heat exposure

heat exposure Although the origins of human life were in the tropical regions, modern man is better able to cope with cold environments than with extreme heat. The skin and subcutaneous tissues of the body can function normally over a wide temperature range, and the temperature of the skin will generally follow the temperature of the surrounding environment. The deeper tissues, and especially the brain, however, must be maintained within a few degrees of the normal body temperature of 37°C if optimum function is to be maintained and irreversible damage prevented. Exposure to a high ambient temperature is a major challenge to the body's ability to keep its temperature within the optimum working range.

Heat is exchanged between the body and its surrounding environment by physical transfer involving conduction, convection, and radiation. When skin temperature is higher than the environmental temperature, heat is lost by these mechanisms, but at high environmental temperatures, heat is gained. Heat exchange can be regulated to some degree by regulating the skin blood flow, which alters the effective insulation layer between the deep tissues and the surface, but behavioural mechanisms, including alterations in the amount of clothing worn and control of the environmental temperature, are more effective. Heat is constantly produced by the metabolic reactions occurring within the body. These are relatively inefficient, and about 75% of the energy appears as heat: this is about the same level of efficiency as the internal combustion engine. At rest, the rate of heat production is low (about 60 W — roughly equivalent to the heat output of a small domestic light bulb), but this rises dramatically during exercise, and the average marathon runner produces more than 1 kW of heat (similar to the output of a small electric fire). When the ambient temperature is low, this can be lost to the environment by physical transfer without any large change in deep body temperature.

When the ambient temperature is high, heat is gained from the environment by physical transfer, and metabolic heat production continues to add to the heat load. Heat loss is promoted by increasing the rate of blood flow to the skin: this allows skin temperature to rise and reduces the thermal gradient, reducing the rate of heat gain. In order to prevent a catastrophic rise in body temperature, an additional heat loss mechanism is invoked as body temperature begins to rise. This involves evaporation of water from the body surface, and active secretion of sweat onto the skin surface will be initiated. The latent heat of vaporization of water is high, and evaporation of this water is an effective way of removing large amounts of heat from the body surface. Water is also lost by evaporation from the respiratory tract, but this is much less important in humans than it is in those animals which have thick coats of hair or wool, as such coats prevent evaporation from the skin by restricting air movement and allowing the air to become saturated with water vapour. A similar problem arises when the humidity of the air close to the skin is high: this is why a hot, humid climate feels more uncomfortable than a hot, dry environment.

At rest, low sweat rates are sufficient to maintain body temperature, but the sweat rate rises dramatically during exercise to balance the increased rate of heat production by the exercising muscles. Industrial workers in hot environments, or atheletes competing in hot climates, can sweat at rates approaching 3 litres/hour. Prolonged exposure to these conditions results in dehydration, and there is a need for fluid replacement if the work is to be sustained. Some of the water lost in sweat is derived from the blood plasma, with the remainder coming from the spaces between and within the body cells. Large sweat losses cause the blood volume to fall by 10% or even more. The reduced blood volume challenges the body's ability to continue to supply sufficient blood flow to the working muscles to provide oxygen and nutrients and to remove waste products, including heat, as well as to supply a high rate of blood flow to the skin to carry heat to the body surface where it can be lost. Heart rate will increase in an attempt to maintain the cardiac output, but eventually skin blood flow will fall, and body temperature will rise.

If body temperature is allowed to rise above about 40°C, a variety of symptoms of heat illness will appear. These include nausea, headache, and dizziness, which should be seen as warning signs. A core temperature of about 42°C or more will result in loss of consciousness, circulatory collapse, and eventually coma and death. The brain, liver, and kidneys, and the blood clotting mechanisms, appear to be particularly sensitive to rises in temperature. The danger of heat illness is obviously increased when both temperature and humidity are high, and is greatest when exercise has to be performed in these conditions. There have been many famous examples of athletes in endurance events collapsing in the later stages of an event, and these collapses normally occur only when the weather is unusually hot. The case of Dorando Pietri, who fell to the track close to the end of the 1908 Olympic Games Marathon held in London, is well known: Pietri was assisted to his feet, and staggered to the finishing line, only to be disqualified on the grounds that he had received assistance during the race.

Sweat contains a variety of salts (especially sodium chloride) as well as a large number of different organic molecules, and replacement of these salts as well as of water is necessary when large sweat losses are incurred. Humans have a limited appetite for salt compared with many animals, but it is normal to feel the need to add extra salt to food when sweat losses are high. Drinks taken to maintain hydration should contain small amounts of sugar and salt to stimulate water absorption in the intestine: this is the basis of the formulation of oral rehydration solutions developed for the treatment of the excessive rates of water loss caused by infectious diarrhoea. Sports drinks used by athletes are formulated along the same principles to achieve effective fluid replacement. The thirst mechanism is not normally sufficient to ensure an adequate rate of fluid to balance the sweat loss, and a conscious effort to drink beyond the level dictated by thirst must be made when sweating rates are high.

On repeated exposure to heat, the body adapts to increase its thermal tolerance. This is achieved by lowering the temperature threshold for the onset of sweating and increasing the sensitivity of the sweating mechanism so that a greater sweat rate is achieved for any given level of body temperature. There is also a more even distribution of sweating over the body surface, ensuring an effective rate of evaporation while minimizing the amount of sweat that drips from the body surface without evaporating. The salt content of the sweat is reduced to compensate for the increased sweating rate, allowing conservation of electrolytes to occur. These adaptations begin within one or two days of exposure to the heat, and the degree of adaptation is related to the amount of heat strain experienced. Adaptations occur more rapidly and more completely if exercise is performed during periods of heat exposure: passive exposure to heat is less effective. Adaptation is essentially complete after about 10–15 consecutive days of exposure to exercise in the heat. After adaptation, heart rate will be lower at any given level of thermal stress, and exercise tolerance is greatly increased. Even after a comprehensive programme of heat acclimatization, however, exercise performance remains impaired relative to that which can be achieved in cool conditions. On returning to a cool environment, the beneficial changes accompanying adaptation are gradually reversed.

R. J. Maughan

Bibliography

Maughan, R. J. and and Shirreffs, S. M. (1998). Fluid and electrolyte loss and replacement in exercise. In Oxford textbook of sports medicine (ed. Harries, Williams, Stanish, and Micheli), 2nd edn pp.97–113. Oxford University Press, New York.
Sutton, J. R. (1994). Physiological and clinical consequences of exercise in heat and humidity. In Oxford textbook of sports medicine, (ed. Harries, Williams, Stanish, and Micheli) pp. 231–8. Oxford University Press, New York.
Wenger, C. B. (1988). Human heat acclimatization. In Human performance physiology and environmental medicine at terrestrial extremes, (ed. K. B. Pandolf, M. N. Sawka, and R. R. Gonzalez) pp. 153–98. Cooper Publishing Group, Carmel.

See also body fluids; metabolism; sweating; temperature regulation.