The most popular theory about what's causing the "allergy epidemic" is the hygiene hypothesis: "[C]lean living isn't necessarily good for us. By depriving our immune system of key infections caused by viruses, bacteria and parasites, we fail to develop the necessary tolerance for ordinarily tame foreign particles. In short, the immune system -- underused and spoiling for a fight -- goes ballistic when finally given the opportunity, no matter how slight the opponent." (Source is the Maclean's article linked above.)
Trust me, my house isn't clean. But I have a kid with severe food allergies.
What I'd really like is to have a researcher to look at the link between allergies and cigarettes.
There's at least one researcher who's found that "Pregnant smokers increases grandkids' asthma risk." And we know that allergies and asthma go hand-in-hand.
And look at the demographics -- smoking increased dramatically in the 50s and 60s. That was the post-war generation, who could afford to smoke as much as they wanted. (Yeah, there were smokers in the war years, but cigarettes were rationed.) So everyone smoked -- at home, in public, in their offices, at parties, everywhere.
And in the course of doing this, they changed their grandchildren's genetic makeup, making them more prone to asthma -- and probably more prone to allergies as well. So here we are, two generations later, and seeing what's being called an allergy epidemic.