Humans, who are warm-blooded animals, maintain their body temperature within 1 or 2 degrees of 98.6° F (37° C) as measured by mouth and 100.4° F (38° C) as measured rectally, despite large fluctuations in external temperatures. This internal temperature range must be maintained for the body to function normally. Body temperature that gets too high or too low can result in serious injury to organs or death.

Temperature Regulation: The body regulates its temperature by balancing heat production and heat loss.

One way the body produces heat is through chemical reactions (metabolism) resulting mostly from the conversion of food into energy. Heat is also produced through the work of muscles during physical activity.

The body cools itself by losing heat, mainly through the processes of radiation and sweating. Radiation, in which heat flows from warmer to cooler areas, is the main source of heat loss when the body is warmer than its environment. Sweating, in which the sweat glands produce moisture that cools the skin as it evaporates, is the main source of heat loss when the environmental temperature approaches body temperature and during exercise. However, humidity (moisture in the air) slows water evaporation, decreasing the effectiveness of sweating. Therefore, heat loss may be difficult in hot, humid weather.

Heat Disorders: There are several types of heat disorder:

• Heat cramps
• Heat exhaustion
• Heatstroke (the most serious)

These types vary by their symptoms, whether (and by how much) body temperature is elevated, and by the severity of body fluid and salt depletion. Body fluid and salt depletion result from excessive sweating and can lead to low blood pressure and painful muscle contractions. Internal organs can be damaged if body temperature is very high for a long time.

Causes

Heat disorders are caused by excessive heat production, ineffective heat loss, or both.

Excessive heat production can be caused by the following:

• Infections that cause fever
• Increased thyroid activity, which speeds up the body's metabolism
• Strenuous muscle activity, which may occur during exercise or physical labor (particularly among obese people) or can result from disorders such as seizures, agitation, or alcohol or drug withdrawal
• Certain stimulant drugs, such as cocaine, amphetamines, methylenedioxymethamphetamine (MDMA, or Ecstasy), monoamine oxidase inhibitors (a type of antidepressant), and phencyclidine (angel dust)
• Overdoses of drugs that contain aspirin, because high doses of aspirin cause cells to produce excessive heat

Ineffective heat loss is most common in hot, humid conditions. The following also greatly interfere with heat loss:

• Heavy, tight, clothing that does not breathe (that is, does not allow air and moisture to pass through easily). Wearing such clothing prevents sweat from evaporating from the skin surface and cooling the body.
• Certain drugs, most often antipsychotic drugs and drugs with anticholinergic effects (see Aging and Drugs: Anticholinergic: What Does It Mean?), may reduce sweating.
• Certain disorders that affect the skin interfere with sweating. These include cystic fibrosis, systemic sclerosis (formerly called scleroderma), psoriasis, eczema, and severe sunburns.
• Obesity interferes with heat loss because a thick layer of fat is a good insulator.
• Mental states that interfere with sensible responses to heat. For example, elderly people with dementia and intoxicated people who are in a hot environment may not move to a cool environment, remove heavy clothing, or turn on an air conditioner.

The chance of developing heat disorders increases when exposure to heat occurs suddenly, such as when a child is left in a closed car on a hot summer day. In hot weather, the interior of a closed car can heat from 80 to 120° F (27 to 49° C) in 15 minutes. When people are gradually exposed to longer periods of heat and humidity, the body adjusts and is better able to maintain normal body temperature. This process is called acclimatization. Acclimatization occurs more rapidly in young or physically active people than in older or physically inactive people.

Factors that increase vulnerability to the effects of most heat disorders include the following:

• Being very old or very young
• Having certain medical conditions, such as those that involve malfunction of the heart, lungs, kidneys, or liver
• Taking diuretics
• Having imbalances in blood chemistries (electrolytes)

Prevention

Using common sense is the best way to prevent heat disorders. For example, children (and pets too) should never be left in enclosed, poorly ventilated spaces, such as a hot car, even for a few minutes. During excessively hot weather, the very old and the young should not remain in unventilated residences without air-conditioning.

During hot, humid weather, it is best to wear light, loose-fitting clothing made of cloth that breathes, such as cotton. Fluids and salts lost through sweating can usually be replaced by consuming water or lightly salted foods and beverages, such as sports beverages, salted tomato juice, or cool bouillon. Alcoholic beverages are not a good fluid replacement.

Exertion in the Heat: Strenuous exertion in a very hot environment should be avoided. When exertion in a hot environment cannot be avoided, drinking plenty of fluids and frequently cooling the skin by misting or wetting it with cool water can help keep body temperature near normal. To replace adequate amounts of fluids, drinking must continue even after thirst is quenched. 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 about 2 pounds (1 kilogram) of starting weight before the next day's exposure. People who lose at least 4% of their body weight should limit their activity for 1 day.

People engaged in outdoor activities who drink large quantities of water without salt may dilute the sodium in the blood (a condition called hyponatremia—see Electrolyte Balance: Hyponatremia), which may cause seizures and even death. Consuming salt, even in salty “junk” food, along with the water can alleviate this problem. Also, many commercially available drinks contain extra salt.

Slowly 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 dangerous. Progressing from 15 minutes per day of moderate activity (enough to stimulate sweating) during a hot time of day to 90 minutes of vigorous activity over 10 to 14 days is typically adequate.

Ways to Help Prevent Heat Disorders
Ensure adequate ventilation or air-conditioning during heat waves, particularly for people who are very old or very young. Avoid leaving children in automobiles in the hot sun, particularly with closed windows. Avoid strenuous exertion in hot environments and poorly ventilated spaces. Avoid inappropriately heavy, insulated clothing. If exertion in heat is unavoidable, wear open-mesh clothing, use a fan, and drink every few hours regardless of thirst. If 2% or more of body weight is lost during exercise or work, drink extra fluids. If 4% or more of body weight is lost during exercise or work, limit activity for 1 day. If large amounts of water are drunk, consume salts in fluids or food. If prolonged exertion in heat is unavoidable, starting 10 to 14 days before maximum exertion is required, begin with moderate activity done for about 15 minutes a day, slowly increasing the intensity of the activity and the time spent doing it.

Last full review/revision February 2009 by James P. Knochel, MD