Heat illness: preventing and treating heat exhaustion and heat stroke

Heat kills more hikers in some regions than cold, lightning, or wildlife combined, and it does it quietly. It starts as a nagging headache and a little fatigue, and if it’s missed it can climb to heat stroke, a true emergency where the body’s cooling system fails and the brain begins to cook. The saving grace is that heat illness is a spectrum you can catch early, and the earlier you act, the easier it is to reverse. This guide covers how to recognize and treat heat cramps, heat exhaustion, and heat stroke in the backcountry, and how to keep them from starting.

The underlying principle is simple: your body sheds heat mainly by sweating, and sweating costs you water and salt. When you lose fluid and electrolytes faster than you replace them, or when it’s so hot and humid that sweat can’t cool you, your core temperature rises. Heat illness is what happens as that balance tips. Catch it at cramps or exhaustion and rest, shade, and fluids fix it; let it reach heat stroke and every minute of delay raises the risk of death. Prevention and early recognition are everything, and they pair closely with our guide to hiking safely in the heat, which is about not getting here in the first place.

The heat-illness spectrum

Think of heat illness as three stages on one road, each more dangerous than the last: heat cramps (salt loss, painful muscles), heat exhaustion (big fluid and salt loss, weakness and clamminess), and heat stroke (the cooling system fails, the body overheats, life is at risk). They don’t always arrive in order. Heat stroke can strike without cramps or exhaustion first, so treat every heat symptom seriously and act before it climbs. The dividing line that matters most is mental status: someone who is confused, disoriented, or unconscious in the heat has heat stroke until proven otherwise, and that is an emergency.

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Heat cramps

Heat cramps come from losing salt through heavy sweating. The signs are painful muscle cramps or spasms, usually in the legs, arms, or abdomen, often starting as a mild ache. They’re a warning shot. The body telling you the balance is tipping.

Treatment: Stop all activity right away. Get into shade, rest, and drink water, ideally lightly salted or with electrolytes. Gently stretch and massage the cramping muscle. Don’t push on through cramps; if you ignore them and keep working, they get worse and can progress to heat exhaustion. Rest until the cramps ease and you’ve rehydrated before doing anything strenuous again.

Heat exhaustion

Heat exhaustion is a large loss of body water and salt, and it’s the stage most hikers actually reach. The person is still sweating, often heavily, but they’re struggling. Watch for:

• Headache and dizziness
• Weakness and fatigue
• Mental confusion or irritability
• Nausea, and sometimes cramps
• Pale, moist, cool, clammy skin

Treatment: Get them into shade immediately, off the hot ground, and have them lie down. Raising the legs onto a pack or pad, roughly 18 inches (45 cm), is reasonable here: in heat the problem is blood pooling in dilated vessels in the skin and legs, which is a different mechanism from the bleeding-related shock where routine leg elevation is no longer taught (see our first aid guide). Loosen or remove excess clothing. Cool them by sprinkling or sponging with water and fanning. Rehydrate with an electrolyte drink rather than plain water where you have one, sipping a few ounces (about 100 ml) every few minutes rather than gulping, which can trigger vomiting and, over hours, contributes to the problem in the next section. Keep them quiet and resting until they clearly recover, which can take a while; do not let them get up and hike on too soon. Heat exhaustion that isn’t treated can tip into heat stroke.

Heat stroke, the emergency

Heat stroke is the breakdown of the body’s heat-regulating system, with the core temperature climbing above about 105°F (40.5°C). The brain is overheating, and the person will die if not cooled fast. Classic signs:

• Mental changes: confusion, disorientation, strange behavior, then delirium and unconsciousness, the key warning sign.
• Hot skin, often dry, though it can still be sweaty in exertional heat stroke on the trail.
• A swollen, beet-red face and reddened whites of the eyes.
• Fast pulse, headache, dizziness, nausea, and vomiting.
• In severe cases, a bluish tinge to lips and nail beds as shock sets in.

Treatment: This is a race. Cool the person as fast as physically possible and get emergency help moving, activate your satellite messenger or beacon and call for evacuation now, while you cool. Don’t wait to see if they improve.

Heat stroke is one of the life threats covered in our guide to backcountry first aid, which walks through the stay-or-go decision, what to tell a rescue service, and how to monitor a casualty while you wait for help.

How to cool someone fast

The goal is to pull heat out of the body by every means available:

• Get them out of the sun into shade or a shelter immediately, and lie them down off the hot ground.
• Immerse them if you can. Dipping them in a cool stream or lake is the fastest whole-body cooling. The single best option if water is nearby.
• If you can’t immerse, wet and fan. Douse them with water (clean or not. Cooling comes first) and fan hard to drive evaporation. Soak the whole body and especially the head; a lot of heat leaves through the scalp.
• Target the big blood vessels. Put cool, wet compresses or cold items on the neck, armpits, and groin, where blood runs close to the surface.
• Keep going until their temperature and mental state improve, then keep them cool and rested.

Expect the person to shiver, struggle, or vomit as they cool, that’s part of it, not a reason to stop. Once they’re conscious and able to swallow, let them sip water. Be ready to treat for shock, and if they stop breathing or have no pulse, be prepared to start CPR.

Water, salt, and the thirst trap

Thirst lags a little behind need: by the time you feel it you are already mildly dehydrated. That is a reason to sip steadily rather than wait for a strong signal, but it is not a reason to drink to a fixed quota, which is how hikers end up with the problem two sections below. Sip small amounts steadily rather than gulping a quart at once, and use thirst together with urine colour as your guide. Working hard in real heat can demand up to about a quart (1 L) an hour, and a person laboring in full sun at 109°F (43°C) can need five gallons (19 L) of water across a day.

Because sweat carries out salt as well as water, replace electrolytes too, especially if you’re drinking a lot. Add an electrolyte mix or lightly salted water, and eat salty snacks. And do not overdo the fluids: steady, moderate intake with electrolytes is the target, not the largest volume you can manage. Sustained plain-water intake much above about 1.5 quarts (1.5 L) an hour is where people get into trouble in the opposite direction, which is the subject of the next section. Our guide to purifying water covers keeping a safe supply.

The one that looks identical: hyponatremia

Everything above assumes the problem is too little water and salt. There is a second condition that produces a nauseated, headachy, dizzy, confused person on a hot day, and it is caused by the opposite. Exercise-associated hyponatremia happens when someone drinks large volumes of plain water over hours while losing salt in sweat, diluting the sodium in their blood until cells, including brain cells, begin to swell.

This is not a rare curiosity. In one study of Grand Canyon National Park, roughly 19% of patients assessed for heat-related illness turned out to show signs of hyponatremia. These are people in the same places, on the same days, with much the same complaints as heat exhaustion.

The distinguishing features are mostly in the history and the urine, not the symptoms:

• Urine. The single most useful sign. Someone who is hyponatremic is typically producing large volumes of clear urine, frequently. Someone dehydrated from heat is producing small amounts of dark urine, or none.
• Thirst. Reduced or absent in hyponatremia. Present, often strongly, in heat exhaustion.
• The drinking history. Ask what they have actually drunk. Hours of steady plain water, or forcing fluids on a schedule “to stay ahead of it”, is the classic setup.
• The trajectory. Someone who is getting worse as they drink is the wrong candidate for more water. Heat exhaustion improves with shade, rest and fluids; hyponatremia does not.
• Skin and temperature. A hyponatremic person usually does not have hot skin or a high core temperature. Hot, dry skin with confusion points to heat stroke instead.
• Severity signs. Seizures, profound confusion, or a marked drop in consciousness point to hyponatremia or heat stroke rather than simple heat exhaustion, and both are emergencies.

Treatment in the field. Stop all fluid intake. If they are fully alert and able to swallow safely, give salty food, such as crisps, jerky, salted nuts or a bouillon cube, rather than a drink. Keep them still, in shade, and monitor them. Do not give plain water, and do not give large volumes of any fluid on the assumption that hydration must help. Mild cases resolve on their own as the body sheds the excess water, which is why doing nothing is often the correct treatment.

Evacuate anyone with confusion, repeated vomiting, a seizure, or a falling level of consciousness. This needs blood testing and hospital management, and it is one of the clearest cases for a satellite messenger on a hot, remote trip. Tell the responders plainly what the person has been drinking and how much, because that single detail changes their treatment.

Prevention is the same advice that prevents heat illness, done in moderation: drink to thirst rather than to a schedule, take salt with your water on long sweaty days, eat properly rather than living on water alone, and be sceptical of anyone insisting you force fluids at a fixed rate regardless of conditions. Our guide to hiking all day covers the same trap on long-mileage days.

Who’s most at risk

Heat illness hits some people harder and faster: children and older adults, anyone already ill, injured, or dehydrated, people carrying heavy loads or unacclimatized to the heat, and anyone under stress with limited water. High humidity multiplies the danger because sweat can’t evaporate to cool you. If you’re traveling with a group, use the buddy system. Watch each other, and treat anyone who lags, complains of tiredness, gets irritable, or wanders off as a possible heat casualty until you’ve checked.

Preventing heat illness

• Hydrate ahead and often. Drink at least once an hour in heat; sip steadily rather than waiting for thirst.
• Rest in the heat of the day. Do hard travel in the cool early morning and evening; shade up at midday and stay off the hot ground.
• Dress smart. Loose, light, full-coverage clothing and a hat protect you and let sweat cool you. Bare skin in the sun heats you faster.
• Replace salt. Eat and use electrolytes, especially on long, sweaty days.
• Check your urine. Pale means you’re drinking enough; dark means catch up now.
• Acclimatize. Ease into hot conditions over several days rather than going all-out on day one.

See our companion guide to hiking safely in the heat for pacing and hydration strategy on the trail.

Common mistakes

• Hiking through the early symptoms. Headache, fatigue, and cramps are the warning. Stop and cool down then, not later.
• Rationing water. Trying to make your water “last” by drinking less is how people become heat casualties. Drink it; ration your effort instead.
• Waiting to cool a heat-stroke victim. Cooling is the treatment and every minute counts. Start before and during evacuation, not after.
• Only drinking plain water on a long sweaty day. Replace salt too, or you risk cramps and, in extreme cases, hyponatremia.
• Treating every heat casualty with more water. If their urine is clear and copious and they are getting worse as they drink, more water is the wrong answer and may be a lethal one.
• Assuming recovery is permanent. Heat stroke can rebound within two days; the person still needs medical care.

What to carry

• Plenty of water and a way to refill: a reliable filter like the Sawyer Squeeze or tablets so you can top up from any source.
• Electrolyte mix or salty snacks: to replace what sweat carries out.
• A first aid kit: with a thermometer if possible, and the knowledge to use it.
• Sun protection: a wide-brim hat, loose light clothing, and sunscreen.
• A satellite messenger or beacon. To summon evacuation for a heat-stroke emergency.

Read our guide to hiking in the heat

Key takeaways

Heat illness is a ladder you can step off at any rung if you catch it early. Heat cramps and heat exhaustion respond to shade, rest, and fluids with electrolytes, so stop and treat them the moment they appear, and don’t push through. Heat stroke, marked by confusion or collapse, is a life-threatening emergency: cool the person as fast as you can by any means, target the neck, armpits, and groin, and evacuate immediately. Drink steadily and replace your salt, but do not force fluids: a hiker with clear, copious urine and no thirst who is getting worse as they drink has hyponatremia rather than heat exhaustion, and giving them more water can kill them. Rest through the hottest hours and watch your companions. Respect the heat and it rarely gets past the first rung.

Adapted in part from the U.S. Army Survival Field Manual (FM 3-05.70 / FM 21-76), Chapter 4, “Environmental Injuries” (heat stroke recognition and cooling) and Chapter 13, “Desert Survival” (heat cramps, heat exhaustion, water and salt, the thirst trap), with modern wilderness-medicine practice, and updated in 2026 against the Wilderness Medical Society Clinical Practice Guidelines for the Prevention and Treatment of Heat Illness (2024 Update) and published research on exercise-associated hyponatremia in hikers. This is general education, not a substitute for medical training or professional care.

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