A human can die really at any cold temperature when the persons body temperature drops below 70F and even before that you can get hypothermia.
So it's important to wear proper clothing during the winter months when it's cold out and get out of the cold weather as much as possible.
Even when you're bundled up really good and you're outside in the cold or in a freezing cold house you could freeze to death even in 30 F weather but when the weather gets below freezing out then your bodies temperature will drop faster and then you'll freeze and die.