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Overview of Heat Illness


David Tanen

, MD, David Geffen School of Medicine at UCLA

Last full review/revision Jun 2019| Content last modified Jun 2019
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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, affects thousands of people each year in the US and can be fatal; it is the second leading cause of death in young athletes. When heatstroke is not treated promptly and effectively, mortality approaches 80%.

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.


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, 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.


Heat disorders are caused by some combination of increased heat input and decreased output (see table Common Factors Contributing to Heat Disorders).

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:

  • Inability to tolerate increased cardiovascular demands (eg, due to aging, heart failure, chronic kidney disease, respiratory disorders, liver failure)

  • Dehydration

  • Electrolyte disturbance

  • Use of certain drugs (see table Common Factors Contributing to Heat Disorders)

The elderly and very young are at increased risk. The elderly 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 the elderly and young children may be relatively immobile and thus have difficulty leaving a hot environment.


Common Factors Contributing to Heat Disorders



Excess heat input

Certain disorders


Salicylate poisoning, severe


High environmental temperatures

Stimulant drugs

Methylenedioxymethamphetamine (MDMA, or Ecstasy)

Strenuous exertion


Physical labor

Withdrawal from certain drugs

Impaired cooling

Heavy clothing

Protective gear for workers and athletes (eg, football pads)

High environmental temperatures

High humidity

Obesity and/or poor cardiovascular fitness

Impaired sweating*

Anticholinergic drugs


Antiparkinsonian drugs




Skin disorders

Burn scars, extensive

Eczema, extensive

Heat rash

Psoriasis, extensive

Systemic sclerosis

*Impaired sweating is a cause of impaired cooling.


Common sense is the best prevention. Physicians should recommend the following measures (1):

  • During excessively hot weather, the elderly and the young should not remain in unventilated residences without air-conditioning.

  • Children 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 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. Acclimatization significantly decreases risk of a heat illness.

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).

Although the WBGT is complex and may not be available, it can be estimated based on only temperature and relative humidity in sunny conditions and when the wind is light (see figure Wet bulb globe temperature based on temperature and relative humidity).

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.


Wet Bulb Globe Temperature and Recommended Activity Levels

Temperature °C (°F)


≤15.6 (≤ 60)

No precautions

>15.6–21.1 (>60–70)

No precautions if adequate hydration maintained

>21.1–23.9 (>70–75)

Unacclimatized: Avoid hiking, sports, and sun exposure

Acclimatized: Heavy to moderate activity permissible with caution

>23.9–26.7 (>75–80)

Unacclimatized: Stop or restrict exercise

Acclimatized: Exercise with caution; rest periods and water breaks every 20 to 30 minutes

>26.7–31.1 (>80–88)

Unacclimatized: Avoid activity

Acclimatized: Limited brief activity permissible, only if fit

≥31.1 (>88)

Avoid activity and sun exposure

General reference

  • Lipman GS, Eifling KP, Ellis MA, et al: Wilderness Medical Society practice guidelines for the prevention and treatment of heat-related illness: 2014 Update. Wilderness Environ Med 25(4 Suppl):S55-S65, 2014. doi: 10.1016/j.wem.2014.07.017.

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 increases and when air movement decreases.

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