Extreme heat and cold can be very dangerous. In high temperatures, the body keeps cool by the evaporation of sweat from the skin. In low humidity evaporation occurs quickly and air, moved by wind or a fan, also increases evaporation. But there is an upper limit to the air temperature we can endure with comfort, even in low humidity. This is because in very high temperatures we may not be able to sweat fast enough to match the rate of evaporation. Also, the body can gain heat from very hot air. Increased wind speed will be even more dangerous because evaporation will be even faster.

When the body cannot lose heat fast enough there is a possibility of heat exhaustion. There is a feeling of tiredness, loss of appetite, and general discomfort. Visual hallucinations are possible and also vomiting. The sufferer should be moved to a cool place and made to drink salt and water to replace the sweat and salts lost through perspiration.

Heatstroke is a much more dangerous condition and, although rare, is more likely to follow from the less obvious condition of heat exhaustion. It can be fatal. Heatstroke is the result of the body's normal cooling mechanism ceasing to function. The skin becomes dry and body temperature starts to rise. The symptoms are burning sensations, dry skin, followed by feverish feelings. These symptoms can develop into restlessness, headache and confusion. Unless treated at once body temperature will rise until the victim becomes unconscious, and death may follow. For a person suffering from heatstroke, medical attention is necessary. The patient must be put in the coolest possible place and cooled as fast as possible. He or she should be splashed with cold water, or better still, iced water. Wrapping him or her in a wet sheet with a fan directed on the body will help to reduce body temperature quickly. Vigorous massage can also help.

There are precautions that can be taken against heat exhaustion and heat stroke. When arriving in a very hot country do not exert the body until you are acclimatized - about a week. Air travel is so fast that, unlike slow sea journeys, the body has no time gradually to acclimatise itself to high temperatures. Also air conditioning in hotels, offices etc. delays the process of acclimatisation. Drink plenty of liquid but not too much alcohol. Take more salt with food, or even salt tablets. Wear comfortable, light clothing and avoid sunburn. Adopt local customs, the inhabitants know what it's like.

In very cold climates we put on more clothes - special clothing in polar regions - and we can keep relatively warm by generating body heat by walking or other activities. But if we stop moving, become tired, and remain in a strong wind below freezing, we can very soon become extremely cold. The main dangers of very cold climates and high mountains, are frostbite and hypothermia. Hypothermia, also called exposure, is the failure to maintain body heat. The possibility is greatly increased if clothes become wet, evaporation from wet clothes causes the body to lose heat even more rapidly. Rain and snow with a strong wind increase the danger. Most cases of hypothermia out of doors occur through lack of proper clothing in mountains or at sea. Climbers and walkers who get lost and are forced to spend all night on a mountainside are risking hypothermia - especially if they are wearing light clothing. Even in a hut where they are out of the wind, hypothermia can occur if there are no blankets. Old people are particularly susceptible. At a very low body temperature (around 25-28 degrees C (77-82 degrees F) the condition becomes critical. To counteract hypothermia, the body should be warmed by any means available. Rapid rewarming in a warm bath of 40-45 degrees C (104-113 degrees F) is very important. If breathing has stopped, then artificial respiration and cardiac massage are required immediately.

Frostbite is the extreme condition when the flesh freezes. It is most likely to affect the face, hands and feet and a bad case may mean the loss of limbs or other permanent injury. The affected parts should be rewarmed as soon as possible. If possible rewarming in water no hotter than 40-44 degrees C (103-111 degrees F) will help a great deal. Water hotter than this increases pain and swelling. There should be no massage, no rubbing and no exercise, and the affected part should not be bandaged.