Gabe Mirkin, M.D.
Every year people pass out and die during exercise from a condition called heat stroke, a sudden uncontrolled rise in body temperature that affects the brain so that it can't function properly.
Heat stroke doesn't just happen. You get plenty of warning. First your muscles are affected, then your circulation and then your brain. As your temperature starts to rise, your muscles feel like a hot poker is pressing against them. As it rises further, the air that you breathe feels like it's coming from a furnace and no matter how rapidly and deeply you try to breathe, you won't be able to get enough air. When this happens, stop exercising. If you continue to exercise, your body temperature will rise further and affect your brain. Your head will start to hurt, you'll hear a ringing in your ears, you may feel dizzy, you may have difficulty seeing and then you will end up unconscious on the ground. Stop exercising when you gasp for air beyond your level of effort.
When a person passes out from heatstroke, his brain is being cooked just like the colorless part of an egg turns white when it hits the griddle. Get medical help immediately. Carry the victim into the shade and place him on his back with his head down and feet up. Cool him by pouring liquids on him or spraying him with a hose. After he wakes up, watch him for more than an hour as his temperature can start to rise to high levels again.