2 things to consider:
1. we insulate buildings now like nobodies business. There's a term for it "Unhealthy Building Syndrome".
2. the doctors are 'looking for asthma' like never before. A few years ago when my niece & nephew were in pre-school there was a rash of "ear tubes" to prevent chronic ear infections. In my day it was tonciles.
Never underestimate societies ability to tie itself in knots.
Bingo.