Kids who swim regularly in outdoor chlorinated pools are up to five times more likely to develop asthma than youngsters who have never been in a chlorinated pool, according to a new study by Belgian researchers in the European Respiratory Journal. The same research team found a few years ago that kids who swim regularly in indoor chlorinated pools also experience an increased asthma risk, but it was thought then that outdoor pools would be less likely to cause the same problems since chemical vapors can more easily escape than in an enclosed indoor pool. However, it turns out that chlorine vapors (and other chemical vapors stemming from chlorine’s interaction with organic matter in the pool) tend to stay near the surface of the water even in outdoor pools. “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,” said lead researcher Alfred Bernard. “Young children are more exposed because they take more water into their airways and their lungs are still developing.”