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
Evaporation of water (mainly sweat)
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. Radiation does not require contact with any objects, for example, as when a light bulb radiates heat into a room.
Evaporation of water draws a large amount of heat from a wet surface. Sweat, moisture produced by the sweat glands, cools the skin as it evaporates. Sweating 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.
The body also cools itself through the processes of
Convection: Heat is transferred to cool water or air passing over the skin
Conduction: Heat is transferred to cooler surfaces in contact with the body, such as when lying on the cold ground
Heat disorders
There are several types of heat disorder:
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 Heat Cramps Heat cramps are severe muscle spasms resulting from a combination of prolonged exercise, heavy sweating, and excessive water replacement in extreme heat. Heat cramps is a mild type of heat disorder... read more . Internal organs can be damaged if body temperature is very high for a long time.
Other temperature-regulation disorders
Malignant hyperthermia Malignant Hyperthermia Malignant hyperthermia is a rare, life-threatening rise in body temperature that occurs in susceptible people who are given certain muscle-relaxing drugs plus an anesthetic gas for surgery.... read more , neuroleptic malignant syndrome Neuroleptic Malignant Syndrome Neuroleptic malignant syndrome is confusion or unresponsiveness, muscle rigidity, high body temperature, and other symptoms that occur when certain antipsychotic (neuroleptic) drugs or anti-vomiting... read more , and serotonin syndrome Serotonin Syndrome Serotonin syndrome is a potentially life-threatening drug reaction that tends to cause high body temperature, muscle spasms, and anxiety or delirium. Serotonin is a chemical that transmits impulses... read more are other disorders in which hyperthermia (elevated body temperature) can be life threatening.
Causes of Heat Disorders
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
Strenuous muscle activity, which may occur during exercise or physical labor (particularly among obese people) or can result from disorders such as seizures Seizure Disorders , agitation, or alcohol or drug withdrawal Substance-Related Disorders
Certain stimulant drugs, such as cocaine Cocaine Cocaine is an addictive stimulant drug made from leaves of the coca plant. Cocaine is a strong stimulant that increases alertness, causes euphoria, and makes people feel powerful. High doses... read more , amphetamines Amphetamines Amphetamines are stimulant drugs that are used to treat certain medical conditions but are also subject to abuse. Amphetamines increase alertness, enhance physical performance, and produce euphoria... read more , methylenedioxymethamphetamine (MDMA or Ecstasy) Methylenedioxymethamphetamine (MDMA) MDMA (3,4-methylenedioxymethamphetamine) is similar to an amphetamine but has both stimulant and hallucinogenic effects. (See also Amphetamines and Overview of Substance-Related Disorders.)... read more , monoamine oxidase inhibitors Monoamine oxidase inhibitors (MAOIs) Several types of medications can be used to treat depression: Selective serotonin reuptake inhibitors (SSRIs) Norepinephrine-dopamine reuptake inhibitors, serotonin modulators, and serotonin-norepinephrine... read more (or MAOIs, a type of antidepressant), and phencyclidine Ketamine and Phencyclidine (PCP) Ketamine and phencyclidine are chemically similar drugs used for anesthesia but are sometimes used recreationally. Ketamine is available in powder and liquid form. The powder can be snorted... read more (angel dust)
Overdoses of drugs that contain aspirin, because high doses of aspirin Aspirin Poisoning Aspirin and related drugs called salicylates, a common ingredient in many prescription and over-the-counter drugs, is safe in normal doses, but severe overdose can cause severe symptoms and... read more 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 medications, most often antipsychotic drugs and those with anticholinergic effects Anticholinergic: What Does It Mean?
, may reduce sweating.Certain disorders that affect the skin interfere with sweating. These disorders include cystic fibrosis Cystic Fibrosis , systemic sclerosis (scleroderma) Systemic Sclerosis Systemic sclerosis is a rare, chronic autoimmune connective tissue disorder characterized by degenerative changes and scarring in the skin, joints, and internal organs and by blood vessel abnormalities... read more
, psoriasis Psoriasis Psoriasis is a chronic, recurring disease that causes one or more raised, red patches that have silvery scales and a distinct border between the patch and normal skin. A problem with the immune... read more 
, eczema Atopic Dermatitis (Eczema) Atopic dermatitis (commonly referred to as eczema) is chronic, itchy inflammation of the upper layers of the skin that often develops in people who have hay fever or asthma and in people who... read more 
, and severe sunburns Sunburn Sunburn results from a brief (acute) overexposure to ultraviolet (UV) light. Overexposure to ultraviolet light causes sunburn. Sunburn causes painful reddened skin and sometimes blisters, fever... read more 
.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, older 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.
Risk factors for heat disorders
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 temperature of a closed car can rise from 80 to 120° F (27 to 49° C) in as little as 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)
Being dehydrated
Prevention of Heat Disorders
The following are some ways to prevent heat disorders:
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 and caffeinated beverages are not a good fluid replacement and may worsen dehydration Dehydration Dehydration is a deficiency of water in the body. Vomiting, diarrhea, excessive sweating, burns, kidney failure, and use of diuretics may cause dehydration. People feel thirsty, and as dehydration... read more .
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 Hyponatremia (Low Level of Sodium in the Blood) In hyponatremia, the level of sodium in blood is too low. A low sodium level has many causes, including consumption of too many fluids, kidney failure, heart failure, cirrhosis, and use of diuretics... read more ), which may cause seizures and even death. Consuming salt, even in salty “junk” food, along with the water can alleviate this problem. Other common ways to maintain normal salt levels include salt tablets (which must be dissolved in adequate amounts of water, for example, a single one-gram salt tablet in a liter or quart of water) and commercially available sports drinks containing 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. 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 and water intake.
