The researchers included Alfred Bernard, PhD, of the public health department at Catholic University of Louvain in Brussels, Belgium.
They studied 341 Belgian schoolchildren who were about 11 years old, on average. The children provided blood samples and had their lung health tested.
Bernard's team noted that 43 of the children had taken infant swimming lessons in indoor in chlorinated swimming pools. The researchers also noted other lung health hazards, including maternal smoking during pregnancy and exposure to secondhand smoke.
The lungs of children who had had infant swimming lessons in chlorinated pools appeared to be predisposed to developing asthma and recurrent bronchitis, according to the study, which appears in Pediatrics.
The study doesn't prove that infant swimming caused those lung problems. But the researchers suggest that chlorine-related gases in indoor pools may have been a significant factor.
Bernard's team doesn't know how much chlorine was in the pools in which the children had their infant swimming lessons since the study was conducted when the kids were already 11 years old on average.
However, the researchers concluded that chlorine-related gases may cause lung irritation that's particularly damaging to infants' lungs, which are still developing.
Bernard and colleagues also note that the American Academy of Pediatrics (AAP) states that children generally aren't ready for formal swimming lessons until they're at least 4 years old and that infant swimming lessons may give parents a false sense of security about their children's water safety, since infant swimming lessons haven't been proven to reduce drowning risk.
The AAP's stance on infant swimming lessons doesn't address any potential risks from chlorine or chlorine-related gases.
The Belgian researchers didn't reach any conclusions about lung risks for babies swimming in outdoor pools due to the small number of children with backyard pools.
Bernard and colleagues call for more studies on the effects of chlorine products on the respiratory tract of very young children.