Humid air can stop sweat from cooling the body
When the air is saturated with moisture, the human body loses its primary defense against overheating, turning an elite athlete into a biological space heater.
An elite athlete in full stride functions like a high-powered machine, generating several hundred watts of internal heat—roughly the same output as a small space heater. To prevent their core temperature from spiking to dangerous levels, the body relies almost entirely on the evaporation of sweat to carry heat away. However, this cooling system hits a hard physical wall at a wet-bulb temperature of about 31 to 35 degrees Celsius. In these conditions, the air is so heavy with moisture that sweat cannot evaporate; it simply beads on the skin, leaving the body with no way to shed the heat it creates.