Muggy heat is more oppressive than the "dry" kind. That is because humans and other mammals cool their bodies by sweating; sweat evaporates off the skin into the air, taking the excess heat with it. It works nicely in the desert. But when the air is already crowded with moisture; think about muggiest days of summer in the city; evaporation off the skin slows down, and eventually becomes impossible. When this cooling process halts, one's core body temperature rises beyond the narrow tolerable range. Absent air conditioning, organs strain and then start to fail. The results are lethargy, sickness and, in the worst conditions, death.