Heat Illness & Hot Weather Travel

Heat illness gets less attention than its cold counterpart in most outdoor education, and the result is that it kills people who would have been fine with a clearer understanding of how it works. The misconception that does the most damage is the idea that heat exhaustion and heatstroke are points on a single line — that heatstroke is just heat exhaustion that got worse. They are related, but they are different physiologies, with different field responses, and confusing them gets people killed. Add the parallel problem of hyponatremia (drinking too much water without enough sodium), which can mimic heat exhaustion but gets worse if you treat it the same way, and the situation is more complicated than the standard "drink water and rest in the shade" advice suggests.

This article covers how heat illness actually develops, the critical distinction between heat exhaustion and heatstroke, hyponatremia as the often-overlooked twin problem, prevention through acclimatization and pacing, and field treatment by stage. The recognition framework and field treatment recommendations are drawn from current Wilderness Medical Society practice guidelines for the prevention and treatment of heat illness. As with the other medical articles in this section, the depth is field-level — deeper protocols belong to formal wilderness first aid training.

The Heat Balance Problem

The body has a narrow operating temperature range and a few mechanisms for shedding excess heat. The main one is sweating, which works through evaporative cooling — sweat leaves the skin as vapor and takes heat with it. Sweating is highly effective in dry conditions and progressively less effective as humidity rises, because evaporation slows when the surrounding air is already saturated. This is why a 95-degree day in the desert can be safer to hike than an 85-degree day in the humid southeast.

Heat illness happens when the body's cooling capacity is overwhelmed by some combination of: environmental heat load (sun, hot air, hot ground radiating upward), exertion (which generates internal heat as a byproduct of metabolism), inadequate hydration (you cannot sweat what you do not have), and lost acclimatization (an unacclimatized body sweats less efficiently and tolerates less heat stress). Individual variation is large. Fitness helps some but is not protective on its own. People who are objectively in great shape die of heatstroke every summer.

Recognition: The Spectrum and the Critical Distinction

Heat illness moves through three recognizable stages. The first two are treatable in the field and usually resolve without lasting harm. The third is a medical emergency that needs aggressive intervention within minutes, not hours.

Heat cramps

Painful muscle cramps, usually in the legs, abdomen, or arms, during or after exertion in heat. The person is mentally normal, sweating, and not in serious distress. Heat cramps are usually about electrolyte loss, particularly sodium, combined with dehydration. They respond well to rest, gentle stretching, fluid with electrolytes, and salty food.

Heat exhaustion

The body is working hard to cool itself and starting to lose the battle. Signs: heavy sweating (often soaked through clothes), pale or flushed skin, headache, dizziness or lightheadedness, nausea, weakness, sometimes vomiting, fast pulse, cool clammy skin in some cases. The person is uncomfortable but alert and oriented. Body temperature is usually normal or mildly elevated. Mental status is clear — they can hold a conversation, follow instructions, describe how they feel. Caught here, recovery is reliable with shade, rest, cooling, and measured fluids.

Heatstroke

The body has lost the ability to regulate its own temperature. Core temperature climbs uncontrollably. This is a medical emergency. The cardinal sign is altered mental status — confusion, irrational behavior, combativeness, slurred speech, loss of coordination, eventually unconsciousness. Body temperature is dangerously high (above 104 degrees Fahrenheit in textbook cases, though field thermometers are rare and unreliable). The person may or may not still be sweating — the old teaching that heatstroke patients always have hot dry skin is wrong, and counting on it has killed people. Exertional heatstroke (the kind most backcountry travelers and athletes get) frequently presents with the person still sweating heavily.

The reason the distinction matters so much: heat exhaustion responds to gradual cooling and rehydration. Heatstroke responds only to aggressive, immediate cooling, and every minute of delay increases the risk of permanent organ damage or death. Treating heatstroke as heat exhaustion — giving them water and telling them to rest in the shade — is a fatal mistake. The treatment for heatstroke is to drop core temperature fast, by any means available, before waiting for evacuation.

Hyponatremia: When the Problem Is Too Much Water

Hyponatremia is low blood sodium, and it can mimic heat exhaustion almost exactly — nausea, weakness, confusion, headache, sometimes vomiting. The cause is opposite: too much water relative to sodium intake, usually from drinking large volumes of plain water during a long effort while not eating enough to replace electrolytes. It kills people in ultramarathons and hot-weather backpacking trips with some regularity, and it is often misdiagnosed as dehydration in the field. Treating it like dehydration — pushing more water — makes it worse, and severe cases can progress to seizures and death.

The differential between hyponatremia and heat exhaustion is mostly about history. The hyponatremic patient has usually been drinking a lot, often multiple liters in a short time, and has been eating very little. The heat exhaustion patient is typically under-hydrated or about right on water but overheated. The practical field response that is safer for both: when someone in hot weather is feeling sick and confused, give salty food (jerky, salty snacks, a salt tablet if you carry them) along with small measured sips of fluid, rather than chugging plain water. This stabilizes a hyponatremic patient and helps a heat-exhausted patient. Push large volumes of pure water and you risk killing the hyponatremic patient while helping the dehydrated one.

Prevention

Almost all heat illness is preventable, and the prevention work splits into three areas: acclimatize before you push, pace for the conditions, and manage fluids and electrolytes together.

Acclimatization is real biology, not toughness. The body adapts to heat over 10 to 14 days of regular exposure, becoming more efficient at sweating, conserving electrolytes, and tolerating higher core temperatures. Flying from sea level to Phoenix in July and going for a long hike the next morning is the recipe for an unacclimatized heat exhaustion case. Build heat tolerance by spending progressive amounts of time in heat for a week or more before any serious hot-weather effort. The same applies in reverse — acclimatization fades within a few weeks of moving back to cooler conditions. The CDC and NIOSH heat stress guidance covers acclimatization schedules in more detail for occupational and recreational contexts.

Pace for the heat. Hot-weather hiking is slower than cool-weather hiking, period. Push the same pace and you will overheat. Drop the pace, take more breaks, find shade aggressively. Start earlier in the day, rest through the heat of mid-afternoon (not noon — the hottest part of a desert day is usually 2pm to 5pm), and resume in the late afternoon or evening if the trip allows it. Night travel is a legitimate desert strategy that experienced desert travelers have used for centuries.

Manage fluids and electrolytes together. Water alone is not enough in serious heat. The body loses sodium and other electrolytes through sweat, and replacing only water dilutes what is left. Drink to thirst plus some, not to a fixed schedule of liters per hour — the old advice to force fluids is what produces hyponatremia. A rough field rule is half a liter to a liter per hour during sustained heat exertion, adjusted upward for hotter or drier conditions and downward for shorter or cooler ones. Eat salty food throughout the day. Electrolyte drinks, drink mixes, or salt tablets all work; plain salty snacks work too. Pee output and color is a useful self-check — clear and frequent is over-hydrated, dark and infrequent is under-hydrated, the middle is right.

Dress for the heat. Light-colored, loose-fitting, long-sleeved clothing outperforms minimal clothing in serious sun and heat. The desert nomads who have lived in this weather for thousands of years did not wear shorts and tank tops, and there is a reason. Loose long sleeves trap a layer of cooler air against the skin, block direct sun from reaching it, and protect against the burns and dehydration that accelerate heat illness. A wide-brimmed hat or sun hoodie is worth more than sunglasses. Cool bandanas, wet shirts, and wet hats add significant evaporative cooling when water is available.

Use shade as a tool. In desert and exposed terrain, shade is a resource you manage like water. Take breaks in shade even when you do not feel hot, especially in the middle of the day. A trekking umbrella is the kind of piece of gear that sounds silly until you have used one in 100-degree sun, and then it becomes obvious why every desert hiking guide who is honest about it carries one.

Field Treatment by Stage

Heat cramps

Rest, gentle stretching, salty food, electrolyte fluids in measured amounts. Cramps usually resolve within 20 to 40 minutes. If they keep recurring, the person needs more rest and more sodium than they have been getting; consider cutting the day short.

Heat exhaustion

Get them out of the sun, into shade. Loosen restrictive clothing. Cool with whatever is available — wet a bandana or shirt and put it on the neck, head, armpits, and groin (where major blood vessels run near the surface), fan them, get their feet in a stream if one is nearby. Give measured fluids with electrolytes. Eat salty food. The person should improve noticeably within 30 to 60 minutes. If they are not improving in that window, or they get worse, treat as heatstroke and start evacuating. Even after a clean recovery, the rest of the day's plan needs reassessment — the person is more vulnerable to a repeat episode for at least the next several hours.

Heatstroke

This is a life-threatening emergency. Activate rescue and begin cooling immediately, in that order. Every minute of elevated core temperature increases the risk of permanent damage. Cooling priority, in rough order of effectiveness:

• Cold water immersion. If a cold stream, lake, or any body of cold water is nearby, get the person into it — up to their neck if possible, supporting their head. This is by far the most effective field cooling method available. Even partial immersion (legs in a stream) is significantly better than nothing.
• Wet skin plus fanning. Soak their clothes and skin with water and fan vigorously. Evaporation is doing the work; the fanning accelerates it.
• Cold packs to the neck, armpits, and groin. Cools blood flowing through major vessels. Less effective than immersion but better than nothing.
• Shade and rest alone. Insufficient for heatstroke. Useful only as a complement to active cooling, not as a primary intervention.

Cool aggressively until mental status normalizes — that is the target, not a specific temperature, since you cannot reliably measure their temperature in the field anyway. The person becoming alert, oriented, and able to hold a conversation is the signal that you have brought core temperature back into survivable range. Stop active cooling at that point to avoid overshooting into hypothermia, but keep them out of further heat exposure.

Heatstroke patients should always be evacuated for medical evaluation, even after they appear to recover. Risk of delayed organ failure (kidney, liver, cardiac) is real and can present hours later. The person is also significantly more vulnerable to a second heat illness episode for days afterward.

Desert-Specific Considerations

Desert travel raises the stakes on everything in this article. Water sources are scarce and sometimes unreliable, distances between shade are large, and the margin between competent and in-trouble is narrower than people from temperate climates expect.

Plan water as a logistics problem, not a preference. Know where every reliable water source on your route is. Carry more than you think you need — running out of water in the desert escalates from uncomfortable to fatal faster than the equivalent in temperate climates. Cache water in advance for long routes when possible. Check water source reliability with current information; sources that show on the map can be seasonal, dry, or contaminated.

Hottest is mid-afternoon, not noon. Plan your trip schedule around this. Start in pre-dawn dark if you have to. Set up camp early, before the worst of the afternoon heat, in shade if available. Hike again in the cool of the late afternoon and evening if the route allows.

Watch for stored heat at night. Sand and rock release the day's stored heat for hours after sunset. A late-evening hike across a sun-baked sandstone bench can be hotter than a midday hike across vegetated terrain.

Slot canyons add a separate hazard. Flash flooding from storms anywhere in the upstream drainage can fill a slot canyon with no warning. Even on a clear hot day, a storm fifty miles away can produce a flood. The relevance to heat illness is that slot canyons offer the most appealing shade in the desert, and the temptation to wait out the heat in one is real. Know the weather pattern of the whole drainage, not just where you are.

Solo travel in desert heat is genuinely riskier. The early signs of heat illness are exactly the signs you cannot reliably notice in yourself — degraded judgment, mild confusion, declining decision quality. A partner sees them before you do.

When to Evacuate

Heat cramps and uncomplicated heat exhaustion that resolves cleanly in the field do not always require evacuation, though the day's plan needs significant revision — the person should be done hiking in hot conditions for the rest of the day, at minimum. Heat exhaustion that does not improve within an hour, or that recurs after apparent recovery, should be evacuated.

Heatstroke is always evacuated, regardless of how well the field cooling seemed to work. Hyponatremia of any meaningful severity is also evacuated; delayed seizure risk is real and the diagnosis is hard to confirm in the field. Both conditions require medical evaluation that is not available at the trailhead.

Related Resources

For trip planning that accounts for water sources, weather, and pacing in hot conditions, see Wilderness Preparedness. For formal training in field treatment of heat illness and the patient assessment that supports it, see First Aid & Wilderness Medicine. For water source assessment and treatment, see Water Safety.