Which environmental factor reduces heat loss via sweating in humid air?

• A. Air temperature
• B. Wind
• C. Sun exposure
• D. Humidity

Answer: (D)

Sweating cools you mainly by evaporation—the sweat on your skin absorbs heat as it turns into vapor. In humid air, there’s already a lot of water vapor present, so the air can’t take up much more vapor from your skin. That small vapor-pressure difference between your skin and the surrounding air slows evaporation, meaning less heat is removed and your body loses heat more slowly. Humidity directly lowers the effectiveness of evaporative cooling, which is why it reduces heat loss via sweating in humid conditions. Wind can help by blowing away humid air to boost evaporation, and higher or lower air temperatures can change the rate, but the factor that specifically reduces sweating-based cooling in humid air is humidity.