Unlike floods, earthquakes, and tornadoes, heat's devastation is insidious, pushing on the human body's limits until it suddenly exceeds them. Above a certain threshold—one that's maddeningly difficult to determine due to multiple variables like humidity, sunlight, breeziness, and weather—heat just makes it harder to exist. The hemoglobin that picks up oxygen and carries it in our blood has a harder time binding to it as temperatures rise, turning respiration into a chore—each breath gains us less oxygen. Meanwhile, sweat tends to desert us if we're very young, elderly, or if the air is already very humid. Sweat cools the body through evaporation, but young children and the elderly tend not to sweat as much, and when the air is already thick with humidity, sweat doesn't evaporate—so our bodies don't cool off.