Asthma and swimming: weighing the benefits and the risks.

Swimming is a healthy exercise that is well tolerated by asthmatics because it induces less severe bronchoconstriction than other forms of physical activity. This protective effect of swimming probably results from the high humidity of inspired air at water level, which reduces respiratory water loss and possibly osmolarity of airways mucus. The horizontal position of the body during swimming might also play a role by altering the breathing pathway and thereby producing less airway resistance than do other sports. Swimming is thus frequently recommended to asthmatics as a safe and enjoyable mean to maintain their lung function, to increase their aerobic capacity and to improve their quality of life. Data reported by Wicher et al.1 in this issue of Jornal of Pediatria add to the evidence that swimming has beneficial effects on the lung function of asthmatics. The authors conducted a follow-up study on 61 children and adolescents with mild atopic asthma who were randomized in swimmers and nonswimmers. The swimming program consisted of a total of 24 sessions of 60 minutes each over a period of 3 months. At the end of the training program, the authors found that the bronchial hyperresponsiveness (BHR) measured by the methacholine test was significantly reduced in swimmers while it did not changed in non-swimmers. The swimming pool attended by swimmers had openings in the walls near the roof and thus was presumably well ventilated. Although most studies investigating the effects of swimming on asthmatics, including that of Wicher et al.,1 were based on follow-up of less than 6 months, it is generally assumed that the benefits of swimming observed in these studies hold for much longer periods of training regardless the type of swimming pool and the method for disinfecting water. The deeply rooted idea that swimming in indoor pools can only be beneficial for asthmatics is however increasingly questioned by reports of respiratory problems among swimming pool workers,2 and competitive3 or recreational4,5 swimmers. In the case of competitive swimmers, the mechanical stress imposed on the airways by the intense training probably plays a role in these problems. Intense exercise, however, cannot be the explanation for the respiratory effects observed in lifeguards or infant swimmers who do not train in the pool. Thus, researchers in the field increasingly relate the poorer respiratory health of swimmers to the irritating effects of chlorine or of its by-products, which depending on type of swimming are inhaled as gases, microaerosols or even as small volumes of water (e.g. by infant swimmers). The current hypothesis is that these chemicals irritate the airways of swimmers and thereby make them more sensitive to environmental stressors such as allergens or infection agents.6 This interpretation is supported by biomarker studies showing that chlorination products can cause acute or chronic disruption of the lung epithelial barriers.7,8 Ironically, and adding to the confusion, the physicians who first alerted the medical community about the risks of pool chlorine had their attention drawn by the effects of these chemicals on the airways of their asthmatic patients.9,10 The vulnerability of airways of asthmatics to chlorination products was clearly demonstrated in a more recent study showing an increased BHR to methacholine in asthmatics after only a 12-minute immersion in a chlorinated whirlpool bath.11