According to Popular Science, when temperatures are high and the air is humid, the mechanisms the body has to cool down are not as effective.
Heat energy moves from warmer to cooler places. When outside temperature is cooler, heat leaves the body. But when outside temperatures are higher, heat can’t escape as well.
As the body tries to cool down, blood is sent to the surface of the skin in order to radiate off heat. This pulls water out of the bloodstream and puts them into the sweat glands.
Because of dehydration, less blood is available to transport oxygen to the brain and other internal organs. Circulation is encumbered as well because the blood has less water.
If those conditions maintain and fluids are not replenished, there is a risk of heat exhaustion.
If someone appears to be confused, or starts to show other neurological symptoms, that is a sign of heat stroke.
Young children and older people are at higher risk for heat-related illnesses.
It’s important to stay properly hydrated in hot, humid weather. Wear loose-fitting and lightweight clothing. If possible, avoid direct sunlight. Avoid alcoholic and caffeinated drinks, which are dehydrating.