200 words on the increase of asthma in the US

The incidence of asthma is increasing in the United States.  There are many theories as to why this is happening.  Please research this on the internet and share your research with the class.  Please pick one of the theories and explain why you agree or disagree that the theory is a major cause.  (Hint:  increased pollution, genetics, too much cleanliness, etc.)

must include a minimum of 200 words

a minimum of 2 references in the APA or AMA format