Research reported in the spring issue of the Irish Medical Journal, reflects a link first identified in Belgian research in 2003 between the prevalence of asthma and the number of years boys had been swimming in chlorinated swimming pools.

A study of 121 six to 12-year-old boys attending a national school in Cork city, carried out by Dr Tony Ryan and colleagues in the Department of Paediatrics at University College Cork, questioned how often the boys went swimming in chlorinated indoor pools. The results showed that most boys did so once a week. They had been swimming for an average of five years at the time of the study. No link was found between prevalence of asthma and parental smoking but a significant link existed between the number of years spent swimming and a diagnosis of asthma, as well as whether a boy had experienced wheezing in the previous 12 months.

The authors of the Irish study say a range of chlorine products are used in swimming pools, including chlorine gas. ‘When organic matter is introduced into chlorinated water (eg urine and sweat), a harmful mixture of by-products is created,’ they say, adding that the most concentrated by-product found is nitrogen trichloride. This is a known respiratory irritant, and the authors suggest that chronic exposure to an indoor, chlorinated environment may be a risk factor in the development of asthma in boys.

While acknowledging their study is relatively small and relies on subjective responses by parents, the Cork paediatricians conclude:
‘Until the relationship between respiratory health and chlorination of swimming pools is investigated further and accurate information is available, every effort should be made to improve pool ventilation and enforce better swimmer hygiene.’
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A further study by Dr. Alfred Bernard, a toxicologist at the Catholic University of Louvain in Brussels suggests that that ‘pool chlorine could be an important factor implicated in the epidemic of allergic diseases affecting the westernised world. The impact of these chemicals on the respiratory health of children and adolescents, they say, appears to be much more important - at least by a factor of five - than that associated with secondhand smoke’.

In the this study, Bernard and colleagues compared the health of 733 adolescents, 13 to 18 years old, who swam in chlorinated outdoor and indoor pools for various amounts of time with that of 114 control adolescents who swam mostly in pools sanitized with a concentration of copper and silver. In children with allergic sensitivities, swimming in chlorinated pools significantly increased the likelihood of asthma and respiratory allergies whereas those who swum on copper-silver sanitised pools had no increased risk.

Among ‘sensitive’ adolescents, the odds for hay fever were between 3.3- and 6.6-fold higher in those who swam in chlorinated pools for greater than 100 hours and the odds of allergic rhinitis were increased 2.2- to 3.5-fold among those who logged more than 1000 hours of chlorinated pool time.

For example, among children and teens who swam in chlorinated pools for 100-500 lifetime hours, 22 children out of 369 (6.0%) had current asthma, compared with those who had spent less than 100 hours (2 of 144, 1.8%). The proportions with asthma rose with longer exposure, to 14 out of 221 (6.4%) who had been swimming for 500-1000 hours, and 17 out of 143 (11.9%) who swam for more than 1000 hours.

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Pediatrics, October 2009.

Comment from Asthma UK

Dr Elaine Vickers, Research Relations Manager at Asthma UK, says: 'There are a few studies which suggest that the chemicals present in indoor swimming pools, like chlorine, may be involved in the development or worsening of asthma and other allergic conditions. This is due to the fact that the chemicals in the water may compromise protective cell barriers within the lungs, meaning people with allergic asthma are more vulnerable to allergens.

'However, asthma develops as a result of a complex mix of genetic and environmental factors, so more research is needed in this area before we can make a conclusive link between asthma and the use of chemicals in swimming pools.

'Swimming is an excellent form of exercise for children and young people with asthma as it can help improve lung capacity and the warm, humid air inside indoor pools is less likely to trigger asthma symptoms. We would therefore advise parents of children with asthma not to worry about letting their child go swimming, unless they develop asthma symptoms in the pool environment.'

Click here for more research on the possible causes of asthma

07/9 and 09/09

First Published in July 2009 Updted in September 2009

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