Hot weather and exertion in the hills can be a hazardous combination for the unwary. Here Cory Jones of First Aid Training Co-operative looks at prevention and treatment for dehydration, heat exhaustion and heatstroke.

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Long summer days in the mountains can be glorious—but they also bring a hidden danger. When the sun is high and the pace is steady, your body is working hard to cool itself, often without you noticing. Without enough fluids or the right precautions, dehydration and heat-related illness can creep up on even the fittest runners, climbers and walkers. Here's what you need to know to spot the signs, manage the risks, and take action if things go wrong.

Dehydration: more than just thirst

Many of us associate dehydration with scorching deserts or endurance events—but it can happen anywhere, at any time of year. At its core, dehydration occurs when fluid loss outpaces fluid intake. This is common during prolonged physical activity, even in cool or overcast conditions.

How it starts

As you hike, run or climb, you lose water through sweat and breath. If you don't replace that fluid, your blood volume drops and your body struggles to regulate temperature. Early symptoms can be subtle: a dry mouth, feeling sluggish, trouble concentrating. Left unchecked, dehydration will impact your performance and decision-making.

A simple way to monitor hydration? Keep an eye on your urine. It should be a pale straw colour. If it's dark yellow or amber, it's time to drink.

How much should you drink?

There's no one-size-fits-all answer, but the NHS recommends 1.2 litres per day as a baseline—and more when you're active or exposed to heat. In the hills, especially in summer, most walkers and climbers should aim for closer to 2–3 litres depending on the intensity and duration of the outing.

Electrolyte drinks can help on longer or hotter days when salt and minerals are lost through sweat. Avoid caffeine and energy drinks, which can make dehydration worse by increasing fluid loss.

Recognising and treating mild dehydration

Look out for:

• Dry mouth or lips
• Headache or dizziness
• Tiredness or weakness
• Dark urine or less frequent urination

What to do:

• Stop and rest in a shaded, cool spot
• Sip water slowly but steadily
• Add electrolytes if available
• Remove hats, open vents in clothing, and help the body cool naturally

From dehydration to heat illness

When the body becomes too hot and can't cool itself effectively, dehydration can escalate into heat exhaustion—and in serious cases, heatstroke. This is where first aid knowledge and early action are critical.

Heat Exhaustion: A Warning Sign

Heat exhaustion is the body's distress call. It happens when core temperature rises, but hasn't yet reached the danger zone of heatstroke.

Typical symptoms include:

• Heavy sweating
• Muscle cramps
• Pale, clammy skin
• Fast, shallow breathing
• Nausea or dizziness
• Feeling faint or unsteady

What to do:

• Move the person to a shaded or cool area
• Loosen clothing and remove excess layers
• Offer small, frequent sips of water or rehydration drinks
• Cool their skin with wet cloths, or fan them to encourage evaporation
• Lay them down with legs elevated if dizzy or faint

With prompt action, most people will begin to feel better within 30 minutes. If symptoms persist or worsen, prepare for a more serious condition.

Heatstroke: a medical emergency

When heat exhaustion isn't treated—or comes on rapidly—the body's temperature regulation system can fail. This results in heatstroke, a life-threatening emergency requiring urgent medical help.

Warning signs:

• Hot, flushed, dry skin (sweating may stop)
• Confusion, disorientation or slurred speech
• Rapid pulse
• Seizures
• Loss of consciousness

Immediate actions:

• Call 999 or 112 for emergency help
• Move the casualty to a shaded, ventilated space
• Remove excess clothing
• Use whatever's available to cool the casualty—wet clothing, cold water bottles, streams, or cool packs
• Fan them while cooling
• Monitor breathing and prepare for CPR if needed

The role of sunburn

While it's often considered a separate issue, sunburn can worsen dehydration and increase the risk of overheating. Burned skin loses moisture more rapidly and can't regulate temperature as effectively.

First aid for sunburn:

• Get the person out of direct sunlight
• Cool the affected skin with tepid water—showers, compresses or baths
• Rehydrate with fluids
• Avoid tight clothing over the burn
• Don't apply ice or greasy lotions
• Seek medical attention for large, blistered or infected burns, or if the casualty feels feverish or unwell

For young children, babies, or those with underlying health issues, even mild sunburn can become serious quickly.

Staying safe in hot weather

Preventing heat-related illness is far easier than treating it. If you're out on the hill in warm weather:

• Plan for hydration—bring enough fluids, and consider water refill points or tablets if in remote areas
• Dress for the sun—lightweight, loose clothing and a wide-brimmed hat help prevent overheating
• Take breaks in the shade and avoid full midday exposure if possible
• Pace yourself—particularly if carrying heavy loads or ascending steep slopes
• Use sunscreen generously and reapply often

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About the Author

Cory Jones has been an International Mountain Leader for two decades, and is a wilderness first aid trainer. He is a founder member of First Aid Training Co-operative, who offer comprehensive first aid courses tailored to outdoor environments, with training built around real-life scenarios that apply to mountain users, from climbers to runners and walkers.

When the unexpected happens, knowing what to do can be the difference between discomfort and a serious emergency. Whether you're leading a group or heading out solo, first aid training tailored for outdoor environments gives you the confidence to respond effectively to heat illness, dehydration, and many other incidents.