NEW YORK (Reuters Health) - Children who start swimming before the age of 2 may be at increased risk of a common infant lung infection, and possibly asthma and respiratory allergies later in life, a new study suggests.

The findings, reported in the European Respiratory Journal, add to evidence that exposure to chlorinated pools may affect children's respiratory health -- particularly if they have a family history of asthma or respiratory allergies like hay fever.

Experts have suspected that the air quality around pools, particularly indoor ones, is to blame. When the chlorine used to disinfect pools combines with swimmers' sweat, saliva or urine, irritating chlorine byproducts are formed, and over time these chemicals may damage the airways.

In the new study, Belgian researchers found that infant swimming -- whether in indoor or outdoor pools -- was linked to a heightened risk of bronchiolitis.

Bronchiolitis is an infection of the lungs' small airways, usually caused by the respiratory syncytial virus, that is common in infants.

In this study, infant swimmers who developed the infection were also at increased risk of developing asthma or respiratory allergies by kindergarten.

"This suggests that chlorinated pool attendance can increase the risk of asthma and respiratory allergies by making the airways more sensitive not only to allergens but also to infectious agents," senior researcher Dr. Alfred Bernard, of Catholic University Louvain in Brussels, told Reuters Health in an email.

He did not advise parents to keep their young children away from pools, since it is an "enjoyable" way for kids to be active.

"Parents should, however, not lose sight that chlorine-based disinfectants and their derivatives are strong irritants not only for the skin but also the airways," Bernard added.

He said that parents should be sure not to over chlorinate their home pools and to try to avoid public pools that are heavily chlorinated. Some clues to the latter, according to Bernard, include an overwhelming chlorine smell, and eye, skin and throat irritation in pool users.

Where possible, the researcher said, parents can also opt for public pools that use alternative disinfecting methods, like ozone treatment -- which has long been used in Europe and is becoming more common in the U.S.

For their study, Bernard and his colleagues assessed 430 Belgian kindergarteners and surveyed parents on their children's health history, swimming habits and other factors.

They found that of children exposed to chlorinated indoor or outdoor pools before age 2, 36 percent had a history of bronchiolitis, compared with 24 percent of their peers.

Among children who had used only indoor pools for more than 20 hours before age 2, the risk of bronchiolitis was 3.5-times higher compared with children who had never been to a chlorinated indoor pool at that age. Meanwhile, children who had spent that much time at an outdoor pool showed a two-fold increase in their risk of the lung infection.

Overall, there were no significant differences in the rates of asthma and allergies among infant swimmers and their peers. But when the researchers looked at children with a history of bronchiolitis, only those who had been infant swimmers showed increased risks of asthma and respiratory allergies.

Among infant swimmers who had contracted the infection, 15 percent later developed asthma. That compared with 4 percent of swimmers with no history of bronchiolitis. The figures were nearly the same when it came to hay fever.

It is "very likely," according to the researchers, that airway irritation from chlorine byproducts makes babies more vulnerable to bronchiolitis.

From there, the infection and chronic chlorine exposure appear to "interact" to increase a child's risk of asthma and allergies later on.

SOURCE: European Respiratory Journal, online January 14, 2010.