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Humans are a homeothermic (warm-blooded) species. Although the temperature of the arms, legs, and superficial areas (acral body parts) may vary greatly, the body maintains a relatively constant deep body (core) temperature. Substantial deviations from normal core body temperatures cause adverse effects ranging from minor annoyance to life-threatening illness. Although far less affected by temperature changes than the core, acral body parts can be adversely affected by cold temperatures, particularly if the exposure is prolonged or repeated.1

Body temperature is affected by five fundamental physical processes:

  1. Metabolism—Heat is generated by the biochemical reactions of metabolism.

  2. Evaporation—Heat is lost by evaporation of moisture from the skin and respiratory passages.

  3. Conduction—Heat is transferred to or from matter with which the body is in contact.

  4. Convection—Heat transfer by conduction is greatly facilitated when the body is immersed in a fluid medium (gas or liquid) because of the ability of substance to flow over body surfaces. Conduction in this context is called convection.

  5. Thermal radiation—Heat may be gained or lost due to thermal radiation. The body radiates heat into cold surroundings or gains heat from objects that radiate infrared and other wavelengths of electromagnetic radiation (for example, the sun or a hot stove). The process is independent of the temperature of matter in contact with the body.1

1 Burns involve acute destruction of skin and other tissues. They are caused by a variety of noxious physical and chemical influences, including both extremely high and extremely low temperatures. Burns present a unique set of problems and issues and thus are not discussed further in this chapter.


Heat Stress

Heat stress may result from alteration of any of the five physical processes involved in determining body temperature. For example, increased metabolic heat production caused by strenuous physical activity may stress the runner in a long-distance race or the soldier undertaking military maneuvers. A steel worker may experience heat stress because of the radiant heat emitted from a furnace at the workplace. At a hazardous waste site, a worker who must wear a heavy, impermeable suit may develop heat stress as the air in the suit becomes humid (decreasing evaporative cooling) and warm (limiting heat loss by conduction/convection).

People seek to relieve heat stress by altering one or more of the processes by which the body gains or loses heat. They may rest (lowering metabolic heat production), move to the shade (avoiding radiant solar heat), sit in front of a fan (increasing convective and evaporative heat loss), or swim (facilitating heat loss by conduction/convection through water).

The acute physiological response to heat stress includes perspiration and dilation of the peripheral blood vessels. Perspiration increases cutaneous moisture, allowing greater evaporative cooling. Peripheral vasodilation reroutes blood flow toward ...

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