Heat-Related Illnesses

A healthy body temperature is maintained by the nervous system. As body temperature increases, the body tries to maintain its normal temperature by transferring heat. Sweating and blood flow to the skin (thermoregulation) help us keep our bodies cool. A heat-related illness occurs when our bodies can no longer transfer enough heat to keep us cool.

A high body temperature (hyperthermia) can develop rapidly in extremely hot environments, such as when a child is left in a car in the summer heat. Hot temperatures can also build up in small spaces where the ventilation is poor, such as attics or boiler rooms. People working in these environments may quickly develop hyperthermia.

High temperature caused by a fever is different from a high body temperature caused by a heat-related illness. A fever is usually the body's normal reaction to infection and other conditions, both minor and serious. Heat-related illnesses produce a high body temperature because the body can't transfer heat effectively or because external heat gain is excessive.

Heat-related illnesses include:

* Heat rash (prickly heat), which occurs when the sweat ducts to the skin become blocked, swell, and cause discomfort and itching.
* Heat cramps, which occur in muscles after exercise because sweating causes the body to lose water and salt and minerals (electrolytes).
* Heat edema (swelling) in the legs and hands, which can occur when you sit or stand for a long time in a hot environment.
* Heat tetany (hyperventilation and heat stress), which is usually caused by short periods of stress in a hot environment.
* Heat syncope (fainting), which occurs from low blood pressure when heat causes the blood vessels to expand (dilate) and body fluids move into the legs because of gravity.
* Heat exhaustion (heat prostration), which generally develops when a person is working or exercising in hot weather and does not drink enough liquids to replace those lost fluids.
* Heatstroke (sunstroke), which occurs when the body fails to regulate its own temperature and body temperature continues to rise, often to 105° (40.6°) or higher. Heatstroke is a medical emergency. Even with immediate treatment, it can be life-threatening or result in serious long-term complications.

Often, environmental and physical conditions can make it difficult to stay cool. Heat-related illness is often caused or made worse by dehydration and fatigue. Exercising during hot weather, working outdoors, and overdressing for the environment increase your risk. Caffeine and alcohol also increase your risk for dehydration.

Many medications increase your risk of a heat-related illness. Some medications decrease the amount of blood pumped by the heart (cardiac output) and limit blood flow to the skin, so your body is less able to cool itself by sweating. Other medications can alter your sense of thirst or increase your body's production of heat. If you take medications regularly, ask your health professional for advice about hot-weather activity and your risk of getting a heat-related illness.

Other factors that may increase your risk of a heat-related illness include:

* Age. Babies lose heat quickly because they have a greater body surface area compared with their weight and do not sweat effectively. Older adults do not sweat easily and usually have other health conditions that affect their ability to lose heat.
* Obesity. People who are overweight have decreased blood flow to the skin, hold heat in because of their insulating layer of fat tissue, and have a greater body mass to cool.
* Summer heat waves. People who live in cities are especially vulnerable to illness during a summer heat wave because heat is trapped by tall buildings and air pollutants.
* Chronic diseases, such as diabetes, heart failure, and cancer.
* Travel to wilderness areas or foreign countries with high outdoor temperatures and humidity.

Most heat-related illnesses can be prevented by keeping the body cool and by avoiding dehydration in hot environments. Home treatment is usually all that is needed to treat mild heat-related illnesses. Heat exhaustion and heatstroke require prompt medical treatment.