'Asthma risk' from swimming

“Children who regularly use indoor swimming pools may be more likely to develop asthma,” the Daily Mail has reported. The news comes from Belgian research that claims that the chlorine used in pools can increase a youngster's risk of asthma up to six-fold. Rates of hayfever and other types of allergies are also said to be increased.

This study of adolescent students appears to have been well designed, comparing a number of asthma measures in several sub-groups with different types of allergies, taken from across three schools. As one of these schools sanitised its swimming pool water without chlorine, the researchers were able to use it to provide a comparison group of swimmers not exposed to chlorine in their pool use.

The findings showed that asthma measures worsened as lifetime exposure to chlorine increased, but only among those who were already sensitised to irritants or allergies. The implications for people who have swum in chlorinated pools for less than 100 hours over a lifetime, or for those who do not have allergies, are less clear.

This research will be of interest in the debate over asthma and chlorine, with other researchers saying that the research was not conclusive enough to make them advise parents against indoor pools. Dr Elaine Vickers, of Asthma UK, for example, is reported as saying that, "Asthma develops as a result of a complex mix of genetic and environmental factors, so more research is needed before we can make a conclusive link with the use of chemicals in swimming pools."

Where did the story come from?

This research was conducted by Dr Alfred Bernard and colleagues from the public health department of Catholic University of Louvain in Brussels. It was supported by the National Fund for Scientific Research in Belgium, and the Agency for Environmental and Occupational Health Safety in France, plus a number of other organisations.

The study was published in the peer-reviewed medical journal Pediatrics.  

What kind of scientific study was this?

The researchers wanted to estimate the strength of the link between chlorinated pool exposure and allergic diseases among adolescents. This link has been suggested before, but the overall influence of the irritant byproducts of chlorine on allergic diseases has not yet been fully evaluated.

This cross-sectional study assessed this link in 847 students recruited from three secondary schools in the southern Belgium. This total represented around 70% of the girls and 72% of the boys originally asked to participate. The participants were selected from all 13- to 18-year-old students at two schools who were known to have attended outdoor or indoor chlorinated pools. The researchers compared these students with a control group from the third school, made up of students who all swam at an indoor pool sanitised through a different process using copper and silver. This copper-silver pool was sanitised with concentrations of copper (0.6 –1.2 mg/L) and silver (2–10 micrograms/L) that were within acceptable levels for drinking water standards.

The students completed a health questionnaire that included questions intended to estimate the total time they had spent in indoor or outdoor chlorinated pools. The researchers then took blood tests to measure levels of overall and aeroallergen-specific immunoglobulin E (IgE), an indicator of whether the child has an allergy to inhaled triggers of asthma. They also screened for exercise-induced bronchoconstriction by making the adolescents run indoors for six minutes, with children considered positive if the exercise caused a decrease in measures of asthma function.

The researchers asked about respiratory symptoms, hayfever, allergic rhinitis and asthma that had been diagnosed at any time (termed 'ever asthma') or was being treated with medication or associated with exercise-induced constriction of the airways (current asthma). They tested 26 potential factors thought to be linked to asthma, including cumulative lifetime chlorinated pool attendance (CPA), which was stratified into four categories: under 100 hours, 100 to 500 hours, 500 to 1,000 hours, or over 1,000 hours. The other information they recorded from the questionnaire included gender, maternal smoking during pregnancy and parental asthma or allergy.

What were the results of the study?

Among the 532 adolescent students who had high levels of IgE (suggesting they were sensitive to allergies) the chance of having asthma symptoms or a diagnosis of current asthma or ever asthma increased with greater number of lifetime hours spent in chlorinated pools.

The researchers tested the significance of these increases in risk in a range of groups. For example, the odds ratio for having current asthma increased with exposure and reached values of 14.9 when lifetime chlorinated pool attendance exceeded 1,000 hours. This means that those students with IgE levels over 30kIU/L were 14 times more likely to have asthma if they had swum in chlorinated pools for more than 1,000 hours over their lifetime. This was a statistically significant trend. 

Some of the allergic adolescents who swam in chlorinated pools for more than 100 hours also had a greater risk of hayfever, and those with attendance of more than 1,000 hours had greater risk of other forms of allergic rhinitis. These links were not found among adolescents who were not allergic, or in those with allergies who had a lifetime exposure to chlorine of less than 100 hours.

What interpretations did the researchers draw from these results?

The researchers say that, “Chlorinated pool exposure seems to contribute significantly to the burden of asthma and respiratory allergies among adolescents.”

What does the NHS Knowledge Service make of this study?

This study has several advantages in that a large number of similar participants were selected from schools where pupils naturally had very different levels of exposure to chlorine swimming pools, while the lack of exposure to chlorine in the copper-silver pool allows for a natural control group to compare against.

The study also showed several dose-response effects, meaning that increasing exposure in terms of lifetime hours exposed to chlorine was linked to increasing amounts of respiratory disease. These features increase confidence in the results, but there are several points that should also be considered:

• Not all the dose-response associations tested by the researchers showed significant trends, and it is possible that some of the significant differences may have arisen by chance.
• The students were reasonably similar across all schools. However, it is also possible, although unlikely, that the effect could be explained by some other feature in the school where pupils swam in the copper-silver pools. For example, students from the Louvain-la-Neuve school with a copper-silver pool had higher socioeconomic status higher, reflected by parental education levels and several lifestyle factors, such as breastfeeding and exposure to tobacco smoke. These may all have influenced the development of asthma.

Overall, this study provides reasonable evidence that pool chlorine is a factor that is associated with various allergies but because of the study design, (outcomes and exposures were recorded at the same time), and the fact that the trends were only seen in some groups, it is not yet certain how important a factor it is.