Dr Sarah Jarvis, MBE
Author: Dr Sarah Jarvis, MBE, General Practitioner (GP)

Dr Sarah Jarvis is the Clinical Director of the Patient Platform, an active medical writer, broadcaster, and the resident doctor for BBC Radio 2.

Originally posted: 15th Jul 2022

We’ve had some extraordinary weather in the UK in recent years, regularly approaching or beating the record for the hottest day ever. Of course, most of us love a balmy summer evening and relish the opportunity to shed the multiple layers required most of the time for life in the UK. But there are both short-term and long-term risks associated with too much sun.

In the short term, heat exhaustion and its more dangerous relative heatstroke (sometimes called sunstroke) should always be on our minds. There are different causes that may occur together: if your main problem is dehydration, symptoms can include (perhaps obviously) feeling extremely thirsty, along with headache and general weakness. But if your mineral balance has been disrupted, especially by excessive sweating, you may also experience muscle cramps, feeling or being sick, and light-headedness.

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Who’s at risk?

Anyone can get heat stroke or heat exhaustion, but along with babies and toddlers you’re at particular risk if you:

  • Are older, especially if you’re frail.
  • Do a lot of physical activity, leading to more sweating and increased body temperature.
  • Take water tablets (commonly prescribed for high blood pressure or heart failure – your pharmacist can advise if you’re taking one).
  • Take certain medications for mental health problems.
  • Have underlying health issues such as kidney, lung or heart disease, diabetes, sickle cell disease or sickle cell trait.
  • Are underweight or very overweight.
  • Have gut problems, including ongoing issues such as Crohn’s disease and ulcerative colitis or short-term tummy bugs.

The same groups are at risk of both heatstroke and heat exhaustion. Heat stroke happens when your body is so overwhelmed by heat and its consequences that it can no longer control its internal temperature.

Fairly obviously, heat exhaustion is closely related to the outside temperature. However, very humid weather (particularly humidity over 60%) increases the risk because it hinders your ability to cool your core body temperature down by sweating. Likewise, if the air is very still (for instance in cities or built-up areas) the sweat won’t evaporate as effectively.

Hot air balloon landing near lake

What are the symptoms?

In the early stages, your body will be working hard to keep your internal temperature and fluid/mineral levels constant. It will try to retain fluid by reabsorbing water from your kidneys, but you’ll also be sweating as this cools your body down.

Early symptoms of heat exhaustion largely relate to dehydration, so you’re likely to be passing only small amounts of dark, concentrated urine as your body tries to retain fluid. You may also experience:

  • Dizziness and feeling faint.
  • Headache (often throbbing).
  • Stomach cramps, nausea, vomiting and being off your food - sometimes accompanied by diarrhoea.
  • Lack of energy.
  • Muscle cramps in your arms and legs.
  • Problems concentrating or confusion.
  • Profuse sweating and pale skin.

If heat exhaustion isn’t recognised and treated, it can progress to heatstroke. This is a medical emergency – your body is no longer capable of keeping your internal environment stable and, for instance, you’ll stop losing heat by sweating. This means your body temperature can rise dangerously, to above 40°C. In addition, you’re likely to experience:

  • More severe dizziness or fainting.
  • A greater degree of confusion or loss of consciousness.
  • Severe, throbbing headache.
  • Nausea and vomiting.
  • Very hot, flushed skin.
  • Worsening muscle cramps or weakness.
  • A rapid pulse which can be very weak or unusually strong.

If untreated, heatstroke can lead to serious damage to your vital organs (including swelling of the brain) and can be fatal.

How do I treat heat exhaustion?

The first step if you have heat exhaustion is to get out of the heat into a cool (preferably air-conditioned) place. Remove tight or restrictive clothing and if possible, take a cool-tepid bath or shower (not too cold as this can cause the blood vessels in the skin to contract, preventing sweating). If there’s no shower available, sponge yourself down with cool water or get someone to do it for you.

When you get out of the shower, don’t dry off completely. Instead, put on a fan to increase sweat evaporation and skin cooling. Lie down and rest until you feel completely better, and ideally stay in the cool for the rest of the day. Drink plenty of non-alcoholic fluid – never drink alcohol if you have heat exhaustion as this can dehydrate you further.

If you don’t feel better within half an hour or so – or if you develop any symptoms of heatstroke – seek urgent medical help.

Even if you do recover, you’re likely to be more sensitive to the heat for several days. Cancel any strenuous activities, stay out of the sun as much as possible, avoid alcohol and make sure you keep topped up with non-alcoholic fluids (you should be passing good amounts of pale straw-coloured urine regularly).

Parasol providing shade on white sandy beach

How do I treat heatstroke?

Heatstroke is a medical emergency. Someone with heatstroke is unlikely to be able to help themselves, so it’s up to you to take prompt action. First, call for emergency medical help (999 in the UK – make sure you know the emergency medical number for any country you’re staying in).

While you’re waiting for help to arrive, move them to a cool place. Remove as much clothing as possible and lie them down – in the recovery position if they’re very faint or have lost consciousness. Stay with them at all times – someone with heatstroke can have a seizure. If they’re capable, help them into a cool bath or shower – if not, moisten their skin with a wet towel then apply a fan.

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How do I avoid heatstroke and heat exhaustion?

A combination of staying hydrated and keeping cool is key to avoiding heat exhaustion, especially if you’re at high risk. Don’t forget that the sun can reflect off pale surfaces or water, so it’s possible to get overheated even in the shade.

Drinking enough non-alcoholic fluids to avoid feeling thirsty is key. Take a bottle of water with you whenever you go outside, and take very regular sips.

Avoid strenuous exercise, especially in the middle of the day when temperatures tend to be highest. If you are exercising, drink about a glass of water every 20 minutes, even if you don’t feel thirsty. It may be worth considering a sports drink as part of your fluid intake, especially if you lose a lot of sweat – this can replace electrolytes lost in sweat.

Wear loose, floaty clothes in natural, breathable materials like cotton or linen. Light-coloured clothing is better than dark because the light colours will reflect sunlight instead of absorbing it. Don’t forget to wear a wide-brimmed hat (especially if you’re a little short of head hair).

Wearing sunscreen won’t reduce your risk of heat exhaustion but it can greatly cut the chance of painful sunburn. Opt for a minimum of factor 15 if you have very dark skin; factor 30 for normal skin; and factor 50 if you burn easily.

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