The chlorine used in swimming pools may in part be causing the dramatic rise in childhood asthma, according to a disturbing new study to be published today, writes Dick Ahlstrom, Science Editor.
The research suggests that the resultant lung damage for frequent swimmers was equivalent to the damage seen in regular smokers.
The study of 226 primary school children, published in the British Medical Journal, shows that regular attendance at indoor swimming pools was consistently and significantly associated with damage to tissues in the deep lung.
Chlorine is used to disinfect swimming pools and by-products from its use can accumulate in the air surrounding indoor pools.
Asthma affects one in 10 children in Scotland and Belgium, according to one of the authors of the report.
The Irish Asthma Society says that Ireland is now ranked among the top four in the world of asthma sufferers, with asthma affecting one in seven Irish children.
The report shows that signs of the damage occurring could be seen after just one hour spent at the pool side, even without swimming.
"We were surprised to find this. We were very cautions about publishing this," according to Prof Alfred Bernard of the Catholic University of Louvain.
"Asthma was a very rare disease a century ago. Now it has become the most common disease in young children," he added yesterday.
"I think it is important to check on this lifestyle factor in asthma incidence. We have to be cautious about very young children, baby swimmers".
Chlorine readily reacts with organic matter such as urine or sweat in the water. This in turn forms a volatile by-product, nitrogen trichloride, which is inhaled while in the pool.
The study involved healthy primary school children from a mix of rural and urban schools.
They all had swum regularly at indoor pools at least fortnightly since early childhood, the authors report.
Blood samples were taken checking for levels of certain lung proteins associated with cellular damage and these "were already significantly increased after one hour on the pool side without swimming", the authors report.
They also found that the damage was cumulative and amongst children who swam most frequently, the damage was similar to that found in regular smokers.
Prof Bernard suggested that use of less chlorine or a different disinfectant or better ventilation to reduce nitrogen tricholoride levels might reduce the damaging effects.
Swimming is sometimes recommended for asthmatics, but this might not be the best approach given the findings, the authors point out.
Levels of nitrogen tricholoride could vary greatly, depending on how crowded the swimming pool, how clean the swimmers and how well ventilated the pool side, they added.