Recreational attendance
Table 2 summarizes studies that have investigated the respiratory effects of NCl3 and other inhalable CBPs among recreational swimmers. The first study was conducted among schoolchildren in Belgium51. While assessing the effects of ambient air pollution, the authors unexpectedly found that indoor chlorinated-pools attendance correlated with lung epithelium hyperpermeability and asthma prevalence. Further studies in Belgium revealed that the asthma risk among adolescents and children using chlorinated-pools stems from an interaction with atopic status52,53. Of interest, also the risk of allergic asthma was most strongly linked to pool attendance during early childhood52. Two studies in Sweden confirmed these observations while showing that the risk of allergic asthma correlated with the cumulative inhalation exposure to NCl354,55.
Associations between asthma and recreational swimming in chlorinated-pools were also reported in Ireland56, Italy57 and Portugal58. Several other studies, however, provided no evidence of an increased asthma risk in recreational swimmers59-62. Of these, one of the most contradictory but also the most influential owing to its large size is the UK prospective birth cohort study (ALSPAC study)61, which was suggestive, if anything, of a protective effect of swimming towards asthma risk. There are several possible explanations for these contrasting observations. A first explanation is that children examined in these negative studies59-62 were too young to detect associations with allergic asthma. Another possible explanation is an underestimation of the exposure with consequently a risk of miss-classifying some categories of swimmers63,64.
Associations have also been reported between indoor chlorinated-pool attendance and the risks of allergic rhinitis53,65, autonomic changes58 and airway inflammation assessed by the exhaled NO test52. Further complicating the issue, some studies suggest that early swimming in chlorinated-pools may increase the risks of allergic sensitization in particular to perennial allergens62,66,67.
Last, respiratory risks when swimming in chlorinated-pools are probably not limited to the exposure to NCl3. Swimming in indoor chlorinated-pools during infancy has also been associated with a higher risk of bronchiolitis or recurrent respiratory infection mainly in children with family history of atopic diseases9,68,69.
Biomarkers studies have revealed that the attendance of chlorinated-pools correlates with a disruption of airway epithelial barrier as reflected by altered serum levels of lung epithelium-specific proteins (pneumoproteins). Several studies have shown that early swimming in chlorinated-pools is associated with decreased serum levels of CC169,62,70,71. Lower levels of circulating CC16 in children are associated with subsequent decreased lung function and increased risks of developing asthma and other respiratory diseases71-74.