Hyperthermia, also known simply as overheating, is a condition in which an individual's body temperature is elevated beyond normal due to failed thermoregulation.

When extreme temperature elevation occurs, it becomes a medical emergency requiring immediate treatment to prevent disability or death.

Accompanying dehydration can produce nausea, vomiting, headaches, and low blood pressure and the latter can lead to fainting or dizziness, especially if the standing position is assumed quickly.

[2] Human heat-loss mechanisms are limited primarily to sweating (which dissipates heat by evaporation, assuming sufficiently low humidity) and vasodilation of skin vessels (which dissipates heat by convection proportional to the temperature difference between the body and its surroundings, according to Newton's law of cooling).

Other factors, such as insufficient water intake, consuming alcohol, or lack of air conditioning, can worsen the problem.

Enzymes involved in metabolic pathways within the body such as cellular respiration fail to work effectively at higher temperatures, and further increases can lead them to denature, reducing their ability to catalyse essential chemical reactions.

This loss of enzymatic control affects the functioning of major organs with high energy demands such as the heart and brain.

This is partly because thermoregulation involves cardiovascular, respiratory and renal systems which may be inadequate for the additional stress because of the existing burden of aging and disease, further compromised by medications.

[2] Those working in industry, in the military, or as first responders may be required to wear personal protective equipment (PPE) against hazards such as chemical agents, gases, fire, small arms and improvised explosive devices (IEDs).

Depending on design, the wearer may be encapsulated in a microclimate,[20] due to an increase in thermal resistance and decrease in vapor permeability.

This is compounded by increased work rates, high ambient temperature and humidity levels, and direct exposure to the sun.

[2] A fever occurs when the core temperature is set higher, through the action of the pre-optic region of the anterior hypothalamus.

Wet-bulb temperature, which takes humidity into account, or more complex calculated quantities such as wet-bulb globe temperature (WBGT), which also takes solar radiation into account, give useful indications of the degree of heat stress and are used by several agencies as the basis for heat-stress prevention guidelines.

[24][25] In cases of heat stress caused by physical exertion, hot environments, or protective equipment, prevention or mitigation by frequent rest breaks, careful hydration, and monitoring body temperature should be attempted.

[26] However, in situations demanding one is exposed to a hot environment for a prolonged period or must wear protective equipment, a personal cooling system is required as a matter of health and safety.

Because of the broad variety of operating conditions, these devices must meet specific requirements concerning their rate and duration of cooling, their power source, and their adherence to health and safety regulations.

For example, active-liquid systems operate by chilling water and circulating it through a garment; the skin surface area is thereby cooled through conduction.

Bomb-disposal technicians wearing special suits to protect against improvised explosive devices (IEDs) use a small, ice-based chiller unit that is strapped to one leg; a liquid-circulating garment, usually a vest, is worn over the torso to maintain a safe core body temperature.

By contrast, soldiers traveling in combat vehicles can face microclimate temperatures in excess of 65 °C (149 °F) and require a multiple-user, vehicle-powered cooling system with rapid connection capabilities.

Mild hyperthemia caused by exertion on a hot day may be adequately treated through self-care measures, such as increased water consumption and resting in a cool place.

In this situation, antipyretics actually may be harmful in patients who develop hepatic, hematologic, and renal complications because they may aggravate bleeding tendencies.

[28] Sitting in a bathtub of tepid or cool water (immersion method) can remove a significant amount of heat in a relatively short period of time.

It was once thought that immersion in very cold water is counterproductive, as it causes vasoconstriction in the skin and thereby prevents heat from escaping the body core.

A research study was published in March 2019 that looked into multiple case reports of drug induced hyperthermia.

[34] A different study was published in June 2019 that examined the association between hyperthermia in older adults and the temperatures in the United States.

Hospitalization records of elderly patients in the US between 1991 and 2006 were analyzed and concluded that cases of hyperthermia were observed to be highest in regions with arid climates.

Regions least affected by heat wave-related hyperthermia causing death were Southern and Pacific Coastal states.

[37] Northern cities in the United States are at greater risk of hyperthermia during heat waves due to the fact that people tend to have a lower minimum mortality temperature at higher latitudes.

[38] In contrast, cities residing in lower latitudes within the continental US typically have higher thresholds for ambient temperatures.

[43] Hyperthermia can also be deliberately induced using drugs or medical devices, and is being studied and applied in clinical routine as a treatment of some kinds of cancer.