Evaporation is the main route of heat loss, and it fails in high humidity
Expansion
Muscle efficiency is only about 20 to 25 per cent, so most energy becomes heat, and core temperature can rise to 39 to 40 degrees in prolonged exercise.
Heat loss routes
- Radiation and convection: effective while ambient temperature is below skin temperature. Above about 35 degrees the gradient reverses and the body gains heat
- Evaporation: the only effective route in hot conditions, and it depends on humidity
- Conduction: minor, except in water
Sweat rates reach 1 to 2 litres per hour, and higher in acclimatised athletes.
Acclimatisation over 7 to 14 days: earlier onset of sweating, greater sweat volume, lower sweat sodium (through aldosterone), expanded plasma volume and lower heart rate for a given workload.
Heat illness progresses from cramps to exhaustion to heat stroke, defined by core temperature above 40 degrees with central nervous system dysfunction. The combination of high heat and high humidity is the dangerous one, since it removes the only remaining route of loss, and rapid cooling is the priority.