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Heat and Humidity


Hot, sunny and tropical climates may expose travellers to a rapid change in temperature and/or humidity which can have an adverse effect on their health. Heat disorders may be minor such as rashes (prickly heat), fainting, swelling of hands, feet or legs and muscles cramps. Major disorders include heat exhaustion, heat stroke and low sodium in the blood as a result of over exertion or exercise.

Exposure to high temperatures can result in loss of fluids and if severe, may lead to heat exhaustion and/or heatstroke. Heat exhaustion and heatstroke are related conditions that can have a serious outcome if not treated promptly.

Risk Factors

Temperature regulation is maintained by the skin and circulation. When the body gets overheated, it is cooled by dilation of the blood vessels which direct blood away from the centre of the body to the skin which in turn causes sweating.

  • Rapid dehydration may occur in hot dry conditions.
  • Humidity can reduce rate of sweat evaporation making it difficult to regulate the body's temperature.
  • The elderly, babies and children, and those with pre-existing medical conditions may be more susceptible to heat disorders.
  • Those undertaking strenuous physical activity such as athletes, hikers and military personnel are at greater risk of major heat disorders.

Reducing the Risks

Prickly heat

Prickly heat is a common condition in hot and humid climates - it particularly affects children. It is caused by the sweat glands becoming congested leading to an unpleasant prickly sensation usually affecting the neck, chest and back. This may be accompanied by a rash and blisters.

  • Regular showering and bathing may help.
  • Application of calamine lotion or zinc and castor oil cream may help relieve the itch.
  • Wearing light loose clothing may help.


Sudden fainting in the heat may occur when travellers are not acclimatised. Fainting occurs when the blood vessels dilate to increase the circulation and radiate heat from the skin. This also has the effect of lowering the blood pressure and reducing the blood supply to the brain.

  • Consciousness should quickly return once the person is lying flat.
  • Oral fluids, relief from heat and rest is usually sufficient treatment for an uncomplicated heat related faint.


Mild swelling of feet and hands may occur when first exposed to hot weather. This effect is more common in female travellers. Swelling occurs when the blood supply to the skin increases which also radiates heat; fluid moves out of the blood vessels and into the tissues causing swelling.

  • Heat related swelling should resolve spontaneously during acclimatisation, it should not be treated with diuretics (water tablets), which may cause dehydration and delay acclimatisation.


Heat related cramps are painful muscle contractions that usually begin an hour or more after exercise, often involving heavily used muscles in the calves, thighs and abdomen.

  • Gentle stretching, rest and oral rehydration solutions may help relieve symptoms.

Heat Exhaustion and Heatstroke

Heat exhaustion can be avoided with careful planning.

  • Symptoms of heat exhaustion include malaise, fatigue, dizziness, headache and feeling faint.
  • Treatment is through rehydration with water and other fluids.
  • Salts often need to be replaced too, so water alone is not always sufficient.
  • Rehydration sachets are available but a simple rehydration solution can be made using, for example, soda, salt and lime juice.
  • Spraying the face and body with a water-spray, or sponging down will help as will use of a fan.

Heatstroke can be avoided with careful planning.

  • Heatstroke is more serious when symptoms of overheating do not resolve with the methods above.
  • Urgent medical attention is needed, usually with rehydration using intravenous fluids.
  • Other additional cooling measures such as using ice packs to reduce the body temperature may be required but these should only be used with expert guidance.

Heat acclimatisation takes time; travellers can take some simple steps to reduce their risk of heat related illness:

  • Travellers should limit physical exertion until they have become acclimatised, most travellers will adapt to higher temperatures in approximately 10 days.
  • Maintain adequate hydration; fluid intake can be supplemented with small (but not excessive) amounts of salt added to food or drink, providing there are no pre-existing medical contraindications.
  • If urine becomes dark then fluid intake needs to be increased - this is a good indicator of hydration status.
  • Clothing should be light and airy, avoid wearing dark or tight fitted clothing.
  • A hand held personal fan may prove invaluable when it is not possible to escape the heat - traditional and small battery operated fans are easily available.
  • Travellers should try to avoid heat exposure during the hottest part of the day, usually between 11am and 3pm - take a siesta in hot countries.
  • Take advantage of air-conditioning and shade where possible.

For further information on the management of sun exposure please refer to the Sun Exposure advice sheet.

Exercise-associated Hyponatremia

Hyponatremia (low sodium level in blood) mostly occurs in those that are undertaking physically strenuous activity, due to the loss of sodium through sweating and fluid replacement with excessive amounts of plain water.

  • Symptoms of hyponatremia may be vague and non-specific; confusion, lethargy, muscle weakness, headache and nausea.
  • Late symptoms (associated with severe or rapid-onset hyponatraemia) may include disorientation, agitation, seizures and coma.
  • Hyponatremia may appear as persistent alteration of mental state without a high temperature and history of large volumes of water taken.
  • If symptoms are severe, hyponatraemia should be considered a medical emergency. Urgent medical treatment should be sought and admission to hospital.

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