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Medical problems caused by exercise in hot weather

The following are extracts from ‘First Aid and Wilderness Medicine’, published in April 2007.

Sample pages as a .pdf

Exercise in a hot weather can cause a wide range of problems that are best avoided by careful preparation and prevention (see Prevention in hot weather, p26). Recognition of early symptoms is essential, as they are not painful and therefore easy to ignore.

Heat exhaustion and heat stroke are due to long periods of physical exercise in hot conditions resulting in loss of body fluid and salts. Heat exhaustion can be thought of as dehydration due to exercise in hot weather. While recovery in mild cases of heat exhaustion can be rapid, weakness may persist for days. If the illness was moderate to severe, the victim may be unwell for days and evacuation may be needed.

Symptoms and signs

Are the same as dehydration: tiredness, weakness, dizziness, headache, etc, PLUS:
muscle cramps, temperature is usually normal but may be up to 39ºC, the victim may feel faint or even faint briefly (see ‘Heat stroke’).
Treatment: move away from hot ground, roll onto side if they have fainted. Give water (the victim may be semi-naked or covered with a wet sheet). If their temperature is raised, apply wet cloths (not cold, no ice) to the neck, armpits, groin and upper abdomen while continuing to fan and spray the victim.

Heat stroke

This is an emergency. Heat stroke is life threatening, causing death by overheating of the brain and other vital organs. The heat loss mechanisms of the body fail and there is a rapid rise in body temperature. It represents the extreme end of the spectrum of heat-induced problems and can develop if early symptoms are undetected or untreated and exercise is continued. Once the initial collapse and over-heating have been dealt with, exclude other causes of fever and unconsciousness (e.g. malaria, septicaemia, meningitis).

Other hot weather problems

Dilutional hyponatraemia (low blood sodium level)
This typically occurs in endurance-type situations where there are long periods of sweating exercise when NO food has been eaten but LOTS of plain water has been drunk. As a result, sodium concentration in the blood can drop too low, with little or no dehydration.

Heat cramps

These muscle cramps happen when people engage in, or soon after, heavy exercise. The cramps may be very painful.


Stretch the cramping muscle gently out to full length.
Give 1 litre of ORS.

Prickly heat

This happens when the sweat glands (in the groin, under the breasts, around the waist, chest or back) get blocked in hot and humid conditions, and itchy areas of redness with spots and blisters appear in the affected areas. If this is widespread, it may predispose to heat exhaustion.
Keep cool, wear loose cotton clothes, avoid scratching. Wash with water only, no soap. Treat as for allergy.

Dehydration occurs when too much water (and salt in some cases) is lost from the body. Water losses are normally replaced by drinks and/or food, and salt loss is replaced by food. In normal conditions, the average adult needs approximately 2.5 litres of liquid a day from drinks and/or food. This may increase to 5 litres or more at altitude and 1 litre per hour (or more) when exercising in hot, dry desert conditions.
You are hydrated if your urine is pale, plentiful and doesn’t smell. You are dehydrated if you pee infrequently, in small amounts, it is strong smelling and darker in colour. (Your kidneys need to pass a minimum of 500ml of urine per day to work efficiently.)

A simple formula to make ORS: (Oral Rehydration Solution)

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