The following is a guest post by Simon Owens, blogger for PBS’s MediaShift and owner/author of Bloggasm, which explores the intersection of new and old media. Publishing this article DOES NOT imply endorsement of the views expressed within by the Asthma Mom blog. I’m simply presenting another side to the chlorine/asthma question and have included my own remarks in the comments below.
Does swimming in a chlorinated pool increase your chances of developing asthma? A 2003 study conducted by a group of Belgian scientists argued as much, but a recent “meta analysis” published in the Journal of Asthma suggests that such a correlation does not exist.
The meta study was conducted by Dr. Michael Goodman, an epidemiologist and pediatric physician who teaches at Emory University. Over a two-year period he reviewed 25 separate studies that dealt with the effects of pool use on asthma.
“The very first time I looked into this issue was … when a colleague of mine brought to my attention one or two articles that were proposing the pool chlorine hypothesis and provided some evidence that supported that hypothesis,” Goodman said in a phone interview. “That’s when I started to look at it and review the literature a little bit closer — trying to understand it — because it struck me as potentially a very important issue.”
Most of the reported evidence of a pool and asthma link stems from a study conducted in Belgium that found that chlorine, which is typically used to disinfect swimming pools, can increase the risk of asthma in children. The study argued that gas formed when the chlorine mixes with bodily fluids causes the increase; the findings sparked concern with parents who were worried about the asthma risk.
Dr. Goodman scored the 25 studies for relevancy and divided them into four categories — Studies of Asthma in Elite Athletes, Studies of Asthma and Swimming During Childhood, Studies of the Effect of Swimming Training on Asthma Patients, and Studies Comparing Asthmagenicity of Swimming and Dry Land Exercises.
The first group of studies focused on elite athletes — typically at the Olympic level — and analyzed the percentage of those with asthma in each sport. The studies found that there is a significantly higher percentage of elite swimmers who have asthma compared to athletes in other sports. But Goodman said this could be an example of “reverse causation” in which asthmatics are simply more likely to participate in swimming because it is perceived as having a lower risk of triggering an asthmatic attack.
“It provides some suggestive evidence that asthma is more commonly found among swimmers,” he said. “But I think this study would be hard pressed to conclude that there’s any evidence that swimming causes asthma in previously healthy individuals.”
The second group of studies focuses on the question of whether previously healthy children experienced an increased risk of asthma after constant indoor pool use. Though the Belgian study suggested such a link, several other studies debunked this claim and showed no such correlation.
“None of the individual point estimates demonstrated a significant increase in asthma prevalence among the exposed,” Goodman’s findings showed.
The last two groups dealt with participants who already had asthma; they attempted to measure the effects of swimming pools on asthmatics. Group three, for instance, compared before-and-after changes in asthma severity among patients participating in swimming programs to non-swimming controls.
“If you do have children with asthma and enroll them in programs, do you see an improvement?” Goodman asked. “There are several ways to mark improvement; one is subjective in that you ask them how they feel, but there are some objective ways — for instance doing a pulmonary function test and see whether there’s any improvement that can be observed in those tests.”
These studies showed that though the objective tests didn’t find any noticeable change, the subjective tests often resulted in children saying they felt better after completing the swimming programs.
The final group of studies compared the immediate respiratory effects of swimming to those of running or cycling among asthma patients. These studies attempted to determine whether swimming triggered a lower rate of exercise-induced bronchospasm — an asthma attack caused by physical activity — compared to other sports.
This group of studies found a four to six fold increase in asthma attacks for both running and cycling compared to swimming.
Asked whether parents should refrain from allowing their children to swim in chlorinated pools, Goodman replied that there is not enough conclusive evidence that pools cause a significant risk.
“Parents should be worried about lack of exercise in children,” he said. “This is by far one of the most important public health problems with children. So exercise is important, and exercise with asthmatics is important. The American Academy of Sciences came out and said that swimming is one of the best sports for asthmatics, and they haven’t changed that recommendation as far as I know … Although the concern about the pool chlorine hypothesis is valid, I would say it’s premature to act upon those concerns.”
Goodman said that more comprehensive studies would have to be conducted before epidemiologists knew for certain that there is no increased risk of asthma. Scientists, he argued, would have to follow children from birth and compare swimmers and non-swimmers before they have a chance to develop asthma, and then compare the frequency of those participants developing asthma afterwards. Scientists are starting to undertake these prospective studies. A group of German scientists recently found no relationship between baby swimming and asthma, for instance.
“That would address the issue of these competing hypotheses,” he said. “Because then we’ll know for sure what factor preceded the other — whether the asthma preceded swimming or whether swimming preceded the asthma.”
Journal of Asthma, October 2008: Study Abstract