Exertional heat illness
Evidence: moderate
Running hard in the heat can push core temperature to dangerous levels. Exertional heat stroke is a medical emergency defined by a core temperature above about 40.5 °C plus confusion or collapse, and survival depends on cooling the body fast, before anything else. Cold water immersion is the treatment that saves lives.
Not medical advice
This is a general knowledge base, not medical or dietary advice. If you are injured, unwell or weighing up a supplement or a change to your diet, speak to a doctor, physiotherapist or registered dietitian who knows your situation.
Heat illness is the health counterpart to heat acclimation, the performance method. Acclimation makes a runner more heat-tolerant; this page is about what happens when heat tolerance is exceeded.
The spectrum, from exhaustion to heat stroke
Heat exhaustion is the inability to keep exercising in the heat. The runner feels weak, dizzy, nauseous and heavily fatigued, often with heavy sweating, but their brain still works normally and core temperature is high but not extreme (Casa et al. 2015). It resolves with rest, cooling and fluids, and is not in itself life-threatening.
Exertional heat stroke is the dangerous one. It is defined by a core temperature above roughly 40.5 °C together with central nervous system dysfunction: confusion, irrational behaviour, slurred speech, collapse or loss of consciousness (Belval et al. 2018). At this temperature proteins and organs start to fail, and the longer the body stays hot the worse the outcome. It can be fatal and is a true emergency.
The skin can mislead you
A common and dangerous myth is that someone with heat stroke has stopped sweating and feels hot and dry. In exertional heat stroke the skin is often still sweaty and may even feel cool to the touch (Belval et al. 2018). Judge by the brain, not the skin. Any runner who becomes confused, aggressive or uncoordinated in the heat should be treated as heat stroke until proven otherwise.
Diagnosis needs a core temperature
The two diagnostic criteria are a core temperature above about 40.5 °C and central nervous system dysfunction (Belval et al. 2018). Core temperature here means rectal temperature. Oral, ear, forehead and skin thermometers all read inaccurately in someone who has just been exercising hard in the heat, and relying on them can miss a genuine heat stroke (Casa et al. 2015). At a race, that is why medical tents at hot events use rectal thermometers.
Risk factors
The main drivers are environmental and personal (Casa et al. 2015):
- High air temperature and especially high humidity, which stops sweat evaporating. Wet bulb globe temperature captures both.
- Lack of heat acclimation; the first hot days of a season are the highest-risk period.
- High running intensity, which generates heat faster than the body can shed it.
- Dehydration, which reduces sweating and blood volume and so impairs cooling.
- Poor fitness, illness, recent infection, certain medications and a previous episode of heat illness.
Sensible hydration and electrolyte practice helps, but it does not make a runner immune. A fit, well-hydrated, acclimatised athlete can still develop heat stroke if they push hard enough in severe conditions.
Treatment: cool first, transport second
The single most important principle is to cool the body as fast as possible, and to do it before transporting the patient to hospital (Belval et al. 2018). Cold water immersion, sitting or lying the person in a tub of cold or iced water, is the gold-standard method and cools fastest. The goal is to bring core temperature down to a safe level within about 30 minutes of collapse, because survival tracks the time spent above the critical temperature, not the time to reach a hospital (Belval et al. 2018).
What to do at the scene
Get the person into shade, remove excess clothing and start aggressive cooling immediately: a tub of cold water if available, or otherwise dousing with water plus ice packs to the neck, armpits and groin and constant fanning. Call emergency services, but cool while you wait and during any delay, do not stop cooling to move them sooner. Cold water immersion used promptly for heat stroke has an excellent survival record (Belval et al. 2018). This is one of the few settings where an ice bath is unambiguously the right tool, in contrast to its mixed role in routine cold water immersion recovery.