Why is humid air more dangerous than heatwaves in India?

Sweat is human body's primary weapon against heat. When sweat evaporates off skin, it carries heat away with it, cooling body down. However, when weather turns humid, air is already so loaded with moisture that sweat cannot evaporate. The body fails to cool itself through sweating. This results in the body overheating, leading to heat stress, exhaustion and heat stroke.