At what relative humidity does evaporative heat loss become almost impossible?

Evaporative heat loss becomes significantly compromised as relative humidity approaches higher levels. At about 75% relative humidity, the air is saturated with moisture, meaning that it cannot hold much more water vapor. This leads to a reduction in the evaporation rate of sweat from the skin. Sweat is a crucial mechanism the body uses to cool itself; as it evaporates, it removes heat from the body, thus maintaining a stable internal temperature.

When relative humidity reaches 85%, the air's capacity to facilitate evaporation is nearly eliminated, making it very difficult for the body to cool itself through sweating. At this point, the body's thermal regulation is severely hindered, which can lead to overheating and heat-related illnesses during physical activity. Recognizing these thresholds is vital for athletes and trainers to manage heat stress effectively during training and competition.