This is by no means an authoritative answer to your question, but I can give some information that may be a part. Allergic response is correlated with BMI. the higher the BMI you have the more likely you are to have a response. Thus, you can “develop” allergies, which are really allergies you always had, but only relize after you get heavy. As american are getting heavier, and generally less healthy to boot, I’m sure this is a big part of the problem. I am severly allergic to grass, and mint.
As a kid, grass bothered me, but growing up in the desert it was only a problem when I had to mow the “lawn” (a tiny patch of grass), I’d be sick for days. I normally weigh about 170, but a few years back after a severe accident, I gained almost 25 pounds due to inactivity. For the next year or so, even a mower cutting the lawn outside was enough to send me into a wheezing, hyperventilating death spiral (I’m being dramatic, I know). But after I lost the weight, the grass is still a nuisance, but I’ll be sure to maintain the lower weight from now on (Besides, I didn’t like the other added inconveniences of the extra weight anyway)!
Anyway, I think the rising overweightedness and obesity are heavily (no pun intended), contributing to the problem. Let the kids go out and play!
There are other factors which can worsen an allergic condition.
Stress is one of the big ones. Anyone with allergies can tell you that when they are under stress of any kind, mental, emotional, physical, it can and will worsen symptoms.
That’s an interesting observation. I think another factor is that once, people with severe anaphylactic shock would have died, rather than passing on the allergy to offspring. And another is that people are exposed to a wider variety of foods in modern society than they were a few generations ago.
I’m allergic to Vietnamese fish sauce, which is not something your average Midwesterner encountered in the past. And one of my daughters recently had lobster for the first time and discovered it doesn’t agree with her.