How heat affects the body


How heat affects the body


How heat affects the body

With Summer here in Toronto and the increase in outdoor physical activity, it is good to know how the hot temperatures, in collaboration with exercise participation can affect you and how you can work to prevent a heat injury from occurring

The effect of heat, such as heat exhaustion, heat stroke and heat cramps are commonly  caused by dehydration, exercising in hot temperatures, or a combination of both. This can consequently result in outcomes varying in severity from dizziness to unconsciousness. Without the appropriate guidance and preparation for exercising in the heat, a good intention to exercise can very quickly become a serious heat injury.  Heat injuries are negative outcomes associated with exercising in hot temperatures and/or dehydration.

What are the effects of heat on the body?

Listed below are the most common effects of heat:

  • Extra heat created by the body during exercise caused by:
    • Increases in muscle activity; and
    • Increases in metabolic rate.
  • Sweating to try lower the body’s internal temperature. More so during exercise (especially in hot temperatures) as the body tries to cool the body at the same rate as it creates heat, regulating internal temperatures.
  • Dehydration is caused by either:
    • Excessive sweating (in heat and/or exercise);
    • Lack of available fluids (especially to replace those lost sweating); and
    • Illness causing inability to maintain fluid levels due to vomiting and diarrhea.

Some may find it easier for their body to cope in hot environments and to be able to exceed performance-wise in comparison to others due to acclimatization. When the body is stressed to maintain a normal body temperature, we as humans have cooling mechanisms, such as sweating, to help cool us.

How does Toronto’s humidity affect our bodies?

In temperatures with a high level of humidity, this mechanisms does not work as efficiently as there is already a high level of moisture in the air, and so evaporation of the sweat from the skin does not occur as quickly.

However, if you have lived and/or trained in an environment which is characteristically both hot and humid, your body would be more trained, or acclimated to cope by:

  • Reducing the amount of sodium contained in the sweat you produce;
  • Initiating the sweating response much earlier than usual; and
  • Sweating more than usual.

Despite this, it is still possible for those not acclimated to these environments to increase their ability to cope in these conditions by living and/or training in these environments to slowly adapt their body’s circulatory, musculoskeletal and respiratory systems.

Symptoms of a heat injury?


There are various detrimental effects of heat and dehydration, causing hyperthermia (high body temperature), however the three most common heat injuries are:

1) Heat cramps:

Cause: In the past, heat cramps were thought to be due to dehydration. However, due to a lack of scientific evidence, it is now thought to be as a result of changes in the spinal neural reflex activity, an outcome of fatigue.

When: At rest or during activity.

Outcome: Intense involuntary muscular contractions of a short duration in the gastrocnemius (calf) and hamstrings (back of the thigh).


  • Intense muscle spasms causing localized pain

2) Heat exhaustion / exercise-associated collapse:

Cause: The term ‘heat exhaustion’ is misleading as dehydration is not the cause. Venous return is significantly impacted when there is a sudden drop in exercise activity. During exercise, the muscles act as a secondary pump to maintain the level of blood flow required to sustain exercise. When exercise suddenly stops, the blood is not being pushed back to the heart at the same rate and so blood pools in the limbs, and reduces the blood flow back to the heart. This effect is further worsened in hot climates as the body works harder to sustain intensity.

When: Occurs post-exercise.

Outcome: The pooling of blood in the limbs, known as postural hypotension (low blood pressure), can result in fainting, or at its worst, can progress to the more severe condition, heat stroke.


  • Nausea
  • Skin that looks pale
  • Intense sweating
  • Headaches
  • Dizziness and/or fainting
  • Cramps in the muscles

3) Heat stroke:

Cause: There are two main causes of heat stroke, the first is dehydration, due to heat, exercise and/or sickness. The other more common cause is exercise participation in collaboration with high levels of humidity. This affects the body by forcing it to work overtime to attempt to regulate the body’s internal temperature. When exercising in high levels of humidity, which is a high level of moisture in the air, creating a difficult environment for the body to regulate body temperature by decreasing the efficiency of the body’s cooling mechanism, sweating, the evaporative uptake of sweating from the skin into the air, consequently cooling the body.

When: Can occur in hot temperatures and/or during exercise. It can also occur when there is a lack of fluids in the body, such as when the patient is ill.

Outcome: The inability to cool the body efficiently and/or the inability to replace lost fluids can have varied results, from high body temperatures to unconsciousness.


  • Behavioural – aggressive or irritative, more commonly confused and disorientated.
  • Seizure
  • High breathing, pulse rate and temperature (rectal temperature 41C)
  • Low blood pressure
  • Unconsciousness
  • Coma

Treatment of a heat injury?

The treatment of a heat injury varies greatly on the level of severity and availability of resources to overcome the effects of the condition.

1) Heat cramps:

  • Self-applied massage and/or slight stretching can help relieve pain

2) Heat exhaustion / exercise-associated collapse:

  • To reduce blood pooling in the limbs and subsequently increase blood pressure to normal levels by asking the patient to lay down and elevate the legs.
  • Drinking a sports drink or moving into the shade or air-conditioning can also help in cooling the body’s internal temperature down.
  • If no improvement is seen after approximately 10 minutes, and heart rate and blood pressure do not regulate back to normal measure, intravenous fluids should be administered.

3) Heat stroke:

  • Heat stroke is a condition which quickly progresses in severity. Emergency services should be called as soon as possible.
  • Reduce the internal temperature of the body as efficiently as possible by using a combination of the following:
    • A cold drink, in particular a sports drink if possible which contains electrolytes. This also helps to replace lost fluids.
    • Assist evaporation of the sweating response by fanning the patient.
    • Immerse the patient in cold temperatures such as air-conditioning, shade, a cool bath, or wet towels on the skin’s surface.

Without the required medical attention or resources to treat the heat injury at a less severe stage, it will commonly progress to the most severe stage, heat stroke. With that being said, it is better to be educated on measures you can take to prevent the occurrence of a heat injury. Some of these measures include:

  • Keep fluids high with a combination of water and sports drinks.
  • Try to stay out of heat where possible, in air-conditioning or the shade.
  • If participating in exercise, especially in a competition, consult with a registered Physiotherapist about the individualized steps you should take not only enhance your performance, but also to avoid the negative effects of heat.

Visit Toronto Physiotherapy for more information about our services and the treatments we provide.

About the Author

Brad SaltzBrad is a Registered Physiotherapist at Ace Physio, a highly respected Physiotherapy clinic in downtown Toronto. Ace Physio provides high quality one on one Physiotherapy that combined state-of-art technology such as; Shockwave Therapy, Laser Therapy, and Spinal Decompression with traditional Physiotherapy.View all posts by Brad Saltz