Here is an article from about.com that outlines the problems that some swimmers may face when using a chlorinated pool
here is the leading paragraph to the story.
Chlorinated indoor swimming pools can cause asthma, according to research from several sources. These findings may explain why swimmers using indoor pools are more prone to Asthma than athletes in other sports.“Our results show, indeed, that nitrogen trichloride (produced by Chlorine) is a cause of occupational asthma in indoor swimming pool workers like lifeguards and swim instructors,” says Dr. K. Thickett of the Occupational Lung Diseases Unit at the Birmingham Heartlands Hospital.
The rest of the article can be found here:
http://swimming.about.com/od/allergyandasthma/a/cl_pool_problem.htm