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The chlorine hypothesis: fact or fiction?
  1. M J Nieuwenhuijsen
  1. Correspondence to:
 Dr M J Nieuwenhuijsen
 Department of Epidemiology & Public Health, Division of Primary Care & Population Health Sciences, Faculty of Medicine, Imperial College London, St Mary’s Campus, Norfolk Place, London W2 1PG, UK;m.nieuwenhuijsen{at}imperial.ac.uk

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Commentary on the paper by Nickmilder and Bernard (see page 37)

In this issue, Nickmilder and Bernard1 report on the relationship between the number of indoor chlorination swimming pools and prevalence of wheeze, asthma, hay fever, rhinitis and atopic eczema in European study centres of the International Study of Asthma and Allergies in Childhood (ISAAC) using an ecological study design with adjustment for several indicators such as social economic and lifestyle factors. They found statistically significant positive relationships for all outcomes for the whole of Europe, but fewer when examining specific regions of Europe. They found no statistically significant relationships within countries, but the number of study centres within a specific country was generally small. The relationship was stronger for 13–14-year-olds than for 6–7-year-olds.

Does this ecological study help to move the field forward? We are all aware of the strength and particularly the limitations of an ecological study design, and the interpretation of the results is generally far from straight forward. Over the recent years several analytical studies have been published that provide some support for a relationship between swimming or attending chlorinated swimming pools and respiratory disease, primarily asthma. Bernard et al2 found …

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