Heat Exhaustion

Normally our core body temperature is around 37C. As we get colder we will die at around 23C. As we get hotter, we can expect death at around 42C. With heat, there is far less margin, and so identifying the heat stricken casualty is hugely important.

What is it?

Hyperthermia simply means high core body temperature. You will have heard of the the terms heat exhaustion and also heat stroke. Heat stroke may follow on from heat exhaustion, but don’t worry too much over the semantics, in real life you need to worry about what it is you’re seeing in your casualty.

What does heat exhaustion look like? What do we see?

Whenever our brains are fed anything other than the perfect diet of blood being at the right temperature with the right saturation of sugar levels and oxygen, the problems will manifest themselves initially in the casualty’s behaviour. In turn, in the context of a hot day, these signs will suggest that the casualty may be suffering from heat exhaustion:

• the casualty will look hot and knackered – excessively sweating
• head-ache
• dizziness and confusion
• loss of appetite and feeling sick
• changes in normal behaviour
• intensely thirsty and possible cramps in arms and legs (suggesting dehydration)

Heat stroke is an escalation from this situation. The casualty’s temperature is now likely to be at or above 40C. The casualty with heat stroke may have:

• dry hot skin (a sign that they are dehydrated and no longer able to cool themselves through sweating)
• be unconscious or become unresponsive
• a seizure

Especially keep an eye on young children or people with medical conditions such as diabetes or heart problems: they can be particularly susceptible to heat exhaustion/illness.

How can it be prevented?

These are my top tips …

• Begin your day drinking as much fluid as you can stomach. It will be easier if it is diluted with squash maybe. Eat cereal with milk over toast.
• Wear a wide-brimmed sun hat and make sure you lather yourself in factor 50 at the start of the day.
• Don’t carry too much gear. Pack for the day and go as light as you can. I will carry a lightweight waterproof instead of a heavy one and not bother with the waterproof trousers. I will wear approach shoes instead of boots. I will carry a survival bag but not a group shelter.
• Carry enough water. (If possible have it chilling in the fridge through the night before). If you know there is a stream ahead, carry water purification tablets instead of an extra litre of fluid.
• When you see a water source – use it. Drink your remaining supply of water up, and refill your bottle. Pour water from the stream/lake over your head and body in order to bring your temperature down.
• Dark colours absorb heat – so leave those clothes at home. Leave the thermals behind too. Choose to wear light coloured loosely fitting clothing.
• Don’t just eat sugar – but include salt in your diet too. Crisps are great! (Though they do make you thirsty).

What can we do for the casualty with heat exhaustion or heat stroke as a first-aider?

Leaders working with clients on endurance events or on expedition need to be aware of people not feeling well or simply looking wrong.

First things first, whatever the casualty is doing, stop them from continuing. If you can, get them to rest in the shade, give them water to drink and if possible cool them down by sprinkling water over them. If water is in a short supply, concentrate its effects by dousing bandages (or something equivalent) in water and loosely wrapping these around the neck and wrists.

If you have any (and on a trek or on expedition I carry loads), offer the casualty oral rehydration salts or electrolytes such as Dioralyte Sachets. Make sure that you dissolve the powder in the prescribed ratio since we’re aiming to offer the casualty the right concentration of salts to fluids.

Monitor your casualty – use a casualty card to record the vital signs. Consider what options you have and start thinking ahead. If the casualty’s condition doesn’t improve inside 30 minutes or begins to gets worse, seek further advice by calling 999 (in the UK) and talking through what you’re facing with a member of the local mountain rescue. (Just because you call 999 doesn’t mean that you’re hitting the panic button and everyone will now be heading in your direction). The emergency services would far rather talk you through a problem if it can save things from escalating.

If the casualty is becoming confused, lie them down on their side in a safe airway position and treat them as an unconscious casualty. (They can’t fall off the floor). If the casualty tells you they are struggling to breathe lying down, let them sit up right. If the casualty has a seizure, protect the head as best as possible and time the duration for which it happens. If the casualty stops breathing, or if breathing is not normal, be prepared to commence CPR.


A broken bone is easily and readily seen as a serious injury to an obvious casualty. No one will fail to act in such a situation. However, the highly motivated athlete who is a casualty affected by the heat may fail to see that they are a casualty and could very well just want to press on. Anticipate this as a leader and think before the day how you can prepare your clients for the heat and what to expect if one of them becomes a casualty because of it.

Author: Will Legon runs Will4Adventure and works professionally as a mountain leader, rock climbing instructor and mountain bike leader. Additionally, Will runs outdoor and expedition first aid courses through sister-company, Will4Adventure First Aid.