Jesus Christ, 158 F heat index is so far out of bounds on that scale it's insane. What's after "Extreme Danger"? "People will die"? "Being outside is suicide"?
A [wet-bulb temperature] reading of 35 °C (95 °F) – equivalent to a heat index of 71 °C (160 °F) – is considered the theoretical human survivability limit for up to six hours of exposure.
It's when it feels really hot and you look at the thermometer and think, "wow is that it? I better arbitrarily add a few more degrees so people know it's really hot"