Heat emergencies represent a continuum of disorders from heat cramps to heat stress that, when severe, culminate in heat stroke. In most circumstances, heat emergencies can be avoided through common sense, public education, and prevention.
The incidence of heat-related emergencies varies with the weather, although this is not an absolute requirement.1 During heat waves and severe droughts, fatality rates spike.1 Heat stroke is likely seen less among persons who live in warmer climates than travelers to these areas because of physiologic acclimatization and cultural adaptation of the former to heat.
From 1999 to 2003, an average of 688 heat-related deaths per year from exposure to extreme heat were reported in the United States.1 The heat wave during the summer of 2003 is estimated to have caused 14,800 deaths in France.2 In the Russian heat wave in July/August 2010, there were an estimated 15,000 deaths, with additional morbidity from associated forest fires and smoke injury.3
Mechanisms of Heat Transfer
Body temperature is regulated through the delicate balance of heat production, accumulation, and dissipation. Heat is generated by cellular metabolism and the mechanical work of skeletal muscle. Heat accumulates from radiation from the sun and direct contact with hot objects and is absorbed when the ambient temperature rises above body temperature. As core temperature rises, the autonomic nervous system is stimulated to promote sweating and cutaneous vasodilatation.
The body has several mechanisms for dissipating heat to the environment, including radiation (the 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), and evaporation (heat loss by vaporization of water, or sweat).
Radiation and evaporation dissipate most body heat at lower ambient temperatures (<35°C [<95°F]). Conduction of heat into a layer of ambient air surrounding the skin ends rapidly as soon as that layer acquires similar 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 by convection. When conduction is coupled with convection, rates of heat energy transfer from the body increase. Conduction of heat into water is many times more efficient than conduction into air of the same temperature.
The effect of wind on heat loss depends on wind velocity. Wind moves heat away from the skin by convection, but above 32.2°C (90°F) and 35% humidity, convection does not remove heat well.4 This is why the use of fans alone is not effective in preventing heat stroke during periods of high environmental temperature and humidity.
When the external temperature rises to >35°C (>95°F), the body can no longer radiate heat to the environment and becomes dependent on evaporation for heat transfer. As humidity increases, the potential for evaporative heat loss decreases. Sweat that drips from the skin does not provide any cooling benefit and only exacerbates dehydration. As a result, the combination of high temperature and high humidity essentially blocks the two main physiologic mechanisms that the body uses to dissipate heat.
The body maintains a core temperature between 36°C and 38°C (96.8°F and 100.4°F). Native thermal regulation mechanisms begin to fail at core temperatures of ...