While few of us would question what a great exercise swimming is, there is a growing body of evidence that seems to indicate that an excessive amount of time spent at indoor pools, where the air circulation might not be optimal, can lead to respiratory problems, including asthma. This is of particular concern for young children whose lungs are still developing.

According to experts, asthma and severe breathing problems are extremely common in competitive swimmers, and what’s most striking is that the respiratory conditions seem to kick in after they take up the sport. In fact, a Canadian study found that children who were active in competitive swimming had far more respiratory problems (including coughing, wheezing, and asthma) than their counterpart who played indoor soccer.

And in two recent studies out of Europe, one in the British Journal of Sports Medicine and another in the European Respiratory Journal, researchers found that many elite athletes who took part in competitive swimming suffered from some sort of breathing condition, including inflamed lung tissue and airway hyper-responsiveness, which can be a precursor to asthma. Even the team physician for USA swimming was quoted as saying that “The US National Team has a bunch of asthmatics on it.”

Conventional wisdom might lead us to believe that the culprit in all this is the chlorine disinfectant in the water, which is used to prevent the transmission of germs but also happens to be lethal to humans in large enough quantities. It turns out, however, that it is not the chlorine itself that is the problem, but rather the by-products known as chloramines.

Chloramines form when free chlorine combines with nitrogen based compounds, usually waste products, that come off of our bodies, which include sweat, body oil, and urine. Chloramines are in fact what give pools their distinctive bleach smell and tend to be at their highest concentrations just above the surface of the water, where swimmers tend to breathe. Interestingly, chloramines actually result from insufficient levels of free chlorine, rather than too much, and can be reduced by either increasing free chlorine to optimal levels or by having people shower (and thus remove sweat and body oil) before they enter the pool.

While it has been suggested that casual swimmers who are in the pool less frequently are not at risk, parents of young children should be advised that some studies have found that young swimmers whose lungs are rapidly developing seem to absorb chloramines more readily. In fact some experts have gone so far as to recommend that babies and toddlers should avoid indoor swimming pools altogether.

For all the concern about noxious chemicals affecting our lungs, the good news is the damage is probably not permanent, and the benefits of swimming still seem to outweigh the risks. Nonetheless, it may not be a bad idea to keep tabs on the air at the indoor pool you frequent, which can be an indicator of chloramines levels, or switch over to an outdoor pool, where ventilation is not a problem.