Chlorinated swimming pools, asthma, and DNA damage:

A guide for the science-minded parent

© 2010 Gwen Dewar, Ph.D., all rights reserved

Are swimming pools a health hazard?

Like most activities, swimming in public pools has a downside.

Despite attempts to disinfect the pool, some pathogens may still lurk in the water. And research suggests that disinfectants may pose their own health risks.

Swimming in chlorinated pools may put kids at higher risk for developing respiratory illnesses, including asthma and hay fever.

In addition, there is evidence that disinfectant by-products—formed when chlorinated water is mixed with microorganisms, human body fluids, cosmetics, and sunscreen—can damage your DNA and increase your risk of cancer.

This sounds rather alarming. Does it mean we should keep kids out of the pool? That depends on the pool. As I explain below, some swimming pools are riskier than others.

And keep this in mind: Most studies concern kids who swim regularly. A few visits to the "wrong" swimming pool are unlikely to cause health problems.

So here is an overview of the latest research on swimming pools, pathogens, and disinfectants. For tips on how to protect your child, check out the end of the article, "What parents can do."

Why swimming pools need to be disinfected

When used properly, chlorination protects swimmers from a wide range of dangerous pathogens, including

• E. coli, Rotavirus, Salmonella, and Shigella (each of which may cause gastrointestinal symptoms and, in some individuals, serious illness)

• Adenoviruses (which are linked with gastroenteritis, respiratory infections, eye infections, and other diseases)

• Pseudomonas aeruginosa (which can cause a variety of diseases, including pneumonia and urinary tract infections)

And before the introduction of effective vaccines, chlorination was one of the only ways to protect swimmers from diseases like polio and hepatitis A.

But no disinfection method is 100% effective. Some pathogens, like the protozoan parasites Giardia and Cryptosporidium, can survive the chlorination process.

Others are destroyed but only after sitting in chlorinated water for 20 minutes or more. If you are unlucky enough to be in the pool immediately after an infected child defecates in the water, your chances of contracting the disease is very high (Kababjian 1995).

And even if we assume that all the swimmers are healthy, there are other sources of waterborne disease. All natural sources of water contain organic matter—derived from decaying leaves, bird droppings, fungi-infested rainwater, and dead organisms.

This might suggest that we’d all be better off swimming in highly-chlorinated pools. But there are health risks associated with chlorination.

Chlorine + humans = harmful disinfectant by-products

Chlorine and chlorine compounds can form harmful by-products when they are mixed with other agents. Unfortunately, these other agents are common in swimming pool water. Perspiration, urine, saliva, hair, skin particles, feces—even cosmetics and sunscreens—all contribute to the formation of disinfectant by-products, or DBPs.

So the typical, chlorinated swimming pool contains DBPs. These can be absorbed through the skin and swallowed. Because the chemicals vaporize into the air, they are also inhaled.

Why is this harmful?

Researchers are still trying to understand how the body reacts to chlorinated water and DBPs.

Like chlorine itself, some DBPs are known respiratory irritants. Maybe long-term exposure to chlorinated water vapor causes inflammation of the air passageways. Maybe it causes oxidative stress, damaging the lungs (Weisel et al 2009).

It also seems that DBPs can damage the DNA.

In a recent study, researchers tested the effects of disinfected swimming pool water on mammalian DNA.

They prepared several solutions—each a different kind of disinfectant mixed with water—and applied them to living mammal cells (Liviac et al 2010).

The results were consistent with other studies (e.g., Woodruff et al 2001). All mixtures induced more DNA damage than plain tap water.

DNA damage is worrying because it increases the risk of cancer. And that’s borne out by research. In laboratory experiments, DBPs have caused cancer in rats (McDonald and Komulainen 2005). And studies have linked exposure to these DPBs with an increased risk of cancer in humans (Richardson et al 2007).

So we have good theoretical reasons to be suspicious about chlorinated pool water.

And there is epidemiological evidence, too.

Kids who spend more time in the swimming pool are more likely to suffer from respiratory diseases

The researchers found kids exposed to chlorinated pools before the age of 2 years were more likely to have developed bronchiolitis, an inflammation of the bronchioles.

Moreover, the effect was dosage-dependent. The more time spent in the pool, the higher the risk of bronchiolitis.

And the more researchers controlled for other risk factors—like attendance in day care or family history of allergy—the larger the effect.

Babies who frequented chlorinated pools were also more likely to become preschoolers with asthma or respiratory allergies.

Were these health problems caused by the chlorine? That’s still not clear.

We need to rule out the possibility that kids were affected by the infectious organisms known to inhabit swimming pools.

We also need to rule out the possibility of reverse causation.

Doctors often recommend that asthmatic children take up swimming. Maybe the link between chlorinated pools and respiratory health problems reflects the fact that swimming is the sport of choice for asthmatic kids.

One recent study has addressed these possibilities by including a control group.

Alfred Bernard and his colleagues examined over 800 Belgian teenagers who frequented swimming pools. Some kids swam in chlorinated swimming pools. Others used pools treated with copper-silver.

The teens who spent longer hours in chlorinated pools were more likely to suffer from asthma, hay fever, and other allergies. There was no increased risk of respiratory allergies in kids who had used copper-silver pools (Bernard et al 2009).

What parents can do

We need more studies that control for prior health problems and the presence of pathogens in the water.

And we need to know how much chlorine is in the water and air. Different countries set different standards for disinfection. A swimming pool in the United States might have ten times as much chlorine as a pool in Germany (Weisel et al 2009).

Does that mean German pools are safer than American pools? Maybe. But nobody’s attempted to answer that question yet.

Indeed, I haven't seen any studies that measure the concentration of disinfectant in the swimming pool water, let alone other factors, like the quality of poolside air.

Perhaps variation among swimming pools can explain why different studies have reported different outcomes. Maybe the studies reporting no effect concerned swimming pools with low levels of disinfectant--or particularly good ventilation.

Meanwhile, concerned parents can follow these guidelines.

• If you can smell the chlorine in the pool environment, there’s too much of it. That’s the rule of thumb offered by researchers Brent S. Rushall and Larry Weisenthal.

• If you frequently swim indoors, find a pool with good ventilation. That means avoiding pools that recirculate air (rather than replace it with fresh air). It also means avoiding pools in rooms with low ceilings.

• Avoid swimming pools that don’t insist on cleanliness—including showers before swimming. As noted above, perspiration and other body products mix with chlorine to produce disinfection by-products. When people wash with soap and water before entering the pool, they help minimize the generation of DBPs.

• Tell kids not to pee or spit in the pool. It isn’t just gross—it contributes to the creation of DBPs.

• Encourage your local pool manager to consider research-based alternatives to traditional chlorination. According to the researchers who conducted the DNA study, the safest approach to disinfection may be a combination treatment: UV irradiation with supplemental chlorination (Liviac et al 2010).

• If you are responsible for a pool or hot tub, don’t skip the disinfection process. As noted above, disinfection serves a crucial purpose. The answer isn’t to stop disinfecting pools. The answer is to find ways to disinfect while minimizing other health risks.

References

Centers for Disease Control and Prevention (CDC). 2010. Violations identified from routine swimming pool inspections--selected states and counties, United States, 2008. MMWR Morb Mortal Wkly Rep. 59(19):582-7.

Kababjian RS. 1995. Disinfection of Public Pools and Management of Fecal Accidents. Journal of Environmental Health 58

Liviac D, Wagner ED, Mitch WA, Altonji MJ, and Plewa MJ. 2010. Genotoxicity of Water Concentrates from Recreational Pools after Various Disinfection Methods Environ. Sci. Technol. 44 (9), pp 3527–3532

McDonald TA and Komulainen H. 2005. Carcinogenicity of the chlorination disinfection by-product MX. J Environ Sci Health C Environ Carcinog Ecotoxicol Rev. 23(2):163-214.

Richardson SD, Plewa MJ, Wagner ED, Schoeny R, and Demarini DM.2007. Occurrence, genotoxicity, and carcinogenicity of regulated and emerging disinfection by-products in drinking water: a review and roadmap for research. Mutat Res. 636(1-3):178-242.

Voisin C, Sardella A, Marcucci F, and Bernard A. 2010. Infant swimming in chlorinated pools and the risks of bronchiolitis, asthma and allergy. Eur Respir J. 36(1):41-7.

Weisel CP, Richardson SD, Nemery B, Aggazzotti G, Baraldi E, et al. 2009. Childhood asthma and environmental exposures at swimming pools: state of the science and research recommendations. Environ Health Perspect. 117(4):500-7.

Woodruff NW, Durant JL, Donhoffner LL, Penman BW, and Crespi CL. 2001. Human cell mutagenicity of chlorinated and unchlorinated water and the disinfection byproduct 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX). Mutat Res. 495(1-2):157-68.

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