When I grew up, I knew one kid with a history of asthma. His ancestry was Italian. My theory is that white devils from the northern parts of Europe self selected themselves to tolerate air pollution from a smoky hearth in their shelters which kept them warm at home.
Your theory may be very close to what I believe to be the correct one. ;) Lack of exposure to various elements in fresh air.
Until fairly recently, children were exposed immediately at birth to fresh air (OMG!), which includes all kinds of things. Beginning in the late seventies, hospitals began to close up their windows. I remember fighting with staff when my son was born. I wanted the window open. They kept closing it. They gave me a humidifier which made the stuffy room worse.
I’m not big on a/c either. I think it causes health problems for children and adults. Homes and businesses are becoming more air-tight and more of them use a/c. More and more people are spending their days inside in more sterile environments - places that are professionally cleaned on a daily basis. No fresh air, no dirt, but plenty of germs from other people.
Since the mid-to-late eighties, kids have been plopped in daycare/preschool where they spend most of their time in an enclosed environment (no open windows). School windows are no longer opened. Mom heads off to her job which is in an enclosed environment (no open windows). Then she picks her kids up and they head home to an enclosed environment. Mom gets pregnant in an enclosed environment and spends the bulk of her pregnancy in an enclosed environment.
No exposure to allergens leads to lack of build up of tolerance to those allergens.
Wanna do one on the obesity problem now?