WARNING: This topic may be controversial.
2003 Study of Indoor Chlorinated Pools
In 2003, Dr. Alfred Bernard (and others) published this article associating indoor chlorinated pool use with a greater incidence of asthma and seeing that nitrogen trichloride may be one of the causes.
This study made some sense in that nitrogen trichloride was shown in other studies to be a strong irritant and since Cyanuric Acid (CYA) was not used in most indoor pools and that many such pools may have had a somewhat higher (1 ppm FC or more with no CYA) chlorine level such that the nitrogen trichloride levels could be unacceptably high. This could be especially true with the lower amount of fresh-air exchange associated with modern energy-efficient indoor pool buildings. There were other studies by others later, but I'll talk about that after discussing a more recent study by Dr. Bernard.
2008 Study of Outdoor Chlorinated Pools
In 2008, Dr. Bernard published this article associating outdoor chlorinated pool use with a greater incidence of asthma. This study showed a large correlative increase between asthma and the number of cumulative lifetime (childhood) hours in chlorinated outdoor pools. The study used questionnaires given to parents of 14-16 year-old students in three French cities. This was a "snapshot-in-time questionnaire" study, not a longitudinal study that tracks what goes on over time to a random sample of subjects.
There were a few strange things reported in the full study (which I paid for and downloaded) which seemed at odds with the study conclusions. First is that "when considering the whole population, no significant association emerged between asthma and the attendance at indoor chlorinated pools, whether cumulative over lifetime or during early childhood." This seems at odds with Dr. Bernard's earlier study. At the town with the indoor copper-silver sanitized pool at school, the ever asthma was 10.6% and current asthma was 5.9% (out of 357 students) while at the other towns with indoor chlorinated pools, the ever asthma was 10.3% and 9.9% and the current asthma was 6.0% and 7.1% (out of 349 and 141 students), respectively. 95+% of the students spent time in indoor chlorinated pools. In addition, roughly half spent time in residential and nonresidential outdoor chlorinated pools and these outdoor pools were the focus of the study in terms of correlations. Also, students in the two towns without copper-silver pools spent (for the median) about 80% more time in their chlorinated indoor pools than those students did in the copper-silver pool (and total time in all pools as well), yet the overall population numbers don't show an increase in asthma. So whatever conclusions this study makes with regard to outdoor chlorinated pools needs to explain 1) why this study seems to contradict the earlier indoor chlorinated pool study and 2) what uniquely makes outdoor pools (both residential and nonresidential) different.
A second strange item was the statement in the study that "the cumulative attendance at either type [residential and nonresidential] of outdoor pools was indeed not significantly different between adolescents diagnosed with asthma and their peers without asthma." This makes absolutely no sense given the study's demonstrated correlation between the number of hours (cumulative attendance) in outdoor pools and the increased percentage of those with ever asthma or current asthma. Correlations are a two-way street: if X is correlated with Y, then Y is correlated with X. Correlations don't determine cause and effect, but if there was a correlation between cumulative time in outdoor pools and odds of asthma, then the converse HAS to be true as well -- that is, adolescents diagnosed with asthma have to have had higher cumulative attendance in outdoor pools. The only thing I can figure here is that the correlation claim between outdoor chlorinated pool time and asthma is done on a much smaller sample using students who had asthma while the converse statement I quoted above is looking at the much larger overall population.
The study talks about why it must be the chlorinated pools causing the asthma and not that children with asthma spent more time in pools by making the quote I gave above and by noting that doctors typically recommend using indoor pools where there is warm moist air and not swimming in general. I found it interesting that apparently the questionnaire didn't ask the parents if they changed their children's time in pools or the type of pools after the diagnosis. Also, there was no attempt at analyzing the pools themselves in terms of their water chemistry. It's true that this is impossible since a historical analysis would be needed, but it would be interesting to at least get an idea of what FC, CYA, etc. levels are at the various pools that were used. Dr. Bernard does note that residential pools tend to use stabilized chlorine products, but that's about all that is said on this subject.
2008 Meta-Analysis of Chlorinated Swimming Pool Studies
In 2008, this meta-analysis (i.e. a study of numerous studies) showed no significant correlation between swimming during childhood and asthma. So the Dr. Bernard studies should be taken in that context. [EDIT] A recent conference described here concluded a suggestive link that was not conclusive, pointed out certain problems with existing studies, and recommended specific protocols to determine clear associations. [END-EDIT]
The Dr. Bernard study published in 2008 speculates that it is the higher Free Chlorine (FC) levels typically found in outdoor pools that is the source of the problem and he speculates that it is hypochlorous acid outgassing from the pools. I had sent Dr. Bernard E-mails describing the chlorine/CYA relationship starting in August, 2007 and most recently in May, 2008 with info regarding the implications to nitrogen trichloride production. He never responded so probably didn't read it and in any event may have been too late for modifying his paper. Though it is not certain that the nonresidential pools used stabilized chlorine since sometimes they do not due to high bather loads that overwhelm the chlorine demand (so CYA isn't as helpful), the residential pools more likely than not used stabilized chlorine since it is not very economical to use chlorine otherwise in low-bather load pools exposed to sunlight. In such pools, the outgassing of hypochlorous acid is reduced by orders of magnitude due to CYA and the disinfection by-products are also reduced significantly due to the low bather load.
So if we take Dr. Bernard's 2008 study at face value (being extra conservative), what could cause asthma in children swimming in these pools? If it's not volatile disinfection by-products (due to low bather load) and it's not outgassing of chlorine itself (due to CYA) and it's not skin absorption (CYA skin absorption is negligible so chlorinated cyanurates are probably similar) and it's not drinking the water (since chlorinated drinking water has not seen these associations), then that pretty much leaves getting chlorinated water directly into the lungs or into breathing passageways. If pool water gets into the body, then the CYA level isn't very relevant and it is the Free Chlorine (FC) level that matters in terms of total exposure. The CYA will slow down reaction rates, but it is the FC level that is the total capacity and therefore total exposure. Though hardly scientific, I have noticed when swimming in public pools that it seems far more likely for young children to 1) get water up their noses or 2) swallow pool water into their lungs leading to coughing. Whether this is due to less coordination of breathing when young or the increased head dunking, splashing, and related activity, I don't know. To be clear, the distinction is between gaseous products that are unlikely to be at issue here vs. aerosols and larger quantities of chlorinated water.
If one wants to minimize exposure (and I'm not recommending this; just laying it out for those that want to know), then one simple thing would be to use nose plugs which would at least reduce the amount of chlorinated water going through that route. I'm not sure what one would do to reduce swallowing/gulping accidentally into the lungs. As for having a lower Free Chlorine (FC) level, that can be done at extra expense by using algicide products and possibly enzyme or non-chlorine shock products if the bather load is higher such that more oxidation is needed. Of course, one never knows what getting those products into the lungs might do so it's probably better to focus on reducing exposure to the lung of any pool water regardless of what is in it.