A new study shows competitive swimmers training at indoor chlorinated swimming pools may have lung changes like what is seen in people with mild asthma.

Researchers compared data from more than 20 elite swimmers in Canada with 10 healthy people, and 10 mild asthmatics. Tissue samples from the swimmers showed similarities with those with asthma, although there is no evidence those changes lead to actual asthma.

According to a report published by Reuters, “the research team, led by Valerie Bougault at the Lille 2 University of Health and Law in France, found that tissue samples 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.”

Changes may be the result of inhaling disinfection byproducts, or some other factor. The study appears in the Journal of Allergy and Clinical Immunology.