Swimming in outdoor chlorinated pools appears to increase the odds a child will develop asthma, Belgian researchers said on Thursday.
Other studies have linked chlorine and asthma but the new findings published in the European Respiratory Journal cast doubt on the idea outdoor pools are safer than indoor ones where chlorine vapors remains trapped inside an enclosed space.
"The more you swim, the higher the risk," said Alfred Bernard, a toxicologist at Catholic University of Louvain in Brussels, who led the study. "What is new in this study is that we looked at outdoor pools for the first time."
Asthma, which affects more than 300 million people worldwide, is the most common pediatric chronic illness. Symptoms include wheezing, shortness of breath, coughing and chest tightness.
Bernard and colleagues showed that outdoor pools are just as or more risky than indoor ones because harmful vapors remain at the pool surface and do not drift away.
And because children tend to spend more time in pools they are more likely to swallow chlorinated water or ingest vapors containing chemicals that attack the cellular barriers protecting the lung from allergens, Bernard said.
"We see that the risk of the outdoor pool is equal and even higher than indoor pools because children tend to spend longer in outdoor pools and they are more chlorinated," he said.
The Belgian team tested 847 students around the age of 15 for allergies and asthma and asked their parents about exposure to asthma risks such as tobacco smoke, pets and pollution, and how much time the children had spent in chlorinated pools.
The researchers determined that the risk for the 50 percent of children predisposed to allergies and asthma was directly related to the amount of time spent in a pool.
Children with the highest pool attendance — one hour per week for 10 years — were five times more likely to be asthmatic than young people who had never swum in a pool, the study found.
"Young children are more exposed because they take more water into their airways and their lungs are still developing," Bernard said in a telephone interview.