What is heat-related illness?

A healthy body temperature is maintained by the nervous system. As the body temperature goes up, the body tries to stay at 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 quickly 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 areas may quickly get hyperthermia.

High temperature caused by a fever is different from a high body temperature caused by a heat-related illness. A fever is 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 as well as it should or because there's too much external heat gain.

Heat-related illnesses include:

• Heat rash (prickly heat). It occurs when the sweat ducts to the skin become blocked or swell, causing discomfort and itching.
• Heat cramps. They occur in muscles during and after exercise because sweating caused the body to lose water, salt, and minerals (electrolytes).
• Heat edema (swelling) in the legs and hands. This can occur when you sit or stand for a long time in the heat.
• Heat tetany (hyperventilation and heat stress). This is usually caused by short periods of stress in a hot environment.
• Heat syncope (fainting). It's caused by low blood pressure when heat causes the blood vessels to expand (dilate). Then body fluids move into the legs because of gravity.
• Heat exhaustion (heat prostration). It most often occurs when a person works or exercises in hot weather and doesn't drink enough liquids to replace those lost liquids.
• Heatstroke (sunstroke). This occurs when the body doesn't regulate its own temperature. Body temperature keeps rising, often to 105 F (40.6 C) or higher. Heatstroke is a medical emergency. Even with immediate treatment, it can be life-threatening or cause serious long-term problems.

Often, environmental and physical conditions can make it hard to stay cool. Heat-related illness is often caused or made worse by dehydration and fatigue. Your risk goes up if you exercise during hot weather, work outdoors, and don't wear lightweight or loose-fitting clothing for the environment. Drinking alcohol also increases your risk of dehydration.

Many medicines increase your risk of a heat-related illness. Some medicines 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 medicines can change your sense of thirst or make your body produce more heat. If you take medicines regularly, ask your doctor for advice about hot-weather activity and your risk of getting a heat-related illness.

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

• Age. Babies don't lose heat quickly, and they don't sweat well. Older adults don't sweat easily. And they often have other health conditions that affect their ability to lose heat.
• Obesity. People who are overweight have decreased blood flow to the skin. They hold heat in because of the insulating layer of fat tissue. And they have more body mass to cool.
• Heat waves. People who live in cities are especially vulnerable to illness during a heat wave. Heat is trapped by tall buildings and air pollutants, especially if there's a high level of humidity.
• Chronic diseases, such as diabetes, heart failure, and cancer. These conditions change the way the body gets rid of heat.
• Travel to wilderness areas or foreign countries with high outdoor temperatures and humidity. When you go to a different climate, your body must get used to the differences (acclimate) to keep your body temperature in a normal range.

Acclimation helps you remain active in a hot environment with less risk of a heat-related illness. You can acclimate yourself to a hot environment by gradually increasing the amount of time you exercise in the heat each day. Do this over 8 to 14 days. Adults usually need daily exercise periods that last 1 to 2 hours to become acclimated. Children need 10 to 14 days to acclimate.

You can also start acclimating while in cooler environments by wearing more clothing when exercising. This will raise the body temperature, which helps the body start sweating.

Acclimation helps you sweat for a longer time at a lower body temperature. Although this increases the amount you sweat, it decreases the amount of salt you lose in sweat or urine.