I fit was simply due to cars....then the entire western world (or at least cities etc) would have a fairly similar no. of cases (%age wise), but this is not true. Last year I moved to a small village near the east coast of Scotland which, as my GP informed me, is the asthma capital of this entire region. This can't be because of car pollution (cos Edinburgh is ca 15 miles away, why wouldn't asthma be worse there??) and can't believe people here are more hygenic than people in the surrounding towns etc. Also, I grew up in Campbeltown...way down on the tip of the Mull of Kintyre....miles from anywhere, yet I have mild asthma. I also have soem allergies...and allergies have increased hugely in the last 20 years, probably linked in some way to the asthma. I've heard/read all the theories, and I worked in contract research for a long while myself, yet never came across a really plausible single explanation. It must be a combination of several factors. Latest thinking is that it may be related to the boom in bottle feeding/general decrease in breast feeding...ie parents today are bottle feeding their kids, and were bottle fed themselves...leading to an overall reduction in immuntiy etc. This was just one study tho, and by no means conclusive, much like most other studies into the matter.