Asthma is more likely caused by the milk subsidies given out by our US government.....
Personally, I think the stats reflect an epidemic of diagnosis or self-diagnosis.
Incrementally, and during the non-pollen seasons, I began expanding her lungs through walks, and eventually running. She began running with me, her sister, and eventually, others. She rollerbladed every day. She swam many miles. She runs 6 miles every other day (and marathons); and she has pretty much outgrown or overcome, pollen affliction. No asthma in 5 years. The regimen worked for her.
I am not suggesting it would work for everyone. Each case is different.
I have seen the "growth" in the asthma cases. One thing, I observed generally in most? Very little physical activity. Which is, in fact a tough call. Pretty hard to run when you have asthma. So, no, I'm not suggesting this applies across the board. Some people are born with weaker lungs. And some aren't.