What actually cools an overheated body? The evidence favours cold water, fast

When someone overheats, the useful question is which cooling method actually works — and the trial evidence gives a sharper answer than most first-aid folklore. A 2020 systematic review and meta-analysis pooled 63 studies of cooling techniques and concluded that immersing an overheated adult in water lowered core body temperature more effectively than leaving them to cool passively [s1]. That is the most consistent finding in the field, and it matters most for the most dangerous form of overheating.

Heat exhaustion is not heat stroke

The two conditions sit on a spectrum, but the line between them changes everything. Heat stroke is defined by hyperthermia — a core temperature above 40 °C (104 °F) — together with dysregulation of the nervous system, such as confusion, agitation, seizures, or collapse [s1]. Heat exhaustion is the milder state that precedes it: heavy sweating, weakness, nausea, headache and dizziness, but without the profound change in mental state that marks the emergency.

That distinction governs the response. Heat exhaustion generally settles with the obvious measures — stopping activity, moving into shade or a cool room, loosening clothing, and rehydrating. Heat stroke is a medical emergency: it is a recognised special circumstance for cardiac arrest and, left untreated, a cause of death [s1]. Anyone showing confusion, loss of consciousness or seizures in the heat needs emergency services immediately, and cooling should begin while help is on the way.

What the cooling evidence shows

The 2020 review searched four databases and included 63 studies — 37 controlled studies, two cohort studies and 24 case series [s1]. Across studies of adults with exertional hyperthermia, water immersion at a range of temperatures — cold water (14–17 °C / 57.2–62.6 °F), colder water (8–12 °C / 48.2–53.6 °F) and ice water (1–5 °C / 33.8–41 °F) — produced faster cooling than passive approaches such as simply resting in the shade [s1]. Notably, no single temperature range within that band was found to cool faster than another; the reviewers concluded that water immersion using 1–17 °C water more effectively lowered core temperature than passive cooling in hyperthermic adults [s1].

An earlier systematic review reached a compatible conclusion. Screening 89 research articles, it found only seven that met its inclusion criteria and a minimum quality score, and judged that ice-water immersion provided the most efficient cooling [s2]. Its practical corollary is the part worth remembering when a bathtub of ice is not to hand: when immersion is not possible, continual dousing with water combined with fanning is a reasonable alternative until more advanced cooling can be used [s2]. The international first-aid consensus that followed likewise reviewed techniques for cooling exertional hyperthermia and heat stroke as a priority topic [s3].

Why "cool first" is the theme

The through-line of this evidence is speed. In heat stroke, the duration of extreme core temperature is what damages tissue, so the intervention that lowers temperature fastest is the one that matters — and that is immersion, not waiting [s1]. This is why the phrase used in sports medicine is "cool first, transport second" for exertional cases where immersion is available: time spent cooling on site is not time lost.

Several caveats belong in plain view. The trials sit largely in exertional hyperthermia — young, otherwise healthy people who overheat during exercise or work — and the evidence base is dominated by controlled studies and case series rather than large randomised trials [s1]. Whether the same immersion protocols suit an elderly person overcome during a heatwave, who may have heart or circulatory conditions, is far less well studied, and aggressive immersion is not automatically appropriate for everyone. The reviews assess cooling rate; they are not a licence for bystanders to manage a collapsing person alone.

What it leaves a reader

The defensible reading is narrow. For heat exhaustion, the low-tech measures — stopping, cooling down, rehydrating — are what the situation calls for, and symptoms that do not improve, or that tip into confusion, are the signal to seek help. For suspected heat stroke, the evidence is unusually aligned: it is an emergency, and rapid whole-body cooling, ideally by immersion in cold water, lowers core temperature faster than anything passive while emergency care is summoned [s1] [s2]. What the studies cannot do is tell any individual bystander how to treat a specific collapsed person — that is a clinical decision made in the moment, and calling for help is the first step, not the last.