Heat stroke

Heat stroke is a severe rise in body temperature that comes with central nervous system dysfunction, meaning the brain and spinal cord stop working properly. It can show up, for example, as delirium, a state of confusion, or as seizures. And it occurs in hot weather or with strenuous physical exertion.

One way the body cools itself is through the evaporation of sweat. When humidity is above 75%, that evaporation becomes less effective. It is the same thing that happens to laundry hung out on a muggy day: it takes a long time to dry because the air is so humid. Other ways of shedding heat also become less effective when the surrounding temperature rises above skin temperature: for example, radiation, the heat the body gives off without touching anything, like a space heater that is on; conduction, the heat that passes into whatever you touch; and convection, the heat carried off by moving air.

There are two forms. The classic form, typical of older adults with chronic illnesses. And the exertional form, which affects otherwise healthy people who exercise intensely in heat or humidity.