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Overview of Heat Illness


David Tanen

, MD, David Geffen School of Medicine at UCLA

Last full review/revision Feb 2021| Content last modified Feb 2021
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Patients with heat exhaustion maintain the ability to dissipate heat and have normal central nervous system (CNS) function. In heatstroke, compensatory mechanisms for heat dissipation fail (although sweating may still be present) and CNS function is impaired. Heatstroke should be considered in patients with hyperthermia and an altered mental status or other CNS dysfunction, regardless of sweating.

Pathophysiology of Heat Illness

Heat input comes from

  • The environment

  • Metabolism

Heat output occurs through the skin via the following:

  • Radiation: Transfer of body heat directly into a cooler environment by infrared radiation, a process that does not require air motion or direct contact

  • Evaporation: Cooling by water vaporization (eg, sweat)

  • Convection: Transfer of heat to cooler air (or liquid) that passes over exposed skin

  • Conduction: Transfer of heat from a warmer surface to a cooler surface that is in direct contact

The contribution of each of these mechanisms varies with environmental temperature and humidity. When environmental temperature is lower than body temperature, radiation provides 65% of cooling. Evaporation normally provides 30% of cooling, and exhalation of water vapor and production of urine and feces provide about 5%.

When environmental temperature is > 35° C, evaporation accounts for virtually all dissipation of heat because the other mechanisms function only when environmental temperature is lower than body temperature. However, effectiveness of sweating is limited. Sweat that drips from the skin is not evaporated and does not contribute to cooling. Effectiveness of sweating is also limited by body surface area and humidity. When humidity is > 75%, evaporative heat loss markedly decreases. Thus, if both environmental temperature and humidity are high, all mechanisms for heat dissipation are lost, markedly increasing risk of heat illness.

The body can compensate for large variations in heat load, but significant or prolonged exposure to heat that exceeds capacity for heat dissipation increases core temperature. Modest, transient core temperature elevations are tolerable, but severe elevations (typically > 41° C) lead to protein denaturation and, especially during hard work in the heat, release of inflammatory cytokines (eg, tumor necrosis factor alpha, IL-1b). As a result, cellular dysfunction occurs and the inflammatory cascade is activated, leading to dysfunction of most organs and activation of the coagulation cascade. These pathophysiologic processes are similar to those of multiple organ dysfunction syndrome Multiple organ dysfunction syndrome (MODS) Shock is a state of organ hypoperfusion with resultant cellular dysfunction and death. Mechanisms may involve decreased circulating volume, decreased cardiac output, and vasodilation, sometimes... read more , which follows prolonged shock.

Compensatory mechanisms include an acute-phase response by other cytokines that moderate the inflammatory response (eg, by stimulating production of proteins that decrease production of free radicals and inhibit release of proteolytic enzymes). Also, increased core temperature triggers expression of heat-shock proteins. These proteins transiently enhance heat tolerance by poorly understood mechanisms (eg, possibly by preventing protein denaturation) and by regulation of cardiovascular responses. With prolonged or extreme temperature elevation, compensatory mechanisms are overwhelmed or malfunction, allowing inflammation and multiple organ dysfunction syndrome to occur.

Heat output is modulated by changes in cutaneous blood flow and sweat production. Cutaneous blood flow is 200 to 250 mL/minute at normal temperatures but increases to 7 to 8 L/minute with heat stress (and facilitates heat loss by convective, conductive, radiant and evaporative mechanisms), requiring a marked increase in cardiac output. Also, heat stress increases sweat production from negligible to > 2 L/hour; however, although sweat that is dripped from the skin does not contribute to cooling, it still contributes to dehydration. Significant sweating can occur less perceptibly in very hot, very dry air, in which sweat evaporates very quickly. With sweat production of > 2 L/hour, dehydration can develop very rapidly. Because sweat contains electrolytes, electrolyte loss may be substantial. However, prolonged exposure triggers physiologic changes to accommodate heat load (acclimatization); eg, sweat sodium levels are 40 to 100 mEq/L (or 40 to 100 mmol/L) in people who are not acclimatized but decrease to 10 to 70 mEq/L (mmol/L) in acclimatized people.

Etiology of Heat Illness

Excess heat input typically results from strenuous exertion, high environmental temperatures, or both. Medical disorders and use of stimulant drugs can increase heat production.

Impaired cooling can result from obesity, high humidity, high environmental temperatures, wearing heavy clothing, and anything that impairs sweating or evaporation of sweat.

Clinical effects of heat illnesses are exacerbated by the following:

Older people and the very young are at increased risk. Older adults are at high risk because they more often use drugs that can increase risk, have higher rates of dehydration and heart failure, and have age-related loss of heat-shock proteins. Children are at high risk due to their greater surface-area-to-body-mass ratio (resulting in greater heat gain from the environment on a hot day), and slower rates of sweat production. Children are slower to acclimatize and have less of a thirst response. Both older adults and young children may be relatively immobile and thus have difficulty leaving a hot environment.


Prevention of Heat Illness

  • During excessively hot weather, older adults and children should not remain in unventilated residences without air-conditioning.

  • Children and older adults should not be left in automobiles in the hot sun.

  • If possible, strenuous exertion in a very hot environment or an inadequately ventilated space should be avoided, and heavy, insulating clothing should not be worn.

  • Weight loss after exercise or work can be used to monitor dehydration; people who lose 2 to 3% of their body weight should be reminded to drink extra fluids and should be within 1 kg of starting weight before the next day’s exposure. If people lose > 4%, activity should be limited for 1 day.

  • If exertion in the heat is unavoidable, fluid should be replaced by drinking frequently, and evaporation should be facilitated by wearing open-mesh clothing or by using fans.


Maintaining adequate levels of fluid and sodium helps prevent heat illnesses. Thirst is a poor indicator of dehydration and the need for fluid replacement during exertion because thirst is not stimulated until plasma osmolality rises 1 to 2% above normal. Thus, fluids should be drunk every few hours regardless of thirst. Because maximum net water absorption in the gut is about 20 mL/minute (1200 mL/hour—lower than the maximum sweating rate of 2000 mL/hour), prolonged exertion that causes very high sweat loss requires rest periods that reduce sweating rate and allow time for rehydration.

The best hydrating fluid to use depends on the expected loss of water and electrolytes, which depends on the duration and degree of exertion along with environmental factors and whether the person is acclimatized. For maximum fluid absorption, a carbohydrate-containing beverage can be absorbed by the body up to 30% faster than plain water. A beverage containing 6 or 7% carbohydrate concentration is absorbed most rapidly. Higher carbohydrate concentrations should be avoided because they can cause stomach cramps and delay absorption. However, for most situations and activities, plain water is adequate for hydration as long as overhydration is avoided. Significant hyponatremia Hyponatremia Hyponatremia is decrease in serum sodium concentration 136 mEq/L ( 136 mmol/L) caused by an excess of water relative to solute. Common causes include diuretic use, diarrhea, heart failure, liver... read more has occurred in endurance athletes who drink free water very frequently before, during, and after exercise without replacing sodium losses. Special hydrating solutions (eg, sports drinks) are not required, but their flavoring enhances consumption, and their modest salt content is helpful if fluid requirements are high.

Laborers, soldiers, endurance athletes, or others who sweat heavily can lose 20 g of sodium/day, making heat cramps more likely; such people need to replace the sodium loss with drink and food. In most situations, consuming generously salted foods is adequate; people on low-salt diets should increase salt intake. For more extreme circumstances (eg, prolonged exertion by unacclimatized people) an oral salt solution can be used. The ideal concentration is 0.1% sodium chloride, which can be prepared by dissolving a 1-g salt tablet or one quarter of a teaspoon of table salt in a liter (or quart) of water. People should drink this solution under moderate to extreme circumstances. Undissolved salt tablets should not be ingested. They irritate the stomach, can cause vomiting, and do not treat the underlying dehydration.

Pearls & Pitfalls

  • Salt tablets should not be swallowed because they can cause gastric irritation. Instead, they are dissolved in water to be drunk.


Successively and incrementally increasing the level and amount of work done in the heat eventually results in acclimatization, which enables people to work safely at temperatures that were previously intolerable or life threatening. To reach maximum benefit, acclimatization usually requires spending 8 to 11 days in the hot environment with some daily exercise (eg, 1 to 2 hours/day with intensity increased from day to day). Acclimatization markedly increases the amount of sweat (and hence cooling) produced at a given level of exertion and markedly decreases the electrolyte content of sweat and the risk of a heat illness. People who are not acclimatized are more likely to suffer from heat cramps or other heat illness during prolonged exertion and may need to increase their sodium intake.

Pearls & Pitfalls

  • Unacclimatized people are more likely to suffer from heat cramps and heat illnesses during prolonged exertion and may need to increase their sodium intake.

Moderation of activity level

When possible, people should adjust their activity level based on the environment and any heat loss-impairing gear (eg, firefighting or chemical protective outfits) that must be worn. Work periods should shorten and rest periods increase when

  • Temperature increases

  • Humidity increases

  • Workload is heavier

  • Sun gets stronger

  • There is no air movement

  • When protective clothing or gear is worn

The best indicator of environmental heat stress is the wet bulb globe temperature (WBGT), which is widely used by the military, industry, and sports. In addition to temperature, the WBGT reflects the effects of humidity, wind, and solar radiation. The WBGT can be used as a guide for recommended activity (see table Wet Bulb Globe Temperature and Recommended Activity Levels Wet Bulb Globe Temperature and Recommended Activity Levels Heat illness encompasses a number of disorders ranging in severity from muscle cramps and heat exhaustion to heatstroke (which is a life-threatening emergency). Heat illness, although preventable... read more ).

Wet bulb globe temperature based on temperature and relative humidity

Wet bulb globe temperature based on temperature and relative humidity

Values are derived from an approximate formula that depends on temperature and humidity and that is valid for full sunshine and a light wind. Heat stress may be overestimated in other conditions.


General reference

  • 1. Lipman GS, Gaudio FG, Eifling KP, et al: Wilderness Medical Society clinical practice guidelines for the prevention and treatment of heat illness: 2019 Update. Wilderness Environ Med. 30(4S):S33-S46, 2019. doi: 10.1016/j.wem.2018.10.004

Key Points

  • When environmental temperature is > 35° C, cooling relies largely on evaporation, but when humidity is > 75%, evaporation markedly decreases, so when temperature and humidity are both high, risk of heat illness is high.

  • Among the many risk factors for heat illness are certain drugs and disorders (including those that disturb electrolyte balance or decrease cardiovascular reserve) and extremes of age.

  • Prevention includes common sense measures and maintaining and replacing fluids and sodium.

  • Acclimatization, requiring daily exercise for 8 to 11 days, decreases risk of heat illness.

  • Activity levels should be restricted as temperature, humidity, sunlight, and amount of clothing or gear increase and when air movement decreases.

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