Pool chlorine hypothesis

The pool chlorine hypothesis is the hypothesis that long-term attendance at indoor chlorinated swimming pools by children up to the age of about 6-7 years is a major factor in the rise of asthma in rich countries since the late twentieth century. A narrower version of the hypothesis, i.e. that asthma may be induced by chlorine related compounds from swimming pools, has been stated based on a small numbers of cases at least as early as 1995.cite journal
last = Fjellbirkeland
first = L.
authorlink =
coauthors = Gulsvik A, Walloe A
title = (Swimming-induced asthma)
language = Norwegian
journal = Tidsskr Nor Laegeforen
volume = 115
issue = 17
pages = 2051-2053
publisher =
location =
date = 1995
url = http://www.ncbi.nlm.nih.gov/pubmed/7644982
doi =
id =
accessdate =2008-08-08 ] . An empirically motivated statement of the wider form of the hypothesis is first known to have been published on the basis of tests of the effects of nitrogen trichloride above chlorinated water on the lung as well as epidemiological evidence by a group of medical researchers led by Alfred Bernard of the Department of Public Health in the Catholic University of Louvain in Brussels, Belgium in 2003cite journal
last = Bernard
first = Alfred
authorlink =
coauthors = Carbonnelle S, Michel O, Higuet S, De Burbure C, Buchet JP, Hermans C, Dumont X, Doyle I
title = Lung hyperpermeability and asthma prevalence in schoolchildren: unexpected associations with the attendance at indoor chlorinated swimming pools.
journal = Occupational and Environmental Medicine
volume = 60
issue = 6
pages = 385-394
publisher =
location =
date = 2003-06
url = http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=12771389
doi =
id =
accessdate = 2008-08-07] . In the epidemiological studies, the association between chlorinated swimming pools and asthma was found to be more significant than factors such as age, sex, ethnic origin, socioeconomic status, exposure to domestic animals and passive smoking (in a study in Brussels), and independent of altitude, climate, and GDP per capita (in a Europe-wide study of 21 countriescite journal
last = Nickmilder
first = M
authorlink =
coauthors = Alfred Bernard
title = Ecological association between childhood asthma and availability of indoor chlorinated swimming pools in Europe
journal = Occupational and Environmental Medicine
volume = 64
issue =
pages = 37-46
publisher = British Medical Journal Publishing Group Ltd.
location =
date = 2006-07-17
url = http://oem.bmj.com/cgi/content/abstract/64/1/37
doi = 10.1136/oem.2005.025452
id =
accessdate = 2008-08-08] ).

Effects of nitrogen trichloride (trichloramine) on the human lung

Nitrogen trichloride has been directly linked as a factor causing asthma in two lifeguards and a swimming teacher.cite journal
last = K.M.
first = Thickett
authorlink =
coauthors = McCoach JS, Gerber JM, Sadhra S, Burge PS
title = Occupational asthma caused by chloramines in indoor swimming-pool air.
journal = European Respiratory Journal
volume = 19
issue = 5
pages = 827-832
publisher =
location =
date = 2002
url = http://www.ncbi.nlm.nih.gov/pubmed/12030720
doi =
id =
accessdate =2008-08-08 ] A study of 624 swimming pool workers found a significant correlation between upper respiratory symptoms and their total exposure to nitrogen trichloramine. The study also found an excess risk in the workers for the specific symptoms indicative of asthma.cite journal
last = J.H.
first = Jacobs
authorlink =
coauthors = Spaan S, van Rooy GB, Meliefste C, Zaat VA, Rooyackers JM, Heederik D
title = Exposure to trichloramine and respiratory symptoms in indoor swimming pool workers
journal = European Respiratory Journal
volume = 29
issue = 4
pages = 690-698
publisher =
location =
date = 2007
url = http://www.ncbi.nlm.nih.gov/pubmed/17107995
doi =
id =
accessdate =2008-08-08 ] In a study by Alfred Bernard's group, two hours exposure to an average concentration of 0.490 mg/m³ of nitrogen trichloride above a swimming pool was found in both children and adults to significantly increase the levels of the alveolar surfactant associated proteins A and B, which indicate hyperpermeability of lung epithelium. In other words, exposure to nitrogen trichloride was found to weaken the protective nature of the surface of the lungs.

Epidemiological studies

In a study of 341 schoolchildren, Bernard and his colleagues found that long-term attendance at indoor chlorinated swimming pools by the children up to the age of about 6-7 years was a strong predictor of airway inflammation (measured by exhaled nitric oxide) independently of other factors, while for those children susceptible to allergic problems, as defined by having a blood serum level of immunoglobulin E greater than 100 kIU/L, their total time spent at indoor chlorinated swimming pools was a strong predictor of the probability that they would have asthma.cite journal
last = Bernard
first = Alfred
authorlink =
coauthors = Sylviane Carbonnelle, Claire de Burbure, Olivier Michel, Marc Nickmilder
title = Chlorinated Pool Attendance, Atopy, and the Risk of Asthma during Childhood
journal = Environmental Health Perspectives
volume = 114
issue = 10
pages = 1567-1573
publisher =
location =
date = 2006-06-08
url = http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1626429
doi = 10.1289/ehp.8461
id =
accessdate =2008-08-07 ] .

Relations to demographic and environmental variables

In the Bernard group's study of 226 children in Brussels and the Ardenne region in 2003, asthma and exercise-induced bronchoconstriction (a test related to potential breathing difficulties) were not found to have any statistically significant correlation with the demographic and environmental factors of age, sex, ethnic origin, socioeconomic status or exposure to pets or passive smoking alone. However, when the time spent at chlorinated swimming pools (modified for pool height as a statistical way to indicate likely concentrations of chlorine related gases) was adjusted for exposure to pets and passive smoking, the significance of the correlations with asthma increased further. The authors describe this saying that a "very strong argument in [favour] of causality [between pool attendance and asthma] comes from the synergistic action of exposure to pets and [passive smoking] , two well documented risk factors for asthma, which together considerably increase the strength of the associations, to levels largely above those usually observed in asthma epidemiology."

In a later study by the Bernard group of 190,000 childrencite news | first=Medeleine | last=Brindley | pages= | language =| title=Pool chlorine may give children asthma | date=2006-07-18 | publisher=WalesOnline | url= http://www.walesonline.co.uk/news/wales-news/tm_objectid=17400353&method=full&siteid=50082&headline=swimming-pool-chlorine-may-give-children-asthma--name_page.html |accessdate=2008-08-08] in 21 countries in Europe, it was found that 13 to 14 year old children were 2% to 3.5% more likely to have or have had asthma for every additional indoor chlorinated pool per 100,000 inhabitants in their place of residence. Other atopic diseases such as hay fever or atopic dermatitis were found not found to be associated with the presence of the pools. The association of asthma with the number of indoor chlorinated swimming pools per 100,000 inhabitants was found by the authors to be independent of altitude, climate, and GDP per capita.

cientific debate on the epidemiological studies

After the publication of Bernard's group's 2003 study, B. Armstrong and D. Strachan described the study as "generally well conducted", but stated that some aspects of the statistical analysis and interpretation were "misleading", to the extent that "the epidemiological association of asthma with swimming pool use [was] not as strong as claimed by the authors".cite journal
last = Armstrong
first = B
authorlink =
coauthors = Strachan D
title = Asthma and swimming pools: statistical issues
journal = Occupational and Environmental Medicine
volume = 61
issue = 5
pages = 475
publisher =
location =
date = 2004-05
url = http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=15090674
doi = 10.1136/oem.2003.011205
id =
accessdate = 2008-08-09]

Following publication of Bernard's group's 2006 study, some concerns by P.A. Eggleston and a response by Bernard's group were published.cite journal
last =
first =
authorlink =
coauthors =
title = Correspondence - Letter: Eggleston PA - Response: Bernard A, Carbonnelle S, Nickmilder M - Erratum
journal = Environmental Health Perspectives
volume = 115
issue = 5
pages =
publisher =
location =
date = 2007
url = http://www.ehponline.org/docs/2007/10060/letter.html
doi =
id =
accessdate = 2008-08-09] For example, Eggleston argued that if "chlorinated compounds at indoor swimming pools could cause asthma", then "frequent and longer exposures at home" should be even stronger causes of asthma, in contradiction to the available evidence from a single group of children. Bernard's group's response was that while children at an indoor chlorinated pool "actively inhale [the chlorination products] as gases, aerosols, or even water", they are not usually involved in household cleaning tasks, so they could benefit from the hygienic effects of the chlorine based cleaning products while avoiding any significant contact with the related gases. Members of Bernard's group's declared that they had no potentially conflicting financial interests, while Eggleston declared that he had received money from the United States based group called the Chlorine Chemistry Council. In a "Faculty Disclosure" statement in an asthma-related publication, it was declared that Eggleston is "a consultant for Chlorine Chemistry Council, Church and Dwight, Merck Sharp & Dohme, and Procter & Gamble, and is on the speakers' bureau for AstraZeneca, GlaxoSmithKline, and Merck."cite web| last =| first =| authorlink =| coauthors =| title =Clinical Courier - Maximizing the Management of Pediatric Asthma: Today's Management Tomorrow's Potential| work =| publisher =The Office of Professional Education at National Jewish Medical and Research Center| date =| url =http://www.cmelogix.org/course/info.cfm/course_id/468 |format =| doi =| accessdate =2008-08-09]

Hypothesised mechanistic explanation

Alfred Bernard and colleagues argue that what is common to the pool chlorine hypothesis and epidemiological studies associating chlorine based irritants to atopy could be that frequent, long-term disruption of the epithelium of the lung, which normally provides a protective barrier against various pathogens, could allow allergens to cross this barrier. This process would also cause certain proteins from the lung epithelium to have increased blood serum concentrations.

Relation to the hygiene hypothesis

Independently of their own research results, Bernard and his colleagues have suggested that the hygiene hypothesis still lacks a close causal link with asthma, and that the hygiene hypothesis might be most relevant for a restricted number of types of infections, such as those due to parasites.

ee also

* hygiene hypothesis

References