Hyperthermia, also called overheating, happens when a person's body temperature rises above normal because the body cannot control its temperature properly. This occurs when the body produces or absorbs more heat than it can release. According to the International Emergency Medicine Education Project, severe hyperthermia (a body temperature above 40 °C (104 °F)) is a medical emergency that needs immediate treatment to avoid serious harm or death. Each year, nearly half a million deaths are linked to hyperthermia.

The most common causes are heat stroke and harmful reactions to certain medicines. Heat stroke happens when the body is exposed to extreme heat or a mix of heat and humidity, which overwhelms the body’s ability to regulate temperature. Another cause is a rare side effect of some medicines, especially those that affect the brain. Malignant hyperthermia is a rare complication that can occur during general anesthesia. Hyperthermia can also result from a serious brain injury.

Hyperthermia is different from fever because the body’s normal temperature target does not change. The opposite of hyperthermia is hypothermia, which happens when body temperature drops below what is needed for normal body functions. The word "hyperthermia" comes from Greek, where "hyper" means "above" and "thermos" means "heat."

The highest body temperature ever recorded in a person who survived hyperthermia was 46.5 °C (115.7 °F). This temperature was measured on July 10, 1980, in a man admitted to the hospital with severe heat stroke.

Classification

In humans, hyperthermia is a condition where the body's temperature rises above 37.5–38.3 °C (99.5–100.9 °F), depending on the source, without the body's normal temperature changing.

The typical human body temperature can reach up to 37.7 °C (99.9 °F) in the late afternoon. Hyperthermia happens when the body's temperature is higher than expected. These increases can be mild or severe. Temperatures above 40 °C (104 °F) can be very dangerous and may cause serious harm.

Signs and symptoms

Hyperthermia can begin with a condition called "heat exhaustion" (also known as "heat prostration" or "heat stress"). Symptoms may include heavy sweating, fast breathing, and a weak, rapid heartbeat. If the condition worsens and becomes heat stroke, the skin may feel hot and dry because blood vessels widen to help the body lose heat. In some cases, the body may not be able to cool itself through sweating, leading to dry skin. Hyperthermia caused by neurological diseases may result in little or no sweating, heart or blood vessel problems, and confusion or being disoriented.

Other symptoms can vary. Dehydration may cause nausea, vomiting, headaches, and low blood pressure. Low blood pressure can lead to dizziness or fainting, especially when standing up quickly.

In severe heat stroke, confusion and aggressive behavior may occur. The heart and breathing rates may increase (fast heart rate and fast breathing), while blood pressure drops. The heart tries to keep blood flowing properly. As blood pressure decreases, blood vessels may narrow, causing the skin to appear pale or bluish in serious cases. Young children may experience seizures. Without treatment, severe heat stroke can lead to organ failure, unconsciousness, and death.

Causes

Heat stroke happens when the body's ability to control temperature is overwhelmed by too much heat produced during physical activity, too much heat in the environment, or when the body cannot cool itself properly. This leads to body temperatures that are much higher than normal, sometimes over 40 °C (104 °F). Heat stroke can occur without physical activity (called classic heat stroke) or during physical activity (called exertional heat stroke).

When people do heavy work in hot and humid conditions, their bodies may produce more heat than they can cool down. The body cools itself mainly by sweating (which works best when humidity is low) and by increasing blood flow to the skin (which helps release heat). Other factors, such as not drinking enough water, drinking alcohol, or not having air conditioning, can make it harder for the body to cool itself.

When the body cannot control its temperature, it can harm the body's chemical processes. Enzymes, which help the body perform important tasks like breathing, stop working well at high temperatures. If temperatures rise too much, these enzymes may stop working completely, which can damage organs like the heart and brain. Losing too much water and important minerals can cause heat cramps—muscle spasms that last one to three minutes. These cramps usually happen during heavy physical activity, and body temperature may stay normal or only slightly high.

Situational heat stroke happens even without physical activity. It often affects young children and older adults. In older adults, certain medicines, such as those that reduce sweating or blood flow to the skin, can make it harder for the body to handle heat. This can cause heat stroke even when the person is resting.

During heat waves, more people may die from heat-related causes, especially older adults and those with health problems. This is because the body's systems for controlling temperature, such as the heart, lungs, and kidneys, may not work well enough to handle the extra stress. For example, during a heatwave in Chicago in 1995, at least 700 people died from heat-related causes. People who were bedridden or lived alone were at greater risk, while those with air conditioning or transportation had lower risk. Some deaths may not be correctly reported as heat-related and might be mistaken for other illnesses like stroke or heart attack.

Some medicines can cause the body to produce too much heat. This is more common in areas where these medicines are used more often.

People who work in dangerous jobs, such as in factories, the military, or emergency services, often wear protective gear like special suits, fire gear, or body armor. These suits can trap heat and make it harder for the body to cool itself. This is especially hard during hot weather, when working hard, or when exposed to the sun. This protective gear can increase the risk of heat stress even though it is meant to protect against other dangers.

This problem was seen during the 2014 Ebola outbreak in Africa. Doctors and healthcare workers could only work for about 40 minutes at a time in their protective suits because of the risk of heat stroke.

Other rare causes of overheating include problems with the thyroid gland or a tumor in the adrenal gland. These conditions can cause the body to produce too much heat. Damage to the brain, such as from bleeding, injury, or seizures, can also cause the body to overheat.

Pathophysiology

A fever happens when the body’s temperature control center in the brain, called the pre-optic area of the anterior hypothalamus, raises the body’s temperature set point. For example, during a bacterial or viral infection, some white blood cells release substances called pyrogens. These pyrogens affect the anterior hypothalamus, causing the body’s temperature to increase, similar to turning up the temperature setting on a thermostat.

In contrast, hyperthermia occurs when the body’s temperature rises due to factors outside the temperature control system, such as extreme heat exposure, without the brain’s temperature set point changing.

Some symptoms of acute exertional heatstroke, like vomiting, diarrhea, and gastrointestinal bleeding, may result from damage to the digestive system’s barrier, allowing toxins to enter the bloodstream. Studies show that athletes who engage in ultraendurance activities often have higher levels of endotoxins in their blood. These toxins can trigger the release of inflammatory signals, which may lead to problems in multiple organs. In experiments, monkeys given oral antibiotics before heatstroke did not develop endotoxemia.

Scientific research supports the idea of a temperature set point, which is the body’s preferred temperature needed for essential life processes. Nervous signals from the pre-optic area of the hypothalamus control activities that either cool the body, such as sweating, or warm the body, such as shivering, through the autonomic nervous system. The pre-optic anterior hypothalamus contains neurons that respond to temperature changes. Warm-sensitive neurons increase their electrical activity when the temperature rises above 37 °C (99 °F), while cold-sensitive neurons increase their activity when the temperature drops below 37 °C (99 °F).

Diagnosis

Hyperthermia is usually diagnosed by looking at a very high body temperature and a medical history that suggests hyperthermia rather than a fever. This often happens when the high temperature occurs in very hot and humid conditions (heat stroke) or in people taking a medicine that can cause hyperthermia as a side effect (drug-induced hyperthermia). Doctors also look for signs and symptoms linked to hyperthermia, like those seen in neuroleptic malignant syndrome, and check for the absence of symptoms usually seen with fevers caused by infections.

If medicines that reduce fever lower the body temperature, even if it doesn't go back to completely normal, hyperthermia is ruled out.

Prevention

When the temperature around us is too high, humans and many animals cool their bodies by using sweat (or other liquids like saliva in dogs). This process helps prevent dangerous overheating. How well this cooling works depends on the amount of humidity in the air. A measurement called wet-bulb temperature, which considers humidity, or more detailed measures like wet-bulb globe temperature (WBGT), which also includes sunlight, helps show how much heat stress a person might face. These measurements are used by organizations to create guidelines for avoiding heat-related harm. Wet-bulb temperature is the lowest temperature a person’s skin can reach through evaporative cooling at a specific temperature and humidity level.

If the wet-bulb temperature stays above 35°C (95°F) for a long time, even healthy people without clothing and with a fan can face life-threatening conditions. At this temperature, the body gains more heat than it loses. A 2020 study found that wet-bulb temperatures above 31°C (88°F) to 33°C (91°F) have occurred many times worldwide, and two weather stations recorded temperatures above 35°C (95°F) multiple times.

To prevent or reduce heat stress from physical activity, hot environments, or protective gear, people should take regular breaks, stay hydrated, and check their body temperature. However, in situations where someone must stay in a hot environment for a long time or wear protective equipment, a personal cooling system is needed for health and safety. These systems can be active (using power) or passive (not using power) and are grouped based on their power source and whether they are worn by a person or attached to a vehicle.

Because conditions vary widely, these systems must meet specific standards, such as how quickly and long they can cool, their power source, and compliance with safety rules. Other factors include the user’s need to move freely. For example, active-liquid systems cool water and circulate it through clothing, cooling the skin through contact. These systems have worked well in military, law enforcement, and industrial settings. Bomb-disposal technicians wear suits with a small, ice-based cooling unit on one leg and a liquid-circulating vest over their torso to keep their body temperature safe. Soldiers in combat vehicles, where temperatures can exceed 65°C (149°F), need a cooling system powered by the vehicle that connects quickly for multiple users. Different groups, like hazmat teams, medical workers, and industrial workers, have unique cooling needs.

Treatment

The main cause of the problem must be addressed. Mild overheating from physical activity on a hot day can often be treated with self-care, such as drinking more water and resting in a cool place. Overheating caused by drug use needs the drug to be stopped quickly, and sometimes other medicines may be used to help.

Medicines that lower fever, like acetaminophen, aspirin, or other nonsteroidal anti-inflammatory drugs, do not help treat heatstroke. These medicines work by changing the body’s temperature control center, which is already overwhelmed during heatstroke. They may even be harmful if the person has liver, blood, or kidney problems, as they can worsen bleeding issues.

When body temperature rises very high, mechanical cooling is used to remove heat and help the body regulate its temperature again. Passive cooling methods, such as resting in a cool, shady area and removing clothing, can be done right away. Active cooling methods, like sponging the head, neck, and chest with cool water, help remove heat and bring the body temperature back to normal faster. If immersion in water is not possible, misting the body with water and using a fan can also be effective.

Sitting in a bathtub filled with lukewarm or cool water (immersion method) can quickly remove a large amount of heat. In the past, it was believed that very cold water was not helpful because it might reduce blood flow near the skin, trapping heat inside the body. However, a recent British study found that cooling rates were fastest with the coldest water. This study concluded that ice water immersion is the most effective cooling method for heatstroke caused by physical activity. For heatstroke not caused by physical activity, no better method has been found. Therefore, ice water immersion is considered the best treatment for life-threatening heatstroke.

If body temperature reaches about 40°C (104°F), or if the person is unconscious or confused, hyperthermia is a medical emergency that needs treatment at a hospital. If the person stops breathing or their heart stops, cardiopulmonary resuscitation (CPR) may be needed. In a hospital, stronger cooling methods are available, such as giving fluids through a vein, washing out the stomach with cold salt water, or using hemodialysis to cool the blood.

Epidemiology

Hyperthermia occurs when people cannot control their body temperature, often because of environmental conditions. The main risk is not being able to sweat. People who are dehydrated or older may not sweat enough to cool their bodies. High heat can increase the risk of hyperthermia for certain groups, such as those who work outdoors, like soldiers, construction workers, and factory workers. People with less money may struggle to stay cool if they live in places without air conditioning. Hyperthermia is most common during hot, dry weather in the summer.

Studies have shown that certain medications, like anti-psychotics, antidepressants, and anxiolytics, are linked to higher heat-related deaths compared to other drugs. Another study from 2019 found that older adults in the United States were more likely to experience hyperthermia in areas with dry climates. These cases often happened during early heat waves, when people had not yet learned how to stay cool.

In cities, hyperthermia is more common due to the urban heat island effect, which makes cities hotter than surrounding areas. In the United States, the north-central region (Ohio, Indiana, Illinois, Missouri, Iowa, and Nebraska) had the most hyperthermia-related illnesses between 1991 and 2006. Northeastern states had the next highest rates. Southern and Pacific Coastal states had fewer cases. Northern cities are more at risk during heat waves because people there are more likely to be harmed by even small increases in temperature. In contrast, cities in lower latitudes can handle higher temperatures before health risks increase. In 2015, hundreds of people died in India and Pakistan from heat waves, and tens of thousands died in Europe during a severe heat wave in 2003.

Hyperthermia can be caused by dehydration, certain medications, or the use of drugs like cocaine, amphetamines, or alcohol. A body temperature above 37.5–38.3 °C (99.5–100.9 °F) may indicate hyperthermia. As body temperatures rise or remain high, the risk of serious health problems increases, including heat stroke, organ failure, and death. There are two types of heat stroke: classical and exertional. Classical heatstroke happens during extreme heat, like heat waves, and affects young children, the elderly, or those with chronic illnesses. Exertional heatstroke occurs after intense physical activity and is more common in healthy people aged 15 to 50. Sweating may happen during exertional heatstroke. Heatstroke has a mortality rate of 40 to 64%.

Research

Hyperthermia can be intentionally caused using medicines or medical tools and is used in medical treatments to help fight certain types of cancer. Studies have found that controlled hyperthermia can reduce the size of tumors. This happens because high body temperature harms cancer cells by damaging parts inside the cells. Researchers have also studied whether hyperthermia makes cancerous tumors more likely to be affected by radiation treatment, which has allowed hyperthermia to be used along with other cancer treatments. Different methods of using hyperthermia to treat cancer include targeting specific areas of the body or applying heat to the entire body.