What are the risks of hiking or running in a heatwave?

effects of exercise in extreme heat: runner suffering
Exercise in extreme heat has a profound effect on your body (Image credit: Getty)

Knowing what can happen if you exercise in extreme heat is more pertinent than ever. Across the world, we are seeing longer summers and sustained periods of hot weather. Temperature records continue to be toppled across both the American continent and in Europe as the impact of global warming is felt in a very tangible way.

Love them or hate them, heatwaves are now a fact of life. It might be tempting to get out into the bright sunshine and set forth on an invigorating hike, or hit the local trails in your best trail running shoes to make the most of it. Many of the benefits of exercise in the heat are well documented but, in extreme heat, there are significant dangers.

effects of exercise in extreme heat: hikers in the sun

Hiking with a heavy backpack is not recommended during a heat wave (Image credit: Getty)

If your body’s temperature regulation system is put under too much pressure, you could be at risk of dehydration and heat-related illnesses. These illnesses include: heat cramps, which are painful muscle contractions; heat exhaustion, when you may experience nausea, vomiting, headaches or you may even faint; or heatstroke, a life-threatening condition that can develop when heat exhaustion isn’t treated.

In order to function properly, your body needs to maintain a consistent temperature of around 98.5°F (37°C). In extreme conditions or during exercise, the body works harder to sustain this. Add both extreme heat and exertion together and your body is really scrambling and expending energy in order to regulate itself. This is why heading out in your best hiking boots in the blazing midday summer sun can feel much more taxing than during other seasons.

In this feature, we look at the effects of exercise in extreme heat, taking a look at dehydration, heat exhaustion, heatstroke and sun burn.

Effects of exercise in extreme heat: dehydration

Water is the giver of life. All living things rely on it to survive. It's why astronomers get excited when they discover water on another planet. It's why you can go over a week without food but go without water for more than three days you're a goner. Between 55% and 60% of your body is made up of the stuff. Needless to say, when you go running in hot weather, you should carry plenty.

This is because the average person can expect to lose between 0.8 and 1.4 liters in sweat during normal exercise. The hotter it gets, the more your body needs to sweat to cool down. In extreme heat, you lose it faster and it is far easier to become dehydrated than in normal circumstances.

effects of exercise in extreme heat: woman drinking water

Hydration is key when exercising, especially in hot weather (Image credit: Getty)

Dehydration is when your body loses more fluids than you take in and, if not treated, can be a serious problem. You are more prone to dehydration if you are diabetic, have been drinking alcohol, have a sickness bug or diarrhoea or have been taking diuretics – substances that make you urinate more than usual. Infants, children and elderly people are also more at risk of dehydration.

Exercising without replenishing fluids is a surefire way to become dehydrated, this is why long-distance runners wear hydration packs. The good news is that treatment is super straightforward – simply drink fluids.

The symptoms of dehydration include: 

  • Feeling thirsty
  • Dark yellow, strong smelling urine
  • Dizziness
  • Fatigue
  • A dry mouth, lips and eyes
  • Less frequent urination than usual (less than four times a day)

effects of exercise in extreme heat: hiker with bottle

Have a water bottle ready to avoid dehydration (Image credit: Getty)

Prolonged or repeated dehydration can lead to urinary and kidney problems, such as urinary tract infections, kidney stones or even kidney failure. You lose electrolytes when you sweat and if they become imbalanced can lead to muscle cramps, seizures or even loss of consciousness.

One of the more serious and potentially life-threatening complications includes low blood volume shock – also known as hypovolemic shock. This is when low blood volume leads to a drop in blood pressure, thus leading to a less oxygen in your body.

Heat exhaustion and the more serious heatstroke are another two complications that arise with dehydration...

Effects of exercise in extreme heat: heat exhaustion

Heat exhaustion occurs as a result of your body overheating to between 100.4°F (38°C) and 104°F (40°C). Untreated, it can lead to heatstroke. Fortunately, heat exhaustion is not usually an issue if it's treated within the first half an hour. 

The onset of heat exhaustion can be gradual, perhaps over many days of exposure to high temperatures without proper replacement of fluids. However, exercise in extreme heat can very quickly lead to heat exhaustion. Signs to look out for are:

  • Fatigue and weakness
  • Muscle cramps in arms, legs or stomach
  • Dizziness and confusion
  • Excessive sweating
  • Headaches
  • Nausea
  • Loss of appetite
  • Vomiting
  • Rapid breathing or pulse
  • Body temperature above 100.4°F (38°C)
  • A strong thirst
  • Fainting
  • Cold, clammy skin
  • Dark urine

In the instance of heat exhaustion, the following steps should be followed to cool the person down:

1)    Move them out of the heat and into a cool, shaded spot.

2)    Get them to lie down and raise their feet slightly.

3)    Give them plenty of water or sports hydration drinks.

4)    Cool their skin down using water and/or a fan.

You should stay with the person until their symptoms have subsided.

effects of exercise in extreme heat: hiker pouring water over himself

Pouring or sprinkling water on yourself can help to avoid heat exhaustion (Image credit: Getty)

Effects of exercise in extreme heat: heatstroke

Heatstroke is a life-threatening condition that occurs when your body temperature rises above 104°F (40°C). There are two kinds of heatstroke, exertional and non-exertional. Typically, exertional heatstroke it is the result of too much physical exertion in hot, humid conditions. Non-exertional heatstroke occurs due to other factors like underlying health conditions and age.

Symptoms of heatstroke include those associated with heat exhaustion plus a host of others that indicate a more serious problem. The person in question may initially become irritable, acting irrationally. Vision may become impaired or balance and coordination may be hindered.

In extreme cases, seizures may occur and the affected may become non-responsive or lose consciousness altogether. If left untreated, it can lead to organ failure, brain damage, a coma or even death.

Call the emergency services immediately if you suspect someone has heatstroke and put them in the recovery position if they have lost consciousness.

effects of exercise in extreme heat: recovery position

If someone loses consciousness due to suspected heatstroke, call an ambulance and place them in the recovery position (Image credit: Getty)

Effects of exercise in extreme heat: sunburn

Among the effects of exercise in extreme heat, sunburn can easily be avoided.

Exposure to the suns ultraviolet (UV) rays is harmful to our skin and our eyes. It's why, when it comes to what to wear hiking in hot weather, we recommend a sun hat (check out our guides to the best hiking hats and best running hats you can buy), sunglasses, sun screen and UPF-rated clothing.

Skin that has been exposed to too much sun can become hot, sore and may flake and peel after a few days. This is called sunburn. During a heat wave, the sun's UV rays are very strong and sunburn is more likely. Many people think that cloud cover negates the risk of sunburn but you can still get badly burnt even on a cloudy day.

effects of exercise in extreme heat

Red, flaking skin: the telltale signs of sunburn (Image credit: Getty)

In the worst cases, skin can blister and be extremely painful, though the symptoms usually improve within seven days. However, getting sunburnt increases the risk of skin cancer significantly and severe sunburn can lead to heat exhaustion and heatstroke.

Alex is a qualified Mountain Leader, adventure writer and content creator with an insatiable passion for the mountains. A Cumbrian born and bred, his native English Lake District has a special place in his heart, though he is at least equally happy in North Wales, the Scottish Highlands or the European Alps. Through his hiking, mountaineering, climbing and trail running adventures, Alex aims to inspire others to get outdoors. He is currently the President of the London Mountaineering Club, training to become a Winter Mountain Leader, looking to finally finish bagging all the Wainwright fells of the Lake District and hoping to scale more Alpine 4000ers when circumstances allow. Find out more at www.alexfoxfield.com (opens in new tab)