Lung hyperpermeability and asthma prevalence in schoolchildren: unexpected associations with the attendance at indoor chlorinated swimming pools

A Bernard; S Carbonnelle; O Michel; S Higuet; C de Burbure; J-P Buchet; C Hermans; X Dumont; I Doyle

doi:10.1136/oem.60.6.385

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Lung hyperpermeability and asthma prevalence in schoolchildren: unexpected associations with the attendance at indoor chlorinated swimming pools

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Asthma and swimming pools: statistical issues
Alfred M. Bernard
Published on: 27 April 2016
Asthma and swimming pools: statistical issues
Ben G Armstrong
Published on: 27 April 2016
Asthma in young children
David C Coward
Published on: 27 April 2016

Published on: 27 April 2016
Asthma and swimming pools: statistical issues
- Alfred M. Bernard, Professor and Research Director
- Other Contributors:
  Claire de Burbure
Dear Editor
Although we appreciate the interest of Dr Armtrong and Dr Strachan for our paper on the pool chlorine/asthma risk,[1] we cannot really take on board their reasoning concerning the statistical analyses. When questioning the strength of the associations found in our studies, they seem indeed to attribute much importance to the p values of the associations emerging between cumulated pool attendance and ind...
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Dear Editor
Although we appreciate the interest of Dr Armtrong and Dr Strachan for our paper on the pool chlorine/asthma risk,[1] we cannot really take on board their reasoning concerning the statistical analyses. When questioning the strength of the associations found in our studies, they seem indeed to attribute much importance to the p values of the associations emerging between cumulated pool attendance and indicators of asthma or lung epithelium permeability. The p values, however, are not reliable indicators to judge of the strength of associations found in epidemiology since they are highly dependent on the number of observations. Our assessment of the strength of these associations was therefore based more on the values of r² and on the fact that the associations found with pool chlorine exposure were much stronger than those emerging (and a fortiori not emerging) with other variables classically presented as possible contributors to asthma and lung damage in children (e.g. ETS, pets, outdoor pollution).
As to the third study linking asthma prevalence and pool attendance, we agree of course that this is a retrospective ecological study carried out by aggregating data from each school, which was made possible thanks to the fact that pool attendance is a compulsory activity in Belgian primary schools. However, since this study was not specifically designed to assess the effects of pool chlorine, in our opinion, its major weakness lies less in this school -based aggregation than in the fact that we could not quantify the cumulated pool chlorine exposure of these children on an individual basis, some of them having certainly attended a chlorinated pool with their parents (recreational, baby swimming) or as part of a sport activity. This is the reason why we cautiously concluded our paper by recommending further studies to test this chlorine hypothesis. We have now just completed such a study exploring the links between asthma, lung inflammation, atopy and cumulated exposure of children to pool chlorine. The results clearly show that associations published in OEM were far from having been overestimated (Bernard et al.; manuscript under preparation).
Reference
(1) Armstrong BG, Strachan D. Asthma and swimming pools: statistical issues [electronic response to Bernard et al. Lung hyperpermeability and asthma prevalence in schoolchildren: unexpected associations with the attendance at indoor chlorinated swimming pools] occenvmed.com 2003http://oem.bmjjournals.com/cgi/eletters/60/6/385#86
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Conflict of Interest:
None declared.
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Published on: 27 April 2016
Asthma and swimming pools: statistical issues
- Ben G Armstrong, Reader in Epidemiological Statistics
- Other Contributors:
  David Strachan.
Dear Editor
Bernard et al.[1] presented results from several studies investigating childhood asthma in relation to swimming pool use. Though the studies were generally well-conducted, there are some respects in which the statistical analysis and interpretation are misleading.
The study of asthma prevalence in relation to swimming pool use was essentially an ecologic design - the unit of analysis was...
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Dear Editor
Bernard et al.[1] presented results from several studies investigating childhood asthma in relation to swimming pool use. Though the studies were generally well-conducted, there are some respects in which the statistical analysis and interpretation are misleading.
The study of asthma prevalence in relation to swimming pool use was essentially an ecologic design - the unit of analysis was the school. Though the study of correlations between asthma prevalence and indexes of pool use respects this (the p-values are appropriate), the logistic regression analysis does not. The analysis is carried out as if there were 1881 independent observations of asthma and swimming pool use. In fact, observations are not independent - there is “clustering” of asthma by school - even after allowing for effects of swimming pool use and other covariates. The extremely low p-values in Figure 6 therefore cannot be relied on. The analysis applied to the study of chronic effects on lung epithelium is also limited in not allowing for possible clustering by school. Again, the p-values presented overestimate the strength of evidence for an association.
Caution is also required in interpreting the correlations and p- values in Figures 5C and 5D. These do not test the association of asthma with pool use, but with a composite index of pool use, pets, and passive smoking. It is not possible from the results presented to distinguish the contribution of each. The authors acknowledge this, but reader confusion may arise because the term "adjusted" is more usually used in epidemiology to describe adjustment for confounding of one effect by another - this is not the case here.
Finally, the correlations shown in figures 5A and 5B are selected from a wider range of measures of pool attendance, as shown in figure 4. Among these non-independent indices of exposure, the authors have chosen the one showing the strongest correlation with asthma prevalence. For this reason, the "significance" of the p-value in figure 5B should be interpreted as suggestive rather than definitive.
We conclude that the epidemiological evidence for an association of asthma with swimming pool use is not as strong as claimed by the authors.
Ben Armstrong
Public and Environmental Health Unit
London School of Hygiene and Tropical Medicine
Keppel St
London WC1E 7HT
David Strachan
Department of Public Health Sciences
St. George's Hospital Medical School
Cranmer Terrace
London SW17 0RE
References
(1) Bernard A, Carbonnelle S, Michel O, et al. Lung hyperpermeability and asthma prevalence in schoolchildren: unexpected associations with the attendance at indoor chlorinated swimming pools. Occup Environ Med 2003;60:385-94.
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Conflict of Interest:
None declared.
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Published on: 27 April 2016
Asthma in young children
- David C Coward, Retired
Dear Editor
Having suffered from Asthma all my life I am surprised that common hasards in the daily life of children are not addressed even in the 21st century. Chlorine in swimming baths was always a problem to me as a child, it was used far more concentrated than nowadays.
Another very serious trigger was to be found in cardboard containers of fruit drinks though. I now realise that they contained Sodium...
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Dear Editor
Having suffered from Asthma all my life I am surprised that common hasards in the daily life of children are not addressed even in the 21st century. Chlorine in swimming baths was always a problem to me as a child, it was used far more concentrated than nowadays.
Another very serious trigger was to be found in cardboard containers of fruit drinks though. I now realise that they contained Sodium Metabisulphide as a preserving agent. When the straw was pushed through a small hole in the carton a high dose of sulphur dioxide was passed into the stomach with the drink, soon passing into the lungs in concentrations that caused me great distress. Because of the 'closed loop' nature of the ingestion, there is no way of the preservative to vent into atmosphere first.
I believe this packaging and contents are still sold, mainly to youngsters, who don't understand how damaging it is to the vulnerable. Parents should be made aware of this danger too.
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Conflict of Interest:
None declared.
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