Which relative humidity level is described as impairing the body's ability to evaporate sweat to cool itself?

• A. Humidity level that stops evaporation
• B. Humidity level that impairs evaporation
• C. Humidity level that reduces evaporation rate
• D. Humidity level that enhances cooling

Answer: (B)

High relative humidity reduces the body's ability to evaporate sweat, which is the primary way we cool itself during exercise. Evaporative cooling depends on a vapor pressure gradient: sweat on the skin evaporates into the air, removing heat. When the air is already saturated with moisture, the gradient is small, so sweat evaporates more slowly and cooling is impaired.

Among the options, describing the humidity level as impairing evaporation best matches this overall effect on cooling. Saying it stops evaporation is too absolute—evaporation can still occur at reduced rates even in humid air. Saying it reduces evaporation rate is true, but the phrase that emphasizes impairment aligns most directly with the impact on the body's cooling ability. Saying it enhances cooling is opposite to the physiological effect.