Children who use an indoor swimming pool may be at increased risk of developing asthma, research suggests.
Chlorine exposure may be the key
The Belgian study found rates of childhood asthma and wheeze rose by around 2-3% for every indoor swimming pool per 100,000 people.
The researchers believe the key could be exposure to chlorine, which is used to keep pools clean.
The study, by the Catholic University of Louvain, appears in Occupational and Environmental Medicine.
The researchers say the long-term effects of chlorine by-products on children's respiratory health should be thoroughly evaluated, and that pools should be properly ventilated and levels of chlorine by-products regulated.
A study published three years ago by the same team produced evidence that chlorine in pools can react with sweat or urine to create harmful fumes which can damage lungs.
They found found that children who regularly attend indoor pools accumulate proteins known, in high quantities, to cause damage to cells in the lungs.
In the latest study, they analysed the rates of wheezing, asthma, hay fever, allergic rhinitis and atopic eczema, reported in a study of almost 190,000 13 and 14-year-olds from 21 countries across Europe.
They then compared these rates with the number of indoor chlorinated swimming pools per 100,000 of the population in each of the countries.
The number of indoor pools varied by a factor of 20 between Eastern and Western Europe, ranging from one pool for every 50,000 people in Western Europe to one for every 300,000 people in Eastern Europe.
The results showed that, even after taking account of factors such as wealth, climate and altitude, there seemed to be a strong link between asthma and related disorders and potential access to swimming pools.
The rate of wheezing rose by 3.39% for every additional indoor chlorinated swimming pool. Similarly, the rate of asthma rose by 2.73%.
The researchers argue that the rise of asthma in Western Europe could at least partly be attributed to the increasing exposure of children to the by-products of chlorine in the air and water of indoor swimming pools.
A spokesperson for the charity Asthma UK said: "Swimming is an excellent form of exercise for children with asthma as the warm humid air in the swimming pool is less likely to trigger asthma symptoms.
"We do recognise, however, that the chemicals present in heavily chlorinated pools may be important in making the airways more irritable and further research is needed to understand this association."