Parents should be on alert to a strong smell of chlorine in pools or children complaining of sore eyes or throats after swimming. (Carolyn Kaster/Associated Press)

Swimming in chlorinated swimming pools may increase the risk a child will develop symptoms of asthma, a new European study suggests.

In Monday's online issue of the journal Pediatrics, researchers found teenagers who spent more than 1,000 hours swimming in chlorinated pools showed more than eight times the risk of having asthma as teens who mainly swam in pools disinfected with another method.

"This is an important factor, which might explain the epidemic of the disease in countries like Canada … where you have a lot of swimming pools, indoors or outdoors," said the study's lead researcher, Alfred Bernard, a toxicology professor at Catholic University of Louvain in Brussels.

About half of children are genetically susceptible to the effect, he added.

In the study, Bernard and his colleagues looked at 847 people between the ages of 13 and 18 who all swam in indoor and outdoor swimming pools. Swimming attendance varied. Of the participants, 114 went to pools that used a copper-silver disinfectant, which served as a control.

Allergy, hay fever risk

Teens with the most exposure to chlorine had two to three times greater risk of allergic rhinitis and three to six times greater risk of hay fever, the team found, compared with those with the least exposure, after taking other risk factors such as exposure to tobacco smoke, allergens and parents' education levels into account.

The researchers speculated that chlorinated vapours may facilitate the delivery of allergens across the lining of the airways.

The findings suggest that competitive swimmers like Danielle Poulos and the children she coaches in synchronized swimming are at greatest risk. But Poulos said she hopes that people won't use the research as an excuse not to be active in the pool, given the cardiovascular and other benefits of the exercise.

"The benefits of taking part, in whether it be synchronized swimming, speed swimming or any water sport, far outweighs probably the chance of getting an allergy," said Poulos, who estimated she's spent 20 hours per week in and around a pool over the last 15 years.

Still, parents should be on alert to a strong smell of chlorine or children complaining of sore eyes or throats — signs of too much chlorine, which may pose too great a risk, particularly for very young children, the researchers said.

The research was funded by the NationalFund for Scientific Research in Belgium, the Agency for Environmental and Occupational Health Safety in France, the governments of the Walloon Region and the French Community of Belgium, and the European Union.