What Is Cold Water Immersion?

Cold water immersion (CWI) is the practice of submerging a severely hyperthermic person's entire body in cold water until core temperature returns to a safe level. It is the reference treatment for exertional heat stroke, recognized as such by every major international sports medicine and emergency medicine body.

A simple definition, a demanding protocol

In principle, cold water immersion is straightforward: submerge the whole body, head excluded, in cold water. In practice, its clinical effectiveness depends on three precise variables:

Full-body coverage: immersion must cover the torso and limbs, not the partial contact offered by ice packs
Water temperature: as cold as is practically available in the field, typically 1 to 15°C (34 to 59°F)
Duration: maintained until core temperature drops below 38.6-39°C (101.5-102.2°F), measured rectally

It is not a cold compress, a cold shower, or ice placed on the skin. Full-body submersion is what produces the cooling effect that matters clinically.

Why water, and why immersion, not localized ice

The physics explains the performance gap. Water conducts heat roughly 25 times faster than air, and immersion maintains continuous contact across the entire body surface, avoiding the dead zones left by ice packs positioned at the armpits or groin.

Casa et al. (Exercise and Sport Sciences Reviews, 2007) established that cold water immersion cools the body at 0.35°C per minute, compared with 0.03°C per minute for wet towels or localized ice packs. That tenfold difference directly changes outcomes within the critical 30-minute treatment window.

Exertional heat stroke: the one emergency where this matters most

Exertional heat stroke (EHS) occurs when core body temperature exceeds 40°C (104°F) alongside neurological symptoms: confusion, disorientation, loss of consciousness, seizures. Without immediate cooling, organ and neurological damage becomes irreversible within minutes.

The American College of Sports Medicine (ACSM), 2023 Expert Consensus Statement, and the IOC / BJSM (Hosokawa Y, Racinais S et al., 2021) are unambiguous: immediate cold water immersion is the only validated treatment, and it must begin before transport. This is the "Cool First, Transport Second" principle: cooling takes priority over evacuation.

The Korey Stringer Institute (Dr Douglas Casa, UConn) documents a 100% survival rate across more than 401 treated cases when core temperature drops below 40°C within 30 minutes of onset. Every minute of delay measurably worsens the outcome.

Who uses cold water immersion in the field

Cold water immersion is not a niche practice. It's built into the operational protocols of several organization types that face exertional heat risk directly:

Fire and rescue services, during wildfire operations and crew rehab
Military medical services, on operational deployments and in hot-weather training
EMS and emergency departments, in prehospital and in-hospital care
Civil protection organizations, at civil security operations and mass gatherings
Event medical teams, at trail races, marathons, and Hyrox events
Occupational safety (EHS) teams, in industrial environments with heat exposure

Each context has its own operational constraints, but the clinical action stays the same: full-body, as-cold-as-possible, as-fast-as-possible immersion.

What actually blocks cold water immersion in the field

The protocol isn't the bottleneck: it's well documented and widely taught. What's usually missing is the equipment. Cold water immersion requires a container that can hold a person, water, and ice, and that can be deployed quickly on any terrain. Without it, teams improvise with methods that deliver neither full-body coverage nor the cooling rate the protocol requires.

That's the problem the Kollder emergency cooling tub was built to solve: making cold water immersion possible anywhere, deployed in under 2 minutes by a single operator. Once the equipment is on scene, the protocol can be applied without delay.