, and exactly when to get emergency medical help.

What Is Heat Illness?

Types of Heat Illness: Characteristics and Management Guide

Type of Heat Illness	Key Symptoms	Severity	Immediate Management
Heat Cramps	Painful muscle spasms, heavy sweating, normal body temperature	Mild	Rest in a cool place, drink water or electrolyte beverages, gently stretch affected muscles
Heat Syncope	Sudden fainting or dizziness, pale skin, brief loss of consciousness	Mild to Moderate	Lay person flat, elevate legs, move to cool area, offer fluids if conscious
Heat Exhaustion	Heavy sweating, cool or clammy skin, weakness, nausea, headache, body temperature up to 40°C	Moderate	Move to cool environment, apply cool wet cloths, sip cool water, seek medical attention if symptoms worsen
Heat Stroke (Classic)	Body temperature above 40°C, hot dry skin, confusion, loss of consciousness, no sweating	Severe — Medical Emergency	Call 9-1-1 immediately, rapidly cool the body with ice water immersion or cool wet sheets, do not give fluids
Exertional Heat Stroke	Body temperature above 40°C, excessive sweating or no sweating, altered mental status, seizures	Severe — Medical Emergency	Call 9-1-1 immediately, begin aggressive cooling at once, cold water immersion is the gold standard in Canada

Your body works hard to keep its temperature stable. When the air is very hot and humid, that job becomes much harder. If your body loses too much fluid or simply cannot release heat fast enough, heat illness sets in.

Milder forms usually happen during intense physical activity in the heat. However, severe heat illness — like heat stroke — can strike even people who are just sitting still on a hot day. Children, older adults, and people with chronic health conditions face the greatest risk.

According to Health Canada, extreme heat events are becoming more frequent across the country. Understanding heat illness could save your life or someone else’s.

Heat Cramps: The Mildest Form of Heat Illness

Heat cramps are painful muscle spasms that happen during or after hard physical activity in hot conditions. They most often affect the legs, though they can hit any large muscle group. Your skin will feel cool and damp, and your body temperature stays normal.

Heat cramps are uncomfortable, but they are not dangerous on their own. However, they are your body’s first warning that something is wrong.

How to Treat Heat Cramps

Stop what you are doing and move to a cool area right away. Rest and replace the salt and fluids your body has lost. You can drink a sports electrolyte drink, or mix about one teaspoon of salt into 500 mL of water and sip it slowly.

In rare cases, a doctor may give fluids through an IV line. A moderate amount of normal saline given over a few hours usually clears up the cramps completely. Most people recover well without needing a clinic visit, but do not ignore the cramps and push through the pain.

Heat Exhaustion: A Serious Warning Sign

Heat exhaustion is a more serious form of heat illness. It happens when your body loses too much water and salt through sweating. People who are not yet used to working or exercising in the heat are most at risk.

Symptoms include headache, nausea, vomiting, dizziness, weakness, irritability, and muscle cramps. On examination, the person will be sweating heavily, may have goosebumps, and their blood pressure may drop when they stand up. Body temperature is normal or only slightly raised.

How to Treat Heat Exhaustion

Move the person to a cool, shaded space immediately. Use a fan or cool, damp cloths to help bring their temperature down. Have them drink an oral electrolyte solution — the kind sold at Canadian pharmacies — if they are not vomiting and feel stable.

If the person is vomiting or feels very unwell, they need medical attention. A doctor or paramedic may give one to two litres of normal saline (a saltwater solution) through an IV. Furthermore, the person should avoid exercising in the heat for at least two to three days after recovering. Returning too soon raises the risk of a more dangerous episode.

Heat Syncope: Fainting in the Heat

Heat syncope means fainting — or nearly fainting — because of the heat. It affects people who are not used to hot conditions. Physical activity causes blood vessels near the skin to widen, which pulls blood away from the brain. As a result, the person briefly loses consciousness.

The key difference between heat syncope and heat stroke is recovery speed. A person with heat syncope wakes up quickly once they lie flat, and their body temperature is normal. This is an important distinction — heat stroke is a medical emergency, while heat syncope is generally less severe.

How to Treat Heat Syncope

Lay the person flat in a cool area and elevate their legs slightly. This helps blood return to the brain. Offer cool fluids once they are fully awake and can swallow safely.

In addition, encourage a gradual return to physical activity. Do not rush back into exercise in the heat. If fainting episodes happen more than once, or if the person has a heart condition, a visit to a family doctor or walk-in clinic is a smart next step.

Heat Stroke: A Life-Threatening Emergency

Heat stroke is the most dangerous form of heat illness. The body’s core temperature rises so high — above 40.5°C (104.9°F) — that it begins to damage organs directly. Even with fast treatment, heat stroke can be fatal. Some studies suggest mortality rates can reach 76% when the core temperature climbs above 41.1°C.

There are two types of heat stroke. Classic heat stroke develops over several days of heat exposure. Exertional heat stroke comes on rapidly during intense physical activity in hot, humid conditions. Both types are medical emergencies. Call 911 immediately.

Classic Heat Stroke

Classic heat stroke tends to affect people who are already vulnerable. This includes older adults, people with chronic illnesses, those who are dehydrated, and people taking certain medications. Drugs such as diuretics (water pills), sedatives, antipsychotics, and anticholinergics can raise the risk. High humidity and lack of air conditioning are major contributing factors.

A person with classic heat stroke typically has a core temperature above 40.5°C, is not sweating (anhidrosis), and is confused or unconscious. More than half of patients also have an underlying infection when they arrive at the hospital.

Exertional Heat Stroke

Exertional heat stroke strikes people who push their bodies hard in hot, humid weather without being acclimatised. Athletes, construction workers, and military personnel are at higher risk, especially if they do not have enough water. About half of these patients are still sweating when they are found — which can make it harder to recognise as heat stroke.

Exertional heat stroke also carries a higher risk of serious complications. These include disseminated intravascular coagulation (DIC, a dangerous blood clotting disorder), lactic acidosis, and rhabdomyolysis (muscle breakdown that can damage the kidneys).

Signs of Heat Stroke to Watch For

• Core body temperature at or above 40.6°C (105°F)
• Confusion, disorientation, delirium, or loss of consciousness
• Hot skin — may or may not be sweating
• Rapid heartbeat
• History of prolonged heat exposure or intense exercise in the heat

Emergency Treatment for Heat Stroke

Call 911 right away. While waiting for paramedics, move the person to a cool area and remove excess clothing. Apply ice packs to the neck, armpits, and groin. Use a fan and cool, wet cloths to lower body temperature as quickly as possible.

In hospital, doctors will run blood tests including a complete blood count, kidney function, liver enzymes, clotting factors, blood sugar, and blood gases. Continuous heart monitoring is standard care. If an infection is suspected, doctors will also take blood and urine cultures. The Mayo Clinic explains heat stroke diagnosis and treatment in detail for those who want to learn more.

Who Is Most at Risk for Heat Illness in Canada?

Anyone can develop heat illness, but some people are more vulnerable. Knowing your risk helps you take better precautions during heat warnings issued by Environment and Climate Change Canada.

• Older adults — the body’s cooling system becomes less efficient with age
• Young children — they heat up faster and cannot communicate symptoms clearly
• People with chronic conditions — heart disease, diabetes, kidney disease, and obesity all increase risk
• People taking certain medications — diuretics, antipsychotics, antihistamines, and blood pressure drugs can interfere with sweating or circulation
• Outdoor workers and athletes — prolonged exertion in the heat is a major trigger
• People without air conditioning — heat waves are especially dangerous in homes that stay very hot

The World Health Organization highlights the growing global risk of heat-related illness, particularly for urban populations during extreme heat events.

When to See a Doctor About Heat Illness

Mild heat cramps often resolve with rest and fluids at home. However, you should visit a walk-in clinic or call your family doctor if symptoms do not improve within an hour, or if cramps keep coming back.

Go to an emergency room or call 911 immediately if the person:

• Loses consciousness or cannot be woken up
• Is confused, slurring speech, or behaving strangely
• Has a body temperature above 40°C (104°F)
• Is not sweating despite being very hot
• Has seizures
• Is vomiting and cannot keep fluids down

If you are unsure, it is always better to seek care. Most provincial health plans cover emergency visits, and walk-in clinics across Canada can assess heat exhaustion quickly. You can also call 811 (Health Link or Telehealth Ontario, depending on your province) to speak with a registered nurse any time of day.

Always speak with your family doctor before hot weather arrives if you take regular medications — some drugs increase your sensitivity to heat and may need to be adjusted during heat warnings.

Frequently Asked Questions About Heat Illness

Key Takeaways

• Heat illness ranges from mild cramps to life-threatening heat stroke — know the difference.
• Heat cramps and heat exhaustion can usually be treated with rest, cool surroundings, and electrolyte replacement.
• Heat stroke is a medical emergency — call 911 immediately and begin cooling the person right away.
• Older adults, young children, people with chronic conditions, and those on certain medications face the highest risk.
• Drink plenty of water, seek shade, and take breaks during physical activity on hot days.
• Check in on vulnerable neighbours and family members during heat waves — it can save a life.
• If you are ever unsure whether symptoms are serious, call 811 or visit a walk-in clinic. It is always better to be safe.

According to Health Canada’s guide to extreme heat and heat waves, this information is supported by current medical research.

For more information, read our guide on chronic conditions like thyroid disorders that increase heat sensitivity.

This article is for general information only and does not replace professional medical advice. Always consult your family doctor or a qualified healthcare provider if you have concerns about heat illness or any other health condition.