Did you know that elite swimmers have a higher prevalence of asthma than other athletes? Surprisingly, this fact was only confirmed in a study relatively recently. Chlorine was an obvious candidate to explain this finding and over the last decade, evidence has been mounting to suggest that water chlorination does indeed lead to an increased risk of developing not only asthma, but allergies as well.
Initially, the problem was blamed on trichloramine, the gas that gives indoor pools their characteristic smell. Once it was discovered that outdoor pool exposure was as bad as indoor pool exposure, then the focus shifted to chlorine products in the water or vapors present around the water surface.
A recent study from Belgium published in the journal Pediatrics looked at three groups of adolescents, two of which had utilized only chlorinated pools and one of which utilized a pool decontaminated by means of a copper/silver ionizer. I read the study in detail and it is very complicated and has some potential holes. Nevertheless, I think it does clearly demonstrate that kids who spend a large amount of time in a chlorinated pool are more likely to develop asthma and/or respiratory symptoms of cough and shortness of breath and are more likely to develop allergies.
Given that there are other means of decontaminating pools such as salt water or ionizers, regulatory bodies should reconsider the appropriateness of chlorine as the standard pool decontaminant.