It's far too soon to conclude that these short-term changes lead to long-term health problems, said Alfred Bernard, a toxicologist at the Catholic University of Louvain in Brussels, Belgium.
But he thinks there's already enough reason to swim with caution. His own work suggest that chlorine byproducts are a major risk factor for rising rates of asthma, airway irritation and allergic diseases, especially during windows of sensitivity in babies and young children.
"We have good evidence that you have to be careful with these chemicals," Bernard said. "Maybe chlorine is not the best choice for disinfecting swimming pools."
Alternatives do exist. Some pools use ozone, copper and silver ions, or even moss to kill bacteria.
Even if pools continue to use chlorine, there are ways to reduce how much of it they use, Kogevinas said. Regular emptying and cleaning of the pool helps. So does introducing freshwater.
Swimmers can make a difference, too. By simply showering before they get in the pool, they wash off much of the organic material that reacts with chlorine to produce toxic byproducts. Swimming, Kogevinas added, is still healthier than not exercising.