Heat illness

The spectrum of heat-related illness ranges from mild heat cramps, to life-threatening heat stroke. Regardless of ambient temperature, situations where body heat production is high, such as vigorous exercise, and where evaporative heat loss is limited (e.g. high humidity, impermeable garments) place people at risk of heat illness.

Important factors in preventing heat illness are route planning, including the consideration of the availability of ample water supplies, modifying activity to the cooler morning and evening, and considering humidity and exposure when planning distances and length of days.

Heat illnesses are preventable. Party leaders should be aware of risk factors in the group that predispose to heat illness, adapt activity according to risk factors and environmental conditions, and ensure adequate fluid intake while exercising. Regardless of preparation, training and acclimatisation, no person is immune from heat illness. Prevention of heat illness requires an awareness of the following:

  • Dress in light coloured, loose garments to facilitate sweat evaporation, which is the main means used by the body to dissipate heat. Excess body fat provides insulation which reduces heat loss. The face, scalp and upper torso are the most important areas to provide cooling through sweating, so prudent removal of headgear and towelling off improves heat loss.
  • Modify the level of physical activity according to the heat and humidity, taking frequent rests in the shade.
  • Physical fitness results in improved exercise tolerance in the heat, but even fit individuals may lack conditioning for a particularly strenuous activity or adverse environmental conditions.
  • Acclimatisation, improving the body’s response to heat, occurs with gradual increase in time and intensity of exercise in the heat, usually requiring about 1 hour of exercise per day over a period of 8–10 days. Once acclimatised, the body sweats sooner and in greater volumes, facilitating body cooling. Loss of acclimatisation to heat takes about 1–2 weeks. Acclimatisation is not improved by restricting fluid intake.
  • Fluid intake must be adequate. Remember to be well hydrated before commencing activity, and that relieving thirst alone is not a good marker of sufficient fluid intake. Sweat losses in an acclimatised person can reach up to 1.5 L per hour during exertion. About 250–500 mL water should be drunk 10 to 15 minutes prior to commencing exercise, followed by 250–350 mL every 20–30 minutes during exercise. Deliberate restriction of fluids while exercising in hot conditions is a very hazardous practice.

Susceptible candidates for a heat-related illness are the young or elderly, unfit, obese, exercising in hot, humid conditions, inappropriately dressed, unacclimatised to the heat and with an insufficient fluid intake. Dehydration from any cause, such as inadequate fluid intake during exercise, diarrhoea or vomiting is another a risk factor for heat illness. Nevertheless, remember that healthy young adults exercising strenuously in hot conditions, may be susceptible to heat stroke in the absence of other risk factors. Note that alcohol, which impairs body temperature control, and certain drugs which increase body heat production or impair sweating, predispose to heat illness. Examples include antihistamines, and stimulants such as amphetamines.

Fluids in heat illness
Remember that a person exercising in the heat can readily lose over 1 L of water per hour, and ensuring sufficient fluid replacement may be difficult. Cold oral fluids are absorbed more quickly than warm. Drinking smaller volumes of fluids (e.g. 150–250 mL) is less likely to cause abdominal discomfort or nausea, especially if exercising. The concentration of carbohydrates in fluids should not exceed 6% or absorption from the gut is delayed, and diarrhoea may occur.