According to a recent study swimming in an indoor public pool for as little as 30 minutes every two weeks may result in altered hormone levels in boys.

The effects are most pronounced if the exposure occurs before the age of 7. Boys are particularly sensitive because the skin of the scrotum is susceptible to absorbing chlorination by-products such as those formed when urine or other organic material is found. The lowered hormone levels may contribute to reproductive problems such as reduced sperm, according to an article published by Environmental Health Perspectives, a peer-reviewed journal published by the National Institute of Environmental Health Sciences.

From the story:

The study population consisted of 199 primarily white boys aged 14-18 years who swam regularly in indoor and/or outdoor chlorinated pools, and 162 similar boys who swam most frequently in an indoor pool disinfected with copper–silver ionization (but also swam at times in indoor or outdoor chlorinated pools). The authors compared serum levels of several testicular hormone biomarkers between the two groups: inhibin B, total and free testosterone, sex hormone–binding globulin, luteinizing hormone, follicle-stimulating hormone, and dehydroepiandrosterone sulphate. The boys who swam the most in indoor chlorinated pools had concentrations of inhibin B and total testosterone about 20% lower than those of boys who swam in the pool disinfected with copper–silver ionization, and the former were about 3 times more likely than the latter to have abnormally low concentrations of these hormones. The effects were more pronounced for exposure before age 7 than before age 10 (after which no significant changes were seen), and adverse effects were associated with swimming as little as 30 minutes every 2 weeks. There were no significant hormonal changes in boys who swam in outdoor chlorinated pools. These were primarily backyard pools that study coauthor Alfred Bernard, a professor of toxicology at Catholic University of Louvain, says tend to be less prone than public pools to have elevated concentrations of urine and other organic matter. That means less chlorination by-products are formed.

The study’s findings are preliminary, and the researchers plan more tests to rule out other factors such as bath water that may contribute to lowered hormone levels in boys.