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Public awareness of heat emergencies has increased in the last few years as a result of the heat waves in the U.S. and Europe and the high-profile deaths of several professional athletes. Heat emergencies represent a spectrum of disorders, including heat cramps, heat syncope, heat exhaustion, and heat stroke. The differentiation of disorders along this continuum is somewhat blurred, because individuals respond differently to heat stress and patients can progress rapidly from relatively benign to life-threatening disease. In most circumstances, heat emergencies can be avoided through public education and prevention.

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Epidemiologic data on the incidence of heat stroke is imprecise, because the illness is often underdiagnosed. The male-to-female ratio for heat emergencies is essentially equal, and people of any age can be affected. The incidence of heat-related emergencies varies with the weather.1 During heat waves and severe droughts, fatality rates may spike.1 Heat waves are defined as ≥3 consecutive days of sustained temperatures >32.2°C (>90°F). Heat stroke is seen less among persons who live in countries where hot summers are common because of physiologic acclimatization and cultural adaptation to heat.

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From 1999 to 2003, an average of 688 heat-related deaths per year from exposure to extreme heat were reported in the U.S.1 In an epidemiologic study of heat hospitalizations and deaths in the U.S. Army from 1980 to 2002, the hospitalization rate due to heat emergencies ranged from 20 to 55 cases per 100,000 soldiers per year, and the rate of heat stroke varied from 2 to 14 cases per 100,000 soldiers per year.2During the 1980 heat wave in St. Louis, the incidence of heat stroke varied from 17.6 to 26.5 cases per 100,000 population.3

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Mechanisms of Heat Transfer

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Body temperature is regulated through the delicate balance of heat production, accumulation, and dissipation. Heat is generated as a result of cellular metabolism and the mechanical work of skeletal muscle. Basal metabolism alone can generate up to 100 kcal/h. Moderate work can add 300 to 600 kcal/h. In the absence of heat-dissipating mechanisms, the addition of 70 kcal can increase the core temperature by 0.8°C (1.4°F).4

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Heat is also accumulated through radiation from the sun and direct contact with hot objects, and is absorbed when the ambient temperature rises above body temperature.

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The body has several mechanisms to dissipate heat to the environment, including the following:

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  • Radiation—transfer of heat by electromagnetic waves from a warmer object to a colder object
  • Conduction—heat exchange between two surfaces in direct contact
  • Convection—heat transfer by air or liquid moving across the surface of an object
  • Evaporation—heat loss by vaporization of water (sweat)

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When the temperature is <35°C (<95°F), radiation accounts for approximately 60% of heat dissipation and evaporation accounts for 30%. Conduction of heat energy into solid surfaces usually accounts for <3% of the total heat loss of the body. Conduction of heat into the ambient air surrounding the skin stops rapidly as soon as that layer acquires the same temperature as the skin surface. This results in creation of an “insulator zone” of warmed air through which little heat may be lost. Removing the warmed air next to the skin and replacing it with cooler air may increase conductive heat loss; this process is called convection. When conduction is coupled with convection, rates of heat energy transfer from the body may account for up to 15% of the total heat loss. Conduction of heat into water is 32 ...

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