Exercising in Heat — Illness Prevention and Acclimatization

When you exercise in the heat, the body diverts blood flow to the skin and sweats to shed heat, which adds cardiovascular strain on top of the exercise itself. Performance drops, perceived effort rises, and — if heat gain outpaces heat loss — body temperature can climb into dangerous territory. Humidity makes it worse because sweat evaporates poorly. The two pillars of managing it are heat acclimatization and fluid/electrolyte management, plus knowing when to stop.

Heat acclimatization is the body's adaptation to repeated heat exposure: it occurs over roughly 1–2 weeks of progressively longer, moderate sessions in the heat. The ACSM consensus statement on exertional heat illness (2023) recommends 1–2 weeks of heat acclimatization to induce most of the physiological adaptations that improve performance and lower heat-illness risk. Adaptations include earlier and greater sweating, more dilute sweat (conserving electrolytes), expanded plasma volume, and a lower heart rate and core temperature at a given workload. Until acclimatized, reduce intensity and duration in the heat.

Exertional heat illness ranges from heat exhaustion to heat injury to heat stroke. Common signs include dizziness, headache, nausea, an unsteady walk, weakness, muscle cramps, fatigue, chills, a weak or rapid pulse, and sweaty skin that may feel warm or cool. Exertional heat stroke is a medical emergency defined by central-nervous-system disturbance (confusion, personality change, aggression, collapse, seizure) together with a high core temperature, usually above 40 °C (104 °F) rectal. The skin is often still sweaty, so sweating does not rule it out. It needs immediate on-site whole-body cooling (for example, cold- or ice-water immersion) started as early as possible, plus emergency care. (Exercise-associated muscle cramps on their own are a separate condition.) See Hydration Strategy During Exercise for fluid intake and Exercise-Associated Hyponatremia for the opposite risk of overdrinking plain water.

• Heat + humidity increases cardiovascular strain, core temperature, and fluid loss; performance falls.
• Acclimatize gradually over 1–2 weeks of progressive exposure (ACSM 2023); expect lower HR and core temp once adapted.
• Hydrate with fluids and electrolytes; avoid both dehydration and overdrinking (hyponatremia).
• Know the signs: heat exhaustion (dizzy, nauseated, weak, sweaty) vs heat stroke (altered mental status plus core temperature usually >40 °C; may still be sweating — emergency).
• Higher risk: infants and older adults, the unacclimatized, low fitness or high BMI, recent illness or fever, hypohydration, and medications such as diuretics, anticholinergics, beta-blockers, antihistamines, antidepressants and stimulants.