INTRODUCTION AND EPIDEMIOLOGY
Heat emergencies represent a continuum of disorders from heat cramps to heat stress that, when severe, culminate in heat stroke. The incidence of heat-related emergencies varies with the weather, although other factors may have a greater impact.1 During heat waves and severe droughts, fatality rates spike.1 Physiologic acclimatization and cultural adaptation provide protection from heat stroke for people who live in warmer climates. From 1999 to 2010, there were 8081 heat-related deaths 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
Controlled by the hypothalamus, 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 contact with hot objects. Heat 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). Evaporation is the principal mechanism of heat loss in a hot environment, but this becomes ineffective above a relative humidity of 75%. Radiation and evaporation dissipate most body heat at lower ambient temperatures (<35°C [<95°F]).
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 ambient 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. The combination of high temperature and high humidity essentially blocks the two main physiologic mechanisms that the body uses to dissipate heat. Elevations of temperature are accompanied by an increase in oxygen consumption and metabolic rate, resulting in hyperpnea and tachycardia.
Controlled by the hypothalamus, 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 <35°C (<95°F) and >40°C (>104°F).5 It is ...