Which heat exchange mechanism is the primary cooling method and is impaired at about 65% humidity, stopping near 75%?

• A. Metabolic
• B. Radiant
• C. Evaporative
• D. Conductive

Answer: (C)

Evaporative cooling is the body's main way to shed heat during exercise, especially in hot conditions. Sweat produced by the skin must evaporate to remove heat from the body, which relies on a vapor pressure gradient between the skin and the surrounding air. As relative humidity rises, that gradient shrinks, making evaporation less effective. Around 65% humidity the cooling effect starts to wane, and near 75% humidity evaporation nearly stops, so heat loss through this primary mechanism is greatly reduced. Other heat-loss routes exist—radiative, conductive, and convective—but they don’t compensate fully when evaporation is blocked, increasing the risk of overheating.