A study of Olympic-level athletes concluded that swimmers, especially in endurance classes, are more likely to develop asthma than their aquatic counterparts.

Researchers led by Dr. Margo Mountjoy of McMaster University Waterloo campus in Ontario, Canada, concluded that nearly 25 percent of endurance swimmers had some level of asthma.

The study looked at elite athletes who competed during the 2004 and 2008 Olympic Games and the 2005, 2007 and 2009 FINA World Championships. Nearly 1,500 athletes were studied, representing almost 16 percent of aquatic athletes who competed between 2004 and 2009.

All athletes in the study held therapeutic use exemptions, meaning they had “objective evidence of athlete asthma.” TUEs are given to athletes with conditions requiring support, such as an inhaler.

In part, Mountjoy’s study was meant to compare five water events: swimming, synchronized swimming, water polo, open-water swimming and diving, the only one not considered an endurance sport.

The study noted that non-endurance athletes develop asthma less than endurance athletes. So it’s no surprise that fewer divers have asthma than endurance swimmers. But the researchers also postulate that increased asthma in endurance swimmers is, in part, due to their higher exposure to chloramines. It notes that while open-water swimmers have asthma more than non-aquatic athletes, their risk is less than indoor swimmers, possibly due to the lack of chloramines.

Water polo players also have lower rates of asthma than endurance swimmers. Researchers believe this is because water polo players spend a certain portion of their training away from the chlorinated water, say, lifting weights.