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The health effects of heat

During a heat wave, it is harder to cool down and keep your body temperature within normal limits. At these times, prolonged exposure to heat, excessive physical exertion or very heavy sweating can have certain effects on health.

Symptoms that require monitoring changes in health


It is important to monitor any deterioration in the health of an adult who has the following symptoms:

  • Headaches
  • Muscular cramps
  • Swollen hands, feet and ankles
  • Appearance of small red bumps on the skin, called a “heat rash”
  • Unusual fatigue or exhaustion
  • Generalized malaise
  • Signs of dehydration:
    • Intense thirst
    • Less frequent need to urinate
    • Dark urine
    • Dry skin
    • Rapid pulse and breathing
  • Nausea or vomiting

If you experience any of these symptoms, immediately go to a cool place and drink water.

If you have questions about your health, call Info‑Santé 811 or consult a health professional, a pharmacist for instance.

When to consult

Other symptoms, however, require immediate medical attention, meaning within 2 hours. An adult who has one or more of the following symptoms must be taken to the emergency room immediately or 9‑1‑1 must be called on their behalf:

  • Difficulty breathing
  • Convulsions (stiffness of the body and jerky, involuntary muscle contractions)
  • Signs of impaired consciousness:
    • Confusion
    • Unusual behaviour
    • Agitation
    • Hallucinations
    • No response to stimuli
  • Loss of consciousness
  • Signs of heat stroke:
    • Temperature over 39.5 ºC (103.1 ºF) with an oral thermometer or over 40 ºC (104 ºF) with a rectal thermometer
    • Dry, red, hot skin or pale, cold skin
    • Dizziness and vertigo
    • Confused and illogical speech
    • Aggressive or bizarre behaviour
    • Generalized malaise

Heat stroke is the most serious effect of heat. It can occur suddenly and quickly lead to death if not treated.

Babies or children

Certain symptoms may indicate the presence of heat-related complications:

  • Dry skin, lips or mouth
  • Abnormal skin colour (red or pale)
  • Headaches
  • Sunken eyes with dark rings
  • Dark and smaller quantity of urine
  • Vomiting, diarrhea
  • Unusual restlessness, irritability or confusion, temper tantrums
  • Difficulty breathing
  • Drowsiness, prolonged sleep and difficulty waking up
  • Body temperature over 38.5 ºC (101.3 ºF) with a rectal thermometer or over 37.5 ºC (99.5 ºF) with an oral thermometer (Note: Using an oral thermometer to take the temperature of newborns, babies and children under 5 years old is not recommended.)

When to consult

A child’s health, especially a very young child, can deteriorate quickly and be difficult to notice. When a baby or child shows symptoms, a medical consultation is usually necessary. If you are unsure, you can call Info‑Santé 811. In an emergency, call 9‑1‑1.

Protection and prevention

You can take certain precautions to prevent the effects of heat, such as drinking enough fluids and cooling off often. You will find other advice to ensure your well-being and that of your loved ones during a heat wave on the Preventing the effects of heat page.

Risk factors

The risk of feeling discomfort due to heat is higher:

  • When humidity is high
  • When there is light wind or none at all
  • When a heat wave occurs and the body is not used to the heat, such as early in the season or suddenly after a period of cooler weather
  • When the heat lasts for several days without a break
  • When temperatures are high during the night
  • In cities, where the temperature is generally higher than in the countryside

People at risk

Some people are more at risk of developing complications if they are exposed to heat:

  • Babies and children under 5 years of age
  • Older adults
  • People with reduced autonomy or who live alone:
    • People with reduced mobility
    • People with limited contact with family or friends
  • People suffering from chronic illnesses or severe mental health problems
  • People with drug or alcohol use problems
  • People with physically demanding jobs or who work in the sun or outdoors, such as construction workers or agricultural workers
  • People who work in places where processes emit heat, such as foundries or bakeries
  • People who do intense physical exercise outside in hot weather or indoors in places that do not have air conditioning or are poorly ventilated
  • People who do not have access to cool or air-conditioned places
  • People who take certain medications that can aggravate the effects of heat
  • People who are in poor physical condition or who are overweight
  • People who are sick and have a fever
  • Pregnant women and their fœtus

Last update: June 13, 2023


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