During intense physical activity on hot days the body can lose around 1 litre of sweat per hour or more, a thermoregulatory response that requires regular fluid and electrolyte replacement to maintain performance and health.
Why sweat occurs
Sweating is the body’s primary means of cooling through evaporative heat loss; sweat glands produce fluid that evaporates from the skin surface and removes metabolic heat generated by muscles and absorbed from the environment.
How much can be lost
Under heavy exercise combined with high ambient temperature and humidity, typical whole‑body sweat rates commonly reach several hundred millilitres per hour and can approach or exceed 1 litre per hour in many people, with elite athletes or those working in extreme conditions sometimes losing even more.
Risks of excessive fluid loss
Rapid fluid loss without replacement can cause dehydration, reduced blood volume, impaired thermoregulation, decreased exercise performance, heat exhaustion or heat stroke, and electrolyte imbalances that affect muscle and cardiovascular function.
Practical guidance
Match fluid intake to expected sweat loss by drinking before and during activity, replace electrolytes during prolonged sweating, monitor body mass changes to estimate losses, seek shade and cooling breaks in extreme heat, and acclimate gradually to hot conditions to reduce excessive sweat rates and heat strain.
Takeaway
Sweat rates of about 1 litre per hour are common during intense exertion in hot environments; sensible hydration, electrolyte replacement and heat‑management strategies are essential to maintain safety and performance.