Heat Acclimatisation

Exercise generates a large amount of metabolic heat, which the body sheds mainly by evaporating sweat, with smaller contributions from convection and radiation. In hot, humid conditions this balance is disturbed: high humidity blunts sweat evaporation, so heat is retained and core temperature climbs. The body also diverts blood to the skin to lose heat, which competes with the demand of working muscle, so heart rate rises and endurance capacity falls, while fluid lost as sweat reduces blood volume and adds cardiovascular strain. The same pace therefore feels harder and cannot be held as long, with a measurable drop in performance well before any illness. This applies to any athlete in a hot climate, not only the desert endurance athlete.

Heat illness ranges from mild exertional heat illness, often called heat exhaustion, to severe exertional heat illness or exertional heat stroke, and does not always progress in a simple linear way. Mild exertional heat illness (EHI) brings fatigue, dizziness, nausea, headache and heavy sweating with a clear mental state, and settles with rest, cooling and, only if the athlete is alert, able to swallow and low blood sodium is not suspected, oral fluids. Severe exertional heat illness, including exertional heat stroke, is a high core temperature, generally above 40 degrees Celsius, with central nervous system dysfunction such as confusion, collapse or seizure. Exercise-associated hyponatraemia, a low blood sodium from drinking more than is lost, can mimic it and must be considered, since more fluid would be harmful.

Risk depends on the environment and the individual. The wet-bulb globe temperature (WBGT) combines air temperature, humidity, radiant heat and air movement into one measure, and as a rough guide activity is modified, with extra breaks and reduced intensity, at around 28 to 30 degrees Celsius, and postponement considered above about 30 to 32 degrees Celsius. These figures are a starting point, not a rule on their own: decisions must also weigh sport intensity, clothing and equipment, how well acclimatised the athletes are, and individual susceptibility, which is higher with poor fitness, a large body size, recent illness or fever, some medications and a previous heat illness. Identifying higher-risk athletes early lets their preparation and monitoring be tailored.

Heat acclimatisation is the process of repeatedly exercising in the heat so the body adapts, and it is the most powerful single intervention for safety and performance. Adaptation in a natural hot environment is termed acclimatisation, and the same process induced in a heat chamber is termed acclimation. The adaptations include expanded plasma volume, a lower heart rate and lower core and skin temperature at a given workload, and sweating that starts earlier, is greater and is more dilute as the body conserves sodium, together letting the athlete work harder and more comfortably with less strain.

Because a sustained rise in core temperature drives adaptation, an effective method is controlled hyperthermia, or isothermal training, holding core temperature near 38.5 degrees Celsius for 60 to 90 minutes a session, so the workload must increase as the athlete adapts. Sessions are repeated over one to two weeks, most benefit coming in the first week and fuller adaptation by around two weeks. The most reliable stimulus is training in the destination climate, which means arriving early, or a heat chamber before travel; adding extra clothing in training is a supervised fallback, not an equal substitute. The adaptations fade over days to weeks once regular heat exposure stops, so acclimatisation is best timed close to competition and topped up if there is a gap.

Athletes should begin exercise well hydrated and follow an individual drinking plan that limits large falls in body mass, ideally to no more than about 2 to 3 per cent, while avoiding the opposite error of drinking so much that body mass rises and blood sodium falls. Sweat and sodium losses vary widely, so plans are best based on how body mass changes across sessions, with thirst and urine colour used only as rough guides. Sodium is not needed by every athlete, though heavy or salty sweaters may benefit from replacing roughly 30 to 50 millimoles per litre, around 0.5 to 1.0 grams an hour.

Cooling adds a further margin. Pre-cooling, lowering body temperature before exercise, increases the capacity to store heat and can improve performance; methods include ice slurry, cooling garments and cold water immersion, used before an event or at breaks such as half-time. The value of cooling during exercise depends on the method, sport and setting, and aggressive cooling is unsuitable immediately before or during sprint and power events, where an excessive fall in muscle temperature can impair performance.

Bringing this together, the SEM clinician plans a heat acclimatisation block timed to the event, sets individual hydration guidance, and arranges cooling for training and competition. Just as important is an emergency plan for severe exertional heat illness: a shaded cooling area, ice and ideally a cold water immersion tub, a reliable way to measure core temperature, immediate activation of emergency services, and on-site cooling before transfer, with every severe case going on to hospital. Sessions and, where possible, competition are scheduled for cooler parts of the day, and higher-risk athletes are watched more closely. An athlete who has had exertional heat stroke returns only in a graded way, after clinical recovery, medical review and correction of contributing factors, since a premature return risks another episode.

• •The 2023 International Olympic Committee consensus on sport events in the heat covers heat acclimatisation, hydration, cooling and event-level risk management, and is the principal reference here.
• •The 2024 UK Faculty of Pre-Hospital Care consensus on exertional heat illness guides recognition and immediate on-site management, including cooling before transfer.
• •NHS guidance on heat exhaustion and heatstroke covers recognition and first aid, including when an episode becomes an emergency.
• •The 2015 international consensus on training and competing in the heat remains useful supporting evidence on acclimatisation and hydration.

• •Heat impairs endurance performance through a rising core temperature, cardiovascular strain and fluid loss, and threatens any athlete competing in a hot climate.
• •Heat acclimatisation is the key intervention: repeated heat exercise over one to two weeks, holding core temperature raised for 60 to 90 minutes, expands plasma volume and lowers heart rate and core temperature with earlier, more dilute sweating.
• •Most adaptation comes in the first week and fades within days to weeks once exposure stops, so acclimatise close to competition and top up.
• •Severe exertional heat illness or heat stroke is a high core temperature with central nervous system dysfunction: activate emergency services, cool on site before transfer, and stop cooling around 38.5 to 39 degrees Celsius.
• •Plan hydration individually to limit body-mass loss to roughly 2 to 3 per cent and avoid over-drinking; sodium mainly helps heavy or salty sweaters.
• •Wet-bulb globe temperature guides risk but not alone; weigh intensity, clothing, acclimatisation and susceptibility, and pre-cool with care before sprint and power events.