In the southwest, it's usually dry heat, which means the body can still cool itself by sweating -- but anything over 90F is risky for many people, and over 100F is damn dangerous. 117F can kill a lot of folks. With climate change, heat waves are getting longer, hotter, and more frequent. Eventually, parts of the southeast, where it's both hot and humid, will become uninhabitable. In the west, the problem is water shortage combined with dry heat.
You can see climate change unfolding right in front of you if you know what to watch for.