As a transplanted resident of Paris from Houston, I'd say it's a few things. No one here is used to it, many folks don't have air conditioning, and buildings don't seem to be designed with ventilation in mind. Even buildings that do have air conditioning can't keep up.

I'd also say that Parisiens spend more time walking outside than anyone did in Houston. When it got above 100 in Houston, everyone hunkered down inside until the sun went down. It only takes one time getting sun stroke to learn.

Here they don't get that lesson on a yearly basis. They keep on going, get dehydrated, and then fall over.