Exposure to excessive heat presents wide-ranging and serious health risks.

It can amplify existing conditions and result in premature death, disability and illness, including exacerbation of non-communicable diseases and the effects of air pollution.

  • Prolonged heat imbalance in the body can be life-threatening.
  • Heatstroke is a medical emergency, while heat exhaustion is not usually serious if you can cool down within 30 minutes.
  • Avoiding thermal risk and early recognition of heat stress are the cornerstones of prevention.

How the Body Processes Heat

Heat Balance

The human body tightly regulates the body temperature through a process called thermoregulation, in which the body can maintain its temperature within certain boundaries, even when the surrounding temperature is very different. The core temperature of the human body remains steady at around 36.5-37.5 ºC (or 97.7º-99.5ºF) at rest. This is regulated by the hypothalamus in the brain, which senses changes in the core temperature and operates like a thermostat to induce mechanisms to return the temperature to its normal range. If the body temperature is rises, the hypothalamus can initiate several processes to lower it, including increasing the circulation of the blood to the surface of the body to allow for the dissipation of heat through the skin, and initiation of sweating to allow evaporation of water on the skin to cool the surface of the skin (source).

Heat Imbalance

Failure of thermoregulatory mechanisms or exposure to extreme or sustained temperatures that overwhelm the body’s thermoregulatory capacity can result in potentially life-threatening illnesses (source). These include heat stroke, heat exhaustion, heat fainting, heat edema (swelling of hands, feet and ankles), heat rash and heat cramps (muscle cramps), and are mainly caused by over-exposure to extreme heat or physical overexertion for a person’s age and physical condition.

Understanding Heat Illness

Symptoms of Heat Related Illness

Hyperthermia, defined as a core temperature of > 40°C, may present with sweating, flushing, tachycardia, fatigue, lightheadedness, headache, and paresthesia, progressing to weakness, muscle cramps, oliguria, nausea, agitation, hypotension, syncope, confusion, delirium, seizures, and coma (source).

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Heat Stress/Exhaustion vs Heat Stroke

Changes in mental status can help distinguish 1 potentially fatal heat stroke from heat stress or exhaustion (source). Heat exhaustion is not usually serious if you can cool down within 30 minutes, while heat stroke is a medical emergency (source).


Exertional vs Passive Heat Illness

Exertional heat stroke can develop in able-bodied individuals, such as athletes, soldiers, or laborers, and those performing rigorous physical activities in hot conditions. In contrast, passive or nonexertional heat stroke can develop during low-level physical activities among elderly and very young children and babies, ambulatory individuals with comorbidities including obesity, diabetes, hypertension, heart disease, renal disease, dementia, and alcoholism who for physiological or behavioral reasons are unable to release heat from their bodies in hot conditions (source).

Learn more about managing and adapting to heat in sports and heat in at work.

Heat Stress vs Heat Stroke

Heat Stress / ExhaustionHeat Stroke
SignsEarly warning signs include nausea, light-headedness, fatigue, muscle cramping and dizziness. In addition to the signs of heat stress, someone experiencing heat stroke may also have a headache, confusion, no sweating, rapid heart rate, nausea or vomiting and may lose consciousness
  • Move to an air-conditioned place
  • Take a cold shower or use a cold compress
  • Drink plenty of fluids
  • Remove tight or extra clothing layers
  • Call for medical help immediately
  • Use aggressive cooling such as a cold water bath to get temperature down right away
  • Stop cooling when core temperature of 38 degrees C is reached
  • Do not give them fluids

Management and Adaptation Solutions

Management and adaptation to extreme heat in the body requires an awareness of the key concepts, signs and symptoms of heat-related illness, and an understanding of the measures that can be taken to reduce the initial core temperature and core temperature increase. Avoidance of thermal risk and early recognition of heat stress are the cornerstones of prevention.

Heat Acclimatization

Heat acclimatization is an adaptive response to a hot environment in which an individual’s body “learns” to better tolerate exposure to excessive heat. This adaptation is temporary and may take two weeks when daily exposed to heat, and is lost after some weeks of non-exposure.


The main intention of cooling – such as submerging the body in cold water, or being in a cold room, or drinking cold fluids including ice slushies – is to lower the body’s temperature and reduce the risk of heat related illness. Ice water immersion has been shown to be superior to alternative cooling measures to quickly lower core body temperature (source).

Page Contributors

Jason Lee, Hein Daanen, Nicola Gerrett