Swimming in chlorinated pools, especially frequently and at a young age, has been shown to increase asthma risk. The chlorine in the pools reacts with organic matter such as sweat and urine (swimming anyone?!) to create trichloramine. The researchers believe that "chlorine by-products such as trichloramine disrupt the protective epithelial barriers of the respiratory tract, allowing allergens to enter the lungs." They found that "Cumulative time spent at swimming pools emerged as one of the most consistent predictors of asthma, just after family history of asthma or hay fever and atopy (a genetic tendency toward developing IgE-mediated allergies)." The association was greatest for children using chlorinated pools before age 7. If you're thinking of getting a pool, research the salt-water pool--a choice more popular in Europe. In the meantime, don't pee in the pool! Commentary on the study from Environmental Health Perspectives Volume 114, Number 10, October 2006