In what experts believe is the first study linking asthma to outdoor pools, girls who swam in outdoor pools were shown to be at greater risk for asthma, compared to those who swam in indoor pools.

A recent survey of 303 young swimmers (150 females, 150 males and three unspecified) found that girls aged 9 to 12 who swim in outdoor pools had a higher frequency of asthma than girls or boys who swim in indoor pools only. As the number of hours spent swimming in outdoor pools increased, so did the asthma frequency among the girls. There was no significant difference among boys.

This survey is the latest research showing a correlation between swimming and respiratory problems, generally thought to be associated with chlorine disinfection byproducts.

Mark Siegel, a fourth year undergraduate student at the University of Wisconsin at Madison and former competitive swimmer, and Dr. Charles Siegel, from the University of Missouri-Kansas City School of Medicine, conducted the study. Overall, 30 percent of respondents swam in outdoor pools only and 5 percent said they swam in indoor pools only. The rest swam in both. All participants swim in the Kansas City, Mo. area.

The rate of asthma among respondents was 19 percent, compared with a national average of 9.2 percent, as reported in “Asthma in America,” a survey conducted by research firm Schulman, Ronca & Bucuvalas and funded by GlaxoSmithKline. However, because the 9.2 percent included infants who outgrow asthma, the swimmers may be a skewed population, Siegel said.

He also noted that one possible reason outdoor pools showed a greater correlation with asthma frequency may be that compared with the indoor pools, which are mainly just used for competition, outdoor pools in the Kansas City area are more likely to see greater use, often for recreational purposes. That means greater potential for disinfection byproducts because as those swimmers shed organic compounds including sweat, or even urine.

Results were presented at the American College of Allergy, Asthma & Immunology 2010 Annual Scientific Meeting, and published in the Annals of Allergy Asthma and Immunology; however, experts concur, more research is needed.

“I agree that competitive swimming indoors can create asthma, but I find it difficult to believe outdoor pools are more of the problem,” said aquatics expert Tom Griffiths, founder of Aquatic Safety Research Group in State College, Pa. “We only consider indoor pools as having ‘bad air.’ Still, it is an interesting finding and we need to do more research on both indoor and outdoor pools.”

Along with highlighting the need for greater understanding of how DBPs affect swimmers in indoor and outdoor pools, this study also calls into question the idea that swimming may be healthy for asthmatics.

“The question we always ask is: What is the best sport for people with asthma? We’ve generally said that humidity and warm air tends to be good for them, and we’ve often told our patients to swim, but this study questions this advice,” said John J. Oppenheimer, M.D., associate clinical professor of medicine at New Jersey Medical School, and chairman of the ACAAI abstract review committee.

For his part Siegel said he’d like to partner with an organization such as USA Swimming to survey a larger group.