Yeah, some parts of the US recently experienced something called a wet bulb event. Basically, that’s a phenomenon where the heat and humidity are both so high that your body’s natural cooling mechanism (sweating) stops working entirely and people will overheat simply by being outside. No amount of shade or cool drinks will help, because your body’s primary cooling mechanism has been defeated.

Basically, sweat works by evaporating. When water evaporates, it takes heat with it. This allows sweat to cool you down as it dries. To be able to accurately determine what the temperature feels like, you can’t just use a regular dry thermometer. You need to use something called a wet bulb thermometer. This is basically a thermometer with some wet cotton wrapped around it, to simulate sweat. As the wet cotton dries, it creates a more accurate gauge of what the ambient temperature feels like, the same way sweat cools you.

But a wet bulb event is when the wet bulb thermometer reads above 95°F. At that point, it means the cotton isn’t drying fast enough to cool a person down. At this point, the temperatures are dangerous even to fit and healthy individuals in the shade with fans and cool drinks. Because a breeze won’t even cool you down when it’s that hot outside; A fan will actually heat you up even faster, because the air is adding heat faster than evaporative cooling can remove it.

Non-native English speaker, but I looked it up the other day and it seems that pedantically an A/C only cools things down and heat pumps can both heat and cool.