Researchers from France and Canada compared lung tissue and breathing tests from twenty-three elite Canadian swimmers, whose average age was 21, to ten mild asthmatics and 10 healthy, non-allergic people of the same age. Tissue samples and tests were taken during the off-season when swimmers were not competing.
The team, led by Valérie Bougault at the Lille 2 University of Health and Law in France, found that tissue samples taken from swimmers' lungs had nearly six times as many immune cells associated with asthma and allergies as the lung tissue of healthy subjects -- a similar amount to what was found in the group with mild asthma.
Swimmers and asthmatics also showed evidence of scar tissue in the lungs, while healthy non-swimmers did not.
Lung tissue inflammation was not associated with actual asthma symptoms, such as coughing and wheezing, or with difficulty breathing during a medical test used to determine lung function.
However, previous research has linked exposure to swimming pool chemicals through water and air to respiratory allergies and asthma.
While acting as a disinfectant, chlorine reacts with a wide range of chemicals from human sweat, urine and hair, for example, to form chlorine byproducts -- some of which are hazardous to human health.
These byproducts are very volatile and can escape into the air above the water, according to Ernest Blatchley, an environmental engineer from Purdue University in West Lafayette, Indiana who specializes in water chemistry.
Competitive swimmers are known to inhale large amounts of these chlorine byproducts while doing strenuous exercise in the pool. Exposure to the chlorine compounds in indoor pools may make swimmers more sensitive to allergens such as pet dander, pollen and dust.
There are certain precautions that all swimmers can take at the pool to limit exposure to harmful chemicals, according to Alfred Bernard, a toxicologist at the Catholic University of Louvain in Brussels, Belgium.
He suggested avoiding pools with a strong chlorine smell in the air -- a sign the chemicals in the pool are poorly managed.
One of the best things people can do to reduce exposure to harmful chlorine byproducts is to practice better hygiene, said Blatchley, even in so-called saltwater pools (which are not actually chlorine-free).
"Always taking a shower before entering a pool and not using it as a urinal can cut down on toxic byproducts," he said.