I remember looking into this a while back. I think the cleanliness-leads-to-asthma thesis works generally (Note: families with dogs tend to produce kids with asthma far less than those with them). The one thing that I never heard a good explanation for was the supposedly exploding rate of asthma among inner-city black kids. I don’t mean to disparage anybody, but nobody ever demonstrated that inner-city blacks live in hyper-clean environments. I’m sure there’s more to the story, but that’s about all I remember.