How does heat kill? It confuses your brain. It shuts down your organs. It overworks your heart.

As temperatures and humidity soar outside, what's happening inside the human body can become a life-or-death battle decided by just a few degrees.

The critical danger point outdoors for illness and death from relentless heat is several degrees lower than experts once thought, say researchers who put people in hot boxes to see what happens to them.

With much of the United States, Mexico, India and the Middle East suffering through blistering heat waves, worsened by human-caused climate change, several doctors, physiologists and other experts explained to The Associated Press what happens to the human body in such heat.