Exertional hyperthermia is a pathological rise in core body temperature occurring during or immediately after intense physical exertion. Its most severe form — exertional heat stroke — is a life-threatening emergency that can become fatal within minutes without rapid and appropriate treatment.

Across Europe, the 2023 heatwaves caused over 47,000 deaths, including 9,500 in France according to Santé Publique France. The summer of 2025 was even more deadly, with over 16,500 estimated deaths across Europe — confirming a trend that is accelerating with climate change.

Definition and physiological mechanisms

Hyperthermia occurs when thermoregulation is overwhelmed by a combination of excessive metabolic heat production, environmental heat load, and insufficient or impaired cooling mechanisms.

The human body normally maintains its core temperature around 37°C through several mechanisms: sweating, cutaneous vasodilation, and cardiac output modulation. During intense exertion in a hot environment, these mechanisms can be saturated — particularly in high humidity conditions, which reduce the effectiveness of evaporative cooling.

Two main forms are distinguished:

Classic heat stroke — occurs in sedentary individuals exposed to prolonged ambient heat (heatwave). Primarily affects the elderly, infants, and those with cardiovascular risk factors.

Exertional heat stroke (EHS) — occurs in physically healthy individuals during intense effort: firefighters on operations, military personnel in training, marathon runners, construction workers. This is the most common form in active populations and the most unpredictable.

Critical thresholds

Heat stroke is defined as a body temperature above 40°C combined with neurological signs. Typical complications include seizures, rhabdomyolysis and kidney failure.

Progression is rapid and non-linear:

37–38°C — Normal range. Slight increase in sweating and cardiac output.

38–40°C — Mild to moderate hyperthermia. Fatigue, cramps, dizziness, nausea. Reversible with rest and hydration.

40–41°C — Severe hyperthermia. Confusion, disorientation, possible paradoxical cessation of sweating. Medical emergency — immediate cooling required.

Above 41°C — Imminent multi-organ failure. Brain, liver and kidney damage. Fatal risk if untreated within 30 minutes.

Above 42°C — Possible irreversible brain damage. Very high mortality risk.

At-risk populations

Firefighters face particular exposure during wildland fire operations: thermal protection gear, intense physical exertion, extreme ambient heat and dehydration combine to create extreme risk. Exertional heat stroke is one of the leading non-traumatic causes of death among firefighters in summer operations.

Military personnel are a well-documented population. The mortality risk is below 5% for exertional heat stroke versus up to 65% for classic heat stroke — provided treatment is rapid and appropriate.

Endurance athletes — marathon runners, trail racers, triathletes, Hyrox participants — face exposure during summer competitions. In June 2024, at least 1,301 pilgrims died during the Hajj in Mecca, where temperatures exceeded 51°C, illustrating the lethality of mass hyperthermia without adequate equipment.

Workers in thermally constrained environments — construction, foundries, oil and gas — are subject to increasing regulatory requirements around heat stroke prevention and prehospital response capability.

Recognising the warning signs

Early recognition is decisive. The warning signs of exertional heat stroke are:

Critical point: any athlete who collapses at the finish of a race or during intense exertion in hot conditions must be considered a potential EHS case until proven otherwise, even if consciousness is preserved.

The gold standard treatment: cold water immersion

Treatment of exertional heat stroke is now clearly established by the SFAR, the ACSM and the International Olympic Committee: whole-body cold water immersion, initiated immediately on scene.

The rule is simple: Cool First, Transport Second. Cool before you transport.

This rule rests on a fundamental physiological fact: hyperthermia is a condition whose prognosis depends directly on the duration of exposure to temperatures above 40°C. Transporting to hospital without prior cooling prolongs this exposure by several tens of minutes — often fatally.

Cold water immersion (1.5°C to 15°C) cools at 0.35°C per minute, versus 0.03°C/min for wet towels or ice packs. Dr Douglas Casa (Korey Stringer Institute, University of Connecticut) has documented a 100% survival rate across more than 401 cases of severe EHS treated by immersion within the first 30 minutes.

The logistical challenge: having the equipment on site

Knowing the protocol is not enough. In the field, medical teams frequently face the absence of appropriate equipment: no tub, insufficient cold water, no ice.

This is the logistical problem Kollder was developed to solve. The Kollder cooling tub deploys in under 30 seconds, by one person, with no prior infrastructure. Its food-grade stainless steel frame and high-resistance liner enable whole-body immersion fully compliant with international protocols, with full patient access for all advanced medical procedures.

Folded, it fits in any rescue vehicle, ambulance, pick-up truck or carry bag. Deployed, it enables the Cool First Transport Second protocol to be applied in the most demanding conditions — wildland fire, sporting event, industrial site.

Prevention: what teams can put in place

Prevention of exertional hyperthermia rests on four pillars:

Hydration — Before, during and after exertion. Dehydration significantly increases hyperthermia risk by reducing the blood volume available for thermoregulation.

Acclimatisation — Progressive exposure to heat over 10 to 14 days enables the body to develop thermoregulatory adaptations. Military and elite sports teams systematically integrate this phase before competitions in hot environments.

Monitoring — Identify individual risk factors (diuretic medication, history of hyperthermia, obesity) and monitor the most exposed team members.

Equipment — Have a deployable cooling tub available at every at-risk intervention or event. This is the explicit recommendation of the 2021 IOC consensus (Hosokawa, Racinais et al., BJSM).

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Sources: Santé Publique France — Epidemiological Bulletin Summer 2023, ACSM Expert Consensus Statement on Exertional Heat Illness 2023, IOC Adverse Weather Impact Expert Working Group Tokyo 2020 (Hosokawa, Racinais et al., BJSM 2021), Korey Stringer Institute — Douglas Casa (UConn), 2024 Hajj extreme heat disaster data, Wikipedia Heat stroke / Hyperthermia / Heat illness.

Kollder is the emergency cooling tub that deploys in under 2 minutes, anywhere.

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