Hypothermia and Hyperthermia (Part 2)

Last updated: 28-Jun-16

By Ian Corless

In this article, we look at Hyperthermia which refers to the elevated temperature of the human body due to a failure of thermoregulation and in severe cases can result in death.

Hyperthermia is most often caused by two things: heat stroke and/ or a reaction to drugs. Here, we will discuss the reaction to heat.

Hypothermia occurs when the core temperature drops below 37 ºC, with Hyperthermia the definition is defined as a temperature greater than 37.5 to 38.3 ºC. Hyperthermia can be life threatening if temperatures are close or above 40 ºC.

Rapid breathing, a fast and or weak pulse with heavy sweating are the early signs of Hyperthermia and for many, we would refer to this as heat exhaustion.

Heat stroke takes things up a notch and is the body’s inability to cool down – skin will become hot and dry. A runner, for example, will very likely feel sick, dizzy and may well complain of headaches. As conditions become worse, a runner may well pass out.

If breathing increases and the runner acts a little like a person who has had too much alcohol, chances are the condition has elevated to a severe level. You may well find that you are on the receiving end of hostile behavior. Blood pressure will drop, heart rate will increase and the runner’s skin colour may well change. If this happens, the condition is serious and it can result in death if not treated. Call for help!

How does Hyperthermia happen?

A runner loses heat via sweating. This sweat evaporates on the skin which dissipates heat by convection, assuming that humidity is low enough. Heat stroke will occur when the bodies thermoregulation is overwhelmed.

Overwhelming the body (in simplistic terms) may come from excessive environmental heat (running in a hot climate), running too fast or too hard, being dehydrated, a lack of free flowing air that will help cool the body or a lack of water to splash on the body to cool down.

By way of an example, just think about running in the UK in winter. You are training for the London Marathon. You have spent January, February and March putting in the miles in wet, windy and cold conditions and you know the pace and effort that you want to run at. Race day comes around and a heat wave hits… the temperatures are a good 15-20 degrees more than in training. You or your body is not prepared for this change! If you go out and run at target pace and effort, you will almost certainly stand a good chance of ‘blowing up!’ The only solution for many is to re evaluate the target pace and times and run slower to allow one’s body to regulate to the extreme temperatures.

Heat waves are often followed by a rise in the death rate, and these ‘classical hyperthermia’ deaths typically involve the elderly and infirm. 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. During the July 1995 heat wave in Chicago, there were at least 700 heat-related deaths. The strongest risk factors were being confined to bed, and living alone, while the risk was reduced for those with working air conditioners and those with access to transportation. Even then, reported deaths may be underestimates as diagnosis can be misclassified as stroke or heart attack.- Heat-related deaths during the July 1995 heat wave in Chicago. – Semenza JC, Rubin CH, Falter KH, Selanikio JD, Flanders WD, Howe HL, Wilhelm JL (July 1996).

So how does one avoid Hyperthermia?

We have to remember here that we are talking about Hyperthermia directly relating to running or participating in an event. Therefore, it’s quite simple – when external temperatures rise a runner needs to slow down, take more rest, hydrate, wear protective clothing and monitor constantly how they feel. If water is available, pour it on the back of the neck and take every opportunity to find shade.

Of course, if you enter an event where you know that conditions will be hot, it makes sense to adapt prior to the event by training in hot climates or using the facilities of a heat chamber to simulate possible race conditions.

How does one treat Hyperthermia?

Shade, an ability to stop, apply water and re-hydrate in most scenarios will be fine for mild Hyperthermia. These are often conditions found at races such as the Marathon des Sables – a runner may arrive at a checkpoint with mild Hyperthermia but with some rest, shade, water they are able to continue once recovered.

Should conditions be more severe, active cooling will be required by removing clothing, finding a cool, dark place and then applying water to the body and and applying a wet towel to the head and neck. Ultimately, in all cases of Hyperthermia, the priority is to reduce the heat and return the body back to normal as soon as possible.

Extreme cases are a medical emergency and require the help of a professional. Don’t hesitate to call an emergency number and apply any active cooling that is possible before the emergency services arrive. If possible, submerge the person in water. In a book called Cold water immersion: the gold standard for exertional heatstroke treatment by Casa DJ, McDermott BP, Lee EC, Yeargin SW, Armstrong LE, Maresh CM – submerging a person in ice-water is considered the best method for life threatening heat stroke.

A hospital can provide the best care for extreme cases that include iced saline, gastric lavage and intravenous hydration.


It all sounds scary, huh? The reality is, though, that with a sense of awareness and some common sense, heat stroke can be avoided by being prepared and sensible.

  • If you know you will participate in a hot climate, adapt in advance of the race with multiple heat chamber sessions.
  • Wear a hat.
  • Wear sensible clothing.
  • Keep hydrated and think about salt replacement.
  • Slow down.
  • Take rest breaks.
  • Find some shade.
  • Pour water on your neck and head.

Next time the mercury rises, be sensible, re evaluate targets and thank your lucky stars that you read this article on Hyperthermia, it may well make your next hot weather training session or race a far more pleasurable and safe one!


The opposite problem is Hypothermia which refers to the cooling of the human body which in severe cases can result in death.

In a previous article I looked at what the symptoms are, what to do if it happens and ultimately, how to avoid it!

"Next time the mercury rises, be sensible, re evaluate targets and thank your lucky stars that you read this article"

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