The first electrically powered air-conditioning unit was developed in 1901.  Prior to mid-century the cost of air-conditioning units tended to be prohibitively high for individual households.  Innovations in the latter half of the 1900s lowered costs and allowed for more ubiquitous use.  In many parts of the country most homes now have window units or whole-house air-conditioning systems.

Florida and southern Arizona are two areas where air-conditioning is virtually indispensable.   

I lived in Florida for some months in 1939-40.  Air-conditioned units were extremely rare, and many people had difficulty dealing with the year-round heat.  The state was sparsely populated.  Now virtually all homes and buildings in Florida are air-conditioned, and it is the most densely populated state in the southeastern United States.

Southern Arizona is even hotter than Florida, though not so humid.  A few years ago, I flew into Phoenix at 4 pm, and the temperature measured 118 degrees.  I got out of town as quickly as I could.  No one could live there without air-conditioning.  They have it in their homes, their automobiles, and in their offices.  The city’s population has exploded.

Someone recently described Phoenix as follows:

“Phoenix is such a God-forsaken place. If this had been Sodom, Abraham would have begged for the angels to immediately firebomb it, as there would have been no noticeable difference. It might even have been an improvement.”