At what relative humidity is heat loss through evaporation impaired, and at what level does it stop?

• A. Impaired at 50%, stops at 60%
• B. Impaired at 65%, stops at 75%
• C. Impaired at 70%, stops at 80%
• D. Impaired at 60%, stops at 70%

Answer: (B)

Evaporative cooling relies on sweat evaporating from the skin to transfer heat to the environment. That process depends on a vapor pressure gradient between the skin surface and the surrounding air. As relative humidity rises, the air already contains more water vapor, so the gradient shrinks and evaporation slows. When humidity reaches about mid-60s percent, evaporation becomes noticeably impaired. Once humidity hits about three-quarters of capacity, the air is nearly saturated and evaporation nearly ceases, so heat loss through this route effectively stops. This is why in hot, humid environments, cooling via sweating becomes much less effective, and the body has to rely more on other cooling methods. The best-supported thresholds are impairment around 65% and stopping around 75% relative humidity.