Heat stroke and heat exhaustion

What are heat stroke and heat exhaustion?

Heat illnesses such as heat exhaustion and heat stroke occur when your body can't keep itself cool. As the air temperature rises or you do exercise, your body keeps cool by producing sweat that evaporates. On hot, humid days, you feel uncomfortable because the evaporation of sweat is slowed by the increased moisture in the air.

When you are exposed to high temperatures and humidity for too long then your natural cooling system may be overloaded. You may no longer be able to produce enough sweat to cool yourself or high humidity may prevent the sweat evaporating. This can lead to heat illnesses, ranging from heat cramps to heat exhaustion and, most serious of all, heat stroke, which can be life-threatening.

Heat illnesses are sometimes called sun stroke but can happen even when you are not exposed to the sun.

Who gets heat illnesses?

Anyone can get a heat illness. Babies, children and older people are more susceptible. You may also be more susceptible if you have a condition such as asthma, diabetes, pregnancy, a heart condition, epilepsy, overweight or obesity. Some medicines, illicit drugs and alcohol also increase your risk.

Even young healthy people can develop heat illness if they do strenuous physical activity during hot weather. Importantly, although dehydration may lead to heat illness, both heat exhaustion and heat stroke can occur without you being dehydrated.

What are the symptoms of heat exhaustion and heat stroke?

Common symptoms of heat exhaustion are headache, dizziness, muscle weakness or muscle cramps, and nausea or vomiting. You may have a fast, weak pulse, and your breathing may be fast.

Heat exhaustion can develop into heat stroke, which can be life-threatening. In heat stroke the body’s systems to regulate temperature fail. The skin is usually hot and dry (but can be sweaty if the person is exercising) and the person may become confused and collapse.

In heat stroke the body temperature can rise rapidly and reach above 41 degrees Celsius within 10 to 15 minutes. This can cause death or permanent disability if not treated urgently.

What is the 'feels like' temperature?

Many weather forecasts include a 'feels like' or 'apparent' temperature as well as the air temperature. The feels-like temperature is a measurement of how hot it feels taking into acccount the humidity as well as the air temperature. It is an indicator of heat stress. In humid weather the feels-like temperature is higher than the air temperature. The feels-like temperature is usually given for a person standing in the shade. In the sun, it would be even higher – an extra 8 degrees Celsius when the sun is at its highest in Australia.

The 'wet globe bulb temperature' is another measurement of heat stress that is mostly used in occupational health and safety.

According to Sports Medicine Australia, there is a high risk of heat illness when the air temperature reaches 31 degrees Celsius with 50% humidity (feels-like 35 degrees Celsius). The risk is extreme at an air temperature of 36 degrees Celsius with 30% humidity (feels-like 38 degrees Celsius). Heat illness can occur at lower temperatures if you have a condition that increases your susceptibility.

How can I prevent heat illnesses?

  • Stay indoors in air-conditioned areas when possible.
  • Drink plenty of water before starting an outdoor activity. Drink extra water all day.
  • Drink less tea, coffee and alcoholic beverages as these can contribute to dehydration.
  • Wear lightweight, light-coloured, loose-fitting clothes made of fabric that 'breathes' (lets sweat evaporate).
  • Protect yourself from the sun by wearing a hat or using an umbrella.
  • Increase the time you spend in daily outdoor activities slowly and gradually.
  • Schedule vigorous outdoor activities for cooler times of the day.
  • Try to avoid spending time outdoors during the hottest hours of the day: 10am to 4pm.
  • During an outdoor activity, take frequent breaks and drink water or other fluids every 15 to 20 minutes, even if you don't feel thirsty. If you have clear, pale urine, you are probably drinking enough fluids. (Fluids help your body control its temperature but note that heat illness can still occur when you have drunk enough fluids.)
  • If you have a chronic medical problem, ask your doctor about drinking extra fluids and about the effect of your condition and your medicines.

What should I do if I get signs of heat illness?

Go to a shady, cooler area right away. Lie down with your legs higher than your head. Remove any excess clothing, sponge your body with lukewarm tap water and fan your skin. Slowly sip water or other fluids.

Get medical help right away if you have any of the following warning signs:

  • Hot, dry skin, but not sweaty.
  • Confusion or loss of consciousness.
  • Frequent vomiting.
  • Shortness of breath or trouble breathing.
  • Rapid strong pulse.
  • Body temperature above 38.9 degrees Celsius.


1. Centers for Disease Control and Prevention (US). Extreme heat (updated May 2018). https://www.cdc.gov/disasters/extremeheat/index.html (accessed Sept 2018).
2. MayoClinic.com. Heat exhaustion: first aid (updated 12 Jan 2010). http://www.mayoclinic.com/health/first-aid-heat-exhaustion/FA00020 (accessed Feb 2011).
3. MayoClinic.com. Heatstroke: first aid (updated 12 Jan 2010). http://www.mayoclinic.com/health/first-aid-heatstroke/FA00019 (accessed Feb 2011).
4. MayoClinic.com. Hot-weather exercise: how to keep cool (updated 27 Jun 2009). http://www.mayoclinic.com/health/exercise/HQ00316 (accessed Feb 2011).
5. Australian Government Department of Health and Ageing. 11 December 2009 - heat wave conditions - residential care. Important reminder for aged care providers in heat wave conditions (10 Dec 2009). http://www.health.gov.au/internet/main/publishing.nsf/Content/ageing-mailfax-2009-1112a.htm (accessed Feb 2011).
6. Sports Medicine Australia. Beat the heat: playing and exercising safely in hot weather fact sheet (2008). http://sma.org.au/wp-content/uploads/2009/05/beat-the-heat.pdf (accessed Feb 2011).
7. Australian Government Bureau of Meteorology. Thermal comfort observations (update 5 Feb 2010). http://www.bom.gov.au/info/thermal_stress/ (accessed Feb 2011).