> Why do 90% of Americans have AC while only 20% of Europeans do?

Maybe because the majority of Europe is closer to Canada, latitude-wise, than to Phoenix, AZ, and there is simply less demand? Less wealth is certainly a factor, too, especially considering how the warmest nations in Europe all tend to be weaker economically.

> Why does US have ~4 heat related deaths per million while Europe has ~235 per million?

Maybe its just the higher life expectancy increasing susceptibility? Everyone has to die of something at some point.

It is a statistic, 'treacherous' is a word often lurking around the corner.

No healthy person all of a sudden dies from heat, I am sorry to tell. If that would be the case, everyone would be as panicked as you are. Europe has comparatively older demographics. Heat risk mainly affects infants and the elderly.

Most EU countries have free health care, so even people not caring enough for themselves will have a comparatively higher chance to survive into an old age. But also those who didn't die because of a bad lifestyle are part of this demographic. Like I said, treacherous, because you should look at this demographic and start to ask how many hours of life expectancy is lost. Healthcare keeps finding that the elder people just don't drink enough during these warm days.

I guess that if you want to win back these hours, you have to convince those elderly people to install AC or get them to drink enough during hot days. At this age people have a certain flexibility of mind, complicated by the fact that heat waves these days are really more severe than in their lived past.

Let me assure you: if people think it is too hot for them at home and they don't see an alternative, they will install AC. It is affordable enough. But there might be a cultural difference, people don't think of AC as the first line of defense against the hot days. Environmental awareness is higher; AC's contribute to global warming. Anecdotally, looking around I see there is a preference for sun protection over AC's.

> Why do 90% of Americans have AC while only 20% of Europeans do?

Because Rome is further north than New York and Paris is just south of Ottawa/Montreal.

Most of Europe simply doesn't need an AC. Spain, south of Italy, south of France, parts of the Balkans. But in countries like UK, the Nordics, Germany, etc. you'd need something more than "open windows" for mere days of the year, if that. The people who live in the places that need AC usually have AC. It's actually pretty damn simple.