Earlier quoted context omitted.
> fatal even to fit and healthy people, unclothed in the shade next to a fan; This is scary, what's a good way to survive for someone that doesn't have access to a bunker or AC? Immersing oneself in water in a bathtub?
A tub will work, sure, as long as it's pulled from a large body of water that hasn't warmed up with the air. Hopping in the local lake is an obvious choice. But in a detached demographic sense: effectively everyone lives in a city in the modern world, and while their housing might not be air conditioned most of the major buildings are. In an emergency like this everyone would just crowd into office buildings and shop…
Unless these buildings are designed as emergency shelters for heat waves their air conditioning systems won't be able to cope with all the people.