Extremes of Temperature

CDC Yellow Book 2024

Environmental Hazards & Risks

Howard Backer, David Shlim

International travelers encounter extremes of climate to which they might not be accustomed. Exposure to heat and cold can result in serious injury or death. Travelers should investigate the climate extremes they will face during their journey and prepare themselves with knowledge, proper clothing, and equipment to prevent problems. Travelers should also be aware that climate change is expanding the range and severity of exposure to heat across many travel destinations. Regions with wide temperature fluctuation present risk for both heat and cold problems.

Heat-Related Illness

Risk for Travelers

Heat-related illness is most often seen in occupational, military, and competitive sport activities, but also can occur from recreational activities. Many of the most popular travel destinations are hot tropical or arid areas. Travelers who sit on the beach or by the pool and do only short walking tours incur minimal risk for heat-related illness. People participating in more strenuous activities (e.g., hiking or biking) in hot environments are at greater risk, especially those coming from cool or temperate climates who are not in good physical condition and who are unacclimatized to heat.


Unlike in the cold, where adaptive behaviors play a more important role in body heat conservation, tolerance to heat depends largely on physiologic factors. Heat regulation depends on a combination of physiological and environmental factors. The major means of heat dissipation are radiation while at rest and evaporation of sweat during exercise, both of which become minimal when air temperatures are above 95°F (35°C) and humidity is high.

Cardiovascular status and conditioning are the major physiologic variables affecting the response to heat stress at all ages. Two major organ systems are most critical in temperature regulation: the cardiovascular system, which must increase blood flow to shunt heat from the core to the surface while meeting the metabolic demands of exercise; and the skin, where sweating and heat exchange take place. Many chronic illnesses limit tolerance to heat and predispose people to heat-related illness, most importantly, cardiovascular disease, diabetes, renal disease, certain medications, and extensive skin disorders or scarring that limit sweating.

Apart from environmental conditions and intensity of exercise, dehydration is the most important predisposing factor in heat-related illness. Dehydration reduces exercise performance, decreases time to exhaustion, and increases internal heat load. Temperature and heart rate increase in direct proportion to the level of dehydration. Sweat is a hypotonic fluid containing sodium and chloride. Sweat rates commonly reach 1 liter per hour or more, resulting in substantial fluid and sodium loss.

Clinical Presentations


Mild heat-related problems can be treated in the field and usually do not require medical evaluation or evacuation.

Heat cramps are painful muscle contractions that begin ≥1 hours after stopping exercise and most often involve heavily used muscles in the calves, thighs, and abdomen. Rest and passive stretching of the muscle, supplemented by commercial rehydration solutions or water and salt, rapidly relieve symptoms. Drinking water and eating a salty snack also is sufficient. Travelers can make a simple oral salt solution, as described for heat exhaustion.

Heat edema, another mild heat-related illness, occurs more frequently in women than in men. Characterized by mild swelling of the hands and feet during the first few days of heat exposure, this condition typically resolves spontaneously. Travelers should not treat heat edema with diuretics, which can delay heat acclimatization and cause dehydration.

Prickly heat (miliaria or heat rash) manifests as small, red, raised itchy bumps on the skin and is caused by obstruction of the sweat ducts. Prickly heat resolves spontaneously, aided by relief from heat and avoiding continued sweating. Travelers can best prevent prickly heat by wearing light, loose clothing and avoiding heavy, continuous sweating.


Moderate and severe heat-related illnesses present with collapse (syncope) or inability to continue exertion in heat and are treated similarly with rest, removal from heat or direct sun, and administering fluids and salt.

Heat syncope

Heat syncope—sudden fainting caused by vasodilation—occurs in unacclimated people standing in the heat or after 15–20 minutes of exercise. Consciousness rapidly returns when the patient is supine. Rest, relief from heat, and oral rehydration are mainstays of treatment.

Heat Exhaustion

Most people who experience symptoms associated with exercise in the heat or the inability to continue exertion in the heat are suffering from heat exhaustion. The presumed cause of heat exhaustion is loss of fluid and electrolytes, but there are no objective markers to define the syndrome. Transient mental changes (e.g., irritability, confusion, irrational behavior) might be present in heat exhaustion, but major neurologic signs (e.g., seizures, coma) indicate heat stroke or profound hyponatremia. Body temperature could be normal or mildly to moderately elevated. Heat exhaustion also can develop over several days in unacclimatized people and often is misdiagnosed as “summer flu” because of findings of weakness, fatigue, headache, dizziness, anorexia, nausea, vomiting, and diarrhea.

Most cases of heat exhaustion can be treated with supine rest in the shade or other cool place and oral water or fluids containing glucose and salt; subsequently, spontaneous cooling occurs, and patients recover within hours. Travelers can prepare a simple oral salt solution by adding one-fourth to one-half teaspoon (1/4–1/2 tsp) of table salt (or two 1-g salt tablets) to 1 liter (33 oz) of water. To improve taste, add a few teaspoons of sugar or orange or lemon juice to the mixture. Commercial sports-electrolyte drinks also are effective. Plain water plus salty snacks might be more palatable and equally effective. Without cessation of activity and passive or active cooling measures (see below), heat exhaustion can progress to heat stroke.


Severe heat-related illness requires medical evacuation and emergency medical attention.

Heat stroke

Heat stroke is a medical emergency requiring aggressive cooling measures and hospitalization for support. Heat stroke is the only form of heat-related illness in which the mechanisms for thermal homeostasis have failed, and the body does not spontaneously restore the temperature to normal. Uncontrolled fever and circulatory collapse cause organ damage to the brain, kidneys, liver, and heart. Damage is related to duration and peak elevation of body temperature.

Onset of heat stroke can be acute or gradual. Acute (also known as exertional) heat stroke is characterized by collapse while exercising in the heat, usually with profuse sweating. It can affect healthy, physically fit people. By contrast, gradual or nonexertional (referred to sometimes as classic or epidemic) heat stroke occurs in chronically ill people experiencing passive exposure to heat over several days. Sufferers of nonexertional heat stroke might not perspire. Victims of both exertional and nonexertional heat stroke demonstrate altered mental status and markedly elevated body temperature.

Early symptoms are similar to those of heat exhaustion, including confusion or change in personality, loss of coordination, dizziness, headache, and nausea, but these progress to more severe symptoms. A presumptive diagnosis of heat stroke is made in the field when people have body temperature ≥104°F (≥41°C) and marked alteration of mental status, including delirium, convulsions, and coma; even without a thermometer, people with heat stroke will feel hot to the touch. If a thermometer is available, a rectal temperature is the safest and most reliable way to check the temperature of someone with suspected heat stroke; an axillary temperature might give a reasonable estimation. See Box 4-06 for additional guidance on managing heat stroke.

Heat stroke is life threatening, and many complications occur in the first 24–48 hours, including liver or kidney damage and abnormal bleeding. Most victims have significant dehydration, and many require hospital intensive care management to replace fluid losses. If evacuation to a hospital is delayed, patients should be monitored closely for several hours for temperature swings.

Box 4-06 Heat stroke management

In the field, maintain the airway if victim is unconscious, and immediately institute cooling measures by these methods, if available:

Move the victim to the shade or some cool place out of the sun.

Use evaporative cooling: remove excess clothing to maximize skin exposure, spray tepid water on the skin, and maintain air movement over the body by fanning. Alternatively, place cool or cold wet towels over the body and fan to promote evaporation.

Apply ice or cold packs to the neck, axilla, groin, and as much of the body as possible. Vigorously massage the skin to limit constriction of blood vessels and to prevent shivering, which will increase body temperature.

Immerse the victim in cool or cold water (e.g., a nearby pool, natural body of water, bath). An ice bath cools fastest. Always attend and hold the person while in the water.

Encourage rehydration for those able to take oral fluids.

Exercise-induced Hyponatremia

Hyponatremia occurs in both endurance athletes and recreational hikers due to physiologic mechanisms that result in failure of the kidneys to correct salt and fluid imbalances properly. Excess fluid retention occurs when antidiuretic hormone (secreted inappropriately) influences the kidneys to both retain water and excrete sodium. Sodium losses through sweat also contribute to hyponatremia.

In the field setting, altered mental status in a patient with normal body temperature and a history of taking in large volumes of water suggests hyponatremia. Excessive water ingestion is also a major contributor to exercise-associated hyponatremia; the recommendation to force fluid intake during prolonged exercise and the attitude that “you can’t drink too much” is outdated and dangerous. Prevention includes drinking only enough to relieve thirst. During prolonged exercise (>12 hours) or heat exposure, people should take supplemental sodium. Most sports-electrolyte drinks do not contain sufficient sodium to prevent hyponatremia; on the other hand, salt tablets often cause nausea and vomiting. For recreational athletes, food is the most efficient vehicle for salt replacement. Snacks should include not just sweets, but salty foods (e.g., trail mix, crackers, pretzels).

Symptoms of heat exhaustion and early exercise-associated hyponatremia are similar, including anorexia, nausea, emesis, headache, muscle weakness, and lethargy; hyponatremia symptoms can, however, progress to confusion and seizures, and coma. Severe hyponatremia can be distinguished from other heat-related illnesses by persistent alteration of mental status without elevated body temperature, delayed onset of major neurologic symptoms, or deterioration hours after cessation of exercise and removal from heat. Where medical care and clinical laboratory resources are available, clinicians can measure the patient’s serum sodium to diagnose hyponatremia and guide treatment.

Treating clinicians should restrict fluid if hyponatremia is suspected (neurologic symptoms in the absence of hyperthermia or other diagnoses). If the patient is conscious and can tolerate oral intake, clinicians should give salty snacks with sips of water or a solution of concentrated broth (2–4 bouillon cubes in 1/2 cup of water). Obtunded hyponatremic patients require hypertonic saline.



Travelers should wear lightweight, loose, light-colored clothing that allows maximum air circulation for evaporation but also gives protection from the sun (see Sec. 4, Ch. 1, Sun Exposure). In addition, travelers can wear a wide-brimmed hat, which can markedly reduce radiant heat exposure.

Fluid & Electrolyte Replacement

During exertion, fluid intake improves performance and decreases the likelihood of illness. Reliance on thirst alone is not sufficient to prevent mild dehydration, and forcing a person who is not thirsty to drink water increases the risk of hyponatremia. During mild to moderate exertion, electrolyte replacement offers no advantage over plain water. A person exercising for many hours in the heat should replace salt by eating salty snacks or by lightly salting mealtime food or fluids. Salt tablets swallowed whole can cause gastrointestinal irritation and vomiting; tolerability can be improved by dissolving tablets in 1 L of water. Using urine volume and color to monitor fluid needs is most accurate in the morning.

Heat Acclimatization

Heat acclimatization is a process of physiologic adaptation that occurs in residents of and visitors to hot environments. Increased sweating that contains less salt, and decreased energy expenditure with lower rise in body temperature for a given workload, is the result. Only partial adaptation occurs from passive exposure to heat. Full acclimatization, especially cardiovascular, requires 1–2 hours of exercise in the heat each day. With a suitable amount of daily exercise, most acclimatization changes occur within 10 days. Decay of acclimatization occurs within days to weeks if there is no heat exposure.

Physical Conditioning

If possible, all travelers should acclimatize before departing for hot climates by exercising ≥1 hour daily in the heat. Physically fit travelers have improved exercise tolerance and capacity but still benefit from acclimatization. If this is not possible, clinicians should advise travelers to limit exercise intensity and duration during their first week of travel. Travelers also should try to conform to the local practice in most hot regions and avoid strenuous activity during the hottest part of the day.

Cold-related Illness & Injury

Risk for Travelers

Travelers do not have to be in an arctic or high-elevation environment to encounter problems with cold. Humidity, rain, and wind can produce hypothermia with temperatures around 50°F (10°C). Even in temperate climates, people can rapidly become hypothermic in water. Although reports of severe hypothermia in international travelers are rare, people planning trips to wilderness areas should be familiar with the major mechanisms of heat loss (convection, conduction, and radiation) and how to mitigate them by taking shelter from the wind, getting and staying dry, and keeping warm by building a fire.

Being caught without shelter in a wilderness environment represents a significant risk for accidental hypothermia. Many high-elevation travel destinations, however, are not wilderness areas. Local inhabitants and villages offer shelter and protection from extreme cold weather. In Nepal, for example, trekkers almost never experience hypothermia except in rare instances in which they get lost in a storm.

Clinical Presentations


Hypothermia is defined as a core body temperature <95°F (<35°C). When people are faced with an environment in which they cannot keep warm, they first feel chilled, then they shiver, and eventually they stop shivering because their metabolic reserves are exhausted. Body temperature continues to decrease, depending on ambient temperatures. As core body temperature falls, neurologic function decreases; almost all hypothermic people with a core temperature of ≤86°F (≤30°C) are comatose. The record low core body temperature in an adult who survived is 56°F (13°C).

Travelers heading to cold climates should ask questions and research clothing and equipment. Modern clothing, gloves, and particularly footwear have greatly decreased the chances of suffering cold injury in extreme climates. Cold-related illness and injury occurs more often after accidents (e.g., avalanches, unexpected nights outside) than during normal recreational activities.

People engaging in recreational activities or working around cold water face a different sort of risk. Within 15 minutes, immersion hypothermia can render a person unable to swim or float. In these cases, a personal flotation device is critical, as is knowledge about self-rescue and righting a capsized boat.

Other medical conditions associated with cold affect mainly the skin and the extremities. These can be divided into nonfreezing cold injuries and freezing injuries (frostbite).

Nonfreezing Cold Injury

Nonfreezing cold-related injuries include trench foot (immersion foot), pernio (chilblains), and cold urticaria. Trench foot is caused by prolonged immersion of the feet in cold water (32°F–59°F; 0°C–15°C). The damage is mainly to nerves and blood vessels, and the result is pain aggravated by heat and a dependent position of the limb. Severe cases can take months to resolve. Unlike frostbite, avoid rapid rewarming of trench foot, which can make the damage much worse.

Pernio are localized, inflammatory lesions occurring mainly on the hands after exposure to only moderately cold weather. The bluish-red lesions are thought to be caused by prolonged, cold-induced vasoconstriction. Rapid rewarming makes the pain worse; slow rewarming is preferred. Nifedipine can be an effective treatment.
Cold urticaria are localized or general wheals with itching. The rate of change of temperature, not the absolute temperature, induces this form of skin lesion. If cold urticaria occur regularly in a traveler, they can be prevented or ameliorated by prior treatment with antihistamines.

Freezing Cold Injury

Frostbite describes tissue damage caused by direct freezing of the skin. Once severe tissue damage occurs, little can be done. Fortunately, modern equipment and clothing are available to protect adventure tourists from frostbite. The condition now occurs mainly as the result of accidents, severe unexpected weather, or failure to plan appropriately.
Frostbite is usually graded like burns. First-degree frostbite involves reddening of the skin without deeper damage. The prognosis for complete healing is virtually 100%. Second-degree frostbite involves blister formation. Blisters filled with clear fluid have a better prognosis than blood-tinged blisters. Third-degree frostbite represents full-thickness injury to the skin and possibly the underlying tissues. No blisters form, the skin darkens over time and might turn black. If the tissue is completely devascularized, amputation will be necessary.

Severely frostbitten skin is numb and appears whitish or waxy. The generally accepted method for treating a frozen digit or limb is rapid rewarming in water heated to 104°F–108°F (40°C–42°C). Immerse the frozen area completely in the heated water. Use a thermometer to ensure the water is kept at the correct temperature. Rewarming can be associated with severe pain, so analgesics should be given if needed. Once rewarmed, protect frostbitten skin against freezing again. It is better to keep digits frozen a little longer and rapidly rewarm them than to allow them to thaw out slowly or to thaw and refreeze. A cycle of freeze-thaw-refreeze is devastating to tissue, often resulting in amputation.

Once the area has rewarmed, examine for blisters, and note whether the blisters extend to the end of the digit. Proximal blisters usually mean that the tissue distal to the blister has suffered full-thickness damage. For treatment, avoid further mechanical trauma to the area and prevent infection. In the field, wash the area thoroughly with a disinfectant (e.g., povidone iodine), put dressings between the toes or fingers to prevent maceration, use fluffs (expanded gauze sponges) for padding, and cover with a roller gauze bandage. These dressings can be left on safely for up to 3 days at a time. Prophylactic antibiotics are not needed in most situations.

In the rare situation in which a foreign traveler suffers frostbite and can be evacuated to an advanced medical setting within 24–72 hours, there may be a role for thrombolytic agents (e.g., prostacyclin, recombinant tissue plasminogen activator). Clinicians managing a case of frostbite within the first 72 hours should carefully consider the risks and benefits of using these drugs; consultation with an expert is strongly recommended. Beyond 72 hours after thawing, these interventions probably are not beneficial.

Once a patient with frostbite has reached a definitive medical setting, clinicians should not rush to do surgery. The usual time from injury to surgery is 4–5 weeks. Clinicians can use technetium-99m (Tc-99m) scintigraphy and magnetic resonance imaging to define the extent of the damage. Once the delineation between dead and viable tissue becomes clear, clinicians can plan surgery that preserves the remaining digits.

The following authors contributed to the previous version of this chapter: Howard Backer, David Shlim

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