What the Research Says (and What Most People Get Wrong)

Heat illness sits in that uncomfortable category of wilderness medicine where the textbook answer is simple, the field reality is harder, and a lot of what people think they know is wrong.

Here's the short version: exertional heat stroke is one of the few wilderness emergencies where outcome is almost entirely determined by what happens in the first 30 minutes. Cool fast, and survival approaches 100%. Wait to cool, or transport first, and people die.

What Heat Stroke Actually Looks Like

Per the 2024 Wilderness Medical Society guidelines, exertional heat stroke is defined by two things: a core temperature above 104°F, and central nervous system dysfunction — confusion, loss of coordination, slurred speech, combativeness, seizure, or loss of consciousness.

If someone collapses running uphill in 95°F heat and is acting confused, that's heat stroke until you prove otherwise. Don't wait for a number. Get them cooled down.

Cool First. Transport Second.

This is the most important shift in heat illness practice in the last two decades, and it's now the standard of care.

The data is hard to argue with: over 18+ years at the Falmouth Road Race, 454 cases of exertional heat stroke were treated with on-site cold water immersion before transport. Survival rate: 100%. A 2020 systematic review of 521 cases found that when cooling was adequate, mortality was 0%. When it wasn't, mortality was 4.4%.

A patient cooled in the field has a survival advantage that no helicopter speed can match. The goal: get core temp down to around 101.5°F before you stop cooling, to avoid overshooting into hypothermia.

What to Use to Check Temperature (and What Not To)

Best practice is rectal temperature. It's the only field measurement that reliably reflects core temp. Oral, ear, forehead, and armpit readings all underestimate in a heat-sick patient — sometimes by several degrees. Someone running a rectal temp of 105°F might show 101°F on a forehead scanner. You'd miss the diagnosis.

That said: most of us aren't carrying a rectal thermometer, and that's the reality. If you're leading a trip where heat stroke is a plausible risk — desert running, summer mountaineering, endurance events — it's worth adding one to your kit. It's small, cheap, and the only thing that gives you a real number.

If you don't have one, don't let that stop you from treating. A hot, confused patient who just collapsed in the heat gets cooled immediately. You don't need a number to act. You need a number to stop.

The Tap Water Myth

Most people assume you need ice water to treat heat stroke in the field. The evidence says otherwise.

A well-cited lab study tested cooling rates at four different water temperatures. Ice slurry (36°F) was fastest. But water at 57°F, 46°F, and even 68°F all produced cooling rates that met or exceeded the threshold associated with 0% mortality in the research. The differences between those three were not statistically significant.

What that means in the field: a cold mountain stream, a cooler full of melted ice and drinks, a campground hose, a pile of wet bandanas — all of these are legitimate treatments. "Ice or nothing" is not supported by the evidence.

A useful technique if you don't have a tub: the tarp-assisted cooling method (sometimes called TACO). Four people hold the corners of a tarp around the patient, add roughly 100 liters of cold water, and you've created a field immersion bath. Studies show it achieves cooling rates comparable to a proper cold-water immersion tub.

The One That Gets People Killed: Exercise-Associated Hyponatremia

Here's the part that doesn't get enough attention.

A patient who's been working hard in the heat and presents with weakness, nausea, confusion, headache, and dizziness looks exactly like heat exhaustion. The reflex is to push fluids.

That reflex can make things significantly worse.

Exercise-associated hyponatremia (EAH) is low blood sodium caused by overhydration with plain water or most sports drinks — both hypotonic, meaning they contain less sodium than your blood. The body holds onto excess fluid, sodium gets diluted, and the brain swells. The symptoms are nearly identical to heat illness. In some canyon and trail series, roughly 1 in 5 "heat-related" incidents turned out to be EAH. There are documented fatalities from pushing fluids into patients who were already hyponatremic.

How to tell them apart in the field:

If the patient is clearly hyperthermic (hot skin, history of under-drinking, core temp above 104°F if you have a thermometer): treat for heat stroke. Cool aggressively. Hold fluids until they're cooled and reassessed.

If the patient is not running hot, but is confused, nauseated, or showing signs of fluid retention (bloating, puffy hands, weight gain during the activity): assume EAH. Withhold water and hypotonic fluids. Treatment is oral hypertonic saline — a few bouillon cubes dissolved in a small amount of water, repeated as needed.

The critical rule: do not reflexively push water or sports drinks into every confused exerciser in the heat. Temperature is the field discriminator.

What to Carry

You don't need a lot. You need the right things.

A lightweight tarp. A water source. Salt (bouillon cubes travel well). And if heat stroke is a plausible scenario on your route, a rectal thermometer — not a forehead scanner.

You probably already carry a tarp. Rivers and streams are on most trail maps. The bouillon cubes weigh nothing. The thermometer is the one add for serious heat environments, and even then, knowing the protocol matters more than the gear.

The Bottom Line

Heat stroke survival is almost entirely a function of how fast you cool, not how fast you evacuate.
Any water substantially below body temperature gets you into the therapeutic range. A tarp and a cold creek beats a passive carry-out every time.
Stop cooling at around 101.5°F to avoid overshooting.
And if your patient isn't running hot but looks heat-sick, hold the fluids and think about sodium before you hand them a water bottle.
The math of heat illness is unforgiving. But it's also very winnable when you know what to do.

Backcountry Medical Guides teaches WFR, WFA, and custom field medicine courses for guides, educators, and backcountry users. We teach the protocol the evidence supports — not the one the textbook hasn't updated yet.

Selected References

Eifling KP et al. WMS Clinical Practice Guidelines for the Prevention and Treatment of Heat Illness: 2024 Update. Wilderness Environ Med. 2024;35(1_suppl):112S-127S. PubMed
Lipman GS et al. WMS Clinical Practice Guidelines for the Prevention and Treatment of Heat Illness: 2019 Update. Wilderness Environ Med. 2019;30(4S):S33-S46. PubMed
Proulx CI, Ducharme MB, Kenny GP. Effect of water temperature on cooling efficiency during hyperthermia in humans. J Appl Physiol. 2003;94(4):1317-1323. Full text
Clements JM et al. Ice-water immersion and cold-water immersion provide similar cooling rates. J Athl Train. 2002;37(2):146-150. PMC
DeMartini JK et al. Effectiveness of cold water immersion in EHS at the Falmouth Road Race. Med Sci Sports Exerc. 2015;47(2):240-245. PubMed
Belval LN et al. Exertional heat stroke survival at the Falmouth Road Race: 180 new cases. J Athl Train. 2024;59(3):304-310. PubMed
Hosokawa Y et al. Tarp-assisted cooling for hyperthermic individuals. Ann Emerg Med. 2017;69(3):347-352. PubMed
Filep EM et al. EHS, modality cooling rate, and survival outcomes: systematic review. Medicina. 2020;56(11):589. PMC
Douma MJ et al. First aid cooling techniques for heat stroke (ILCOR systematic review). Resuscitation. 2020;148:173-190. ScienceDirect
Hew-Butler T et al. Statement of the Third International Exercise-Associated Hyponatremia Consensus Development Conference. Clin J Sport Med. 2015;25(4):303-320. Full text
Bennett BL et al. WMS Clinical Practice Guidelines for the Management of EAH: 2019 Update. Wilderness Environ Med. 2020;31(1):50-62. PubMed

Heat Illness in the Backcountry