By Kelly A. Reynolds, MSPH, PhD

In the US, there are more than 8.8 million residential and public swimming pools. Representing one of the most common recreational activities among children and adults, there are more than 368-million-person visits to swimming sites per year.1 Microbial hazards, spread via the fecal-oral route, have always been a concern in pools but recently more attention has been focused on the risks of disinfection byproducts (DBPs) and strategies for control.

Urine and feces—how did it get there?
The more people you have in a swimming pool, the higher concentrations of contaminants you will have. Increased bather load contributes additional skin cells/sweat, urine, personal care products (i.e., sunscreen, lotions, perfumes, etc.) and residual feces. In fact, bathers can add several pounds of feces per day in a typical water park.2 The US Centers for Disease Control and Prevention (CDC) estimates that a water park serving 1,000 children a day can accumulate up to 22 pounds of poop.3

While not all feces carry pathogenic organisms, ill individuals can shed millions of harmful microbes during a single swim event. A single fecal accident can contaminate a water park with millions of gallons of waters and be easily spread by swallowing a few mouthfuls of water.1 Surveys indicate that swimmers continued to use public swimming sites even when symptomatic with diarrhea, despite health department recommendations to not go swimming for two additional weeks after symptoms resolve.

Urine is another common contaminant in pools, whether from intentional or accidental events. A person who swims for two hours excretes between 20 and 80 mL of urine into the water.4 In another study, it is estimated that in a 220,000-gallon public pool, there is approximately 20 gallons of urine, making up approximately 0.01 percent of the total water volume (ick!).

Artificial sweetener, a common ingredient in thousands of foods, drinks and medications, provides a tool for monitoring pool water quality.3,4 Certain sweeteners pass through the body, largely unabsorbed and thus are excreted directly in the urine of swimmers. Thus, the more people pee in the pool, the more that sweetener is detected. One study of 31 hotel and recreational facility pools found that 100 percent tested positive for the marker. While urine is not sterile, it does not pose a high risk for disease transmission but can throw off the balance of the pool chemistry required for chlorine efficacy.3

Chlorine efficacy concerns
Chlorine is the most common disinfectant used in swimming pools. In properly maintained pools, chlorine treatment is generally effective against common recreational waterborne viral (norovirus, rotavirus, adenovirus) and bacterial (Pseudomonas, Shigella, E. coli) hazards. Nearly two-thirds of recreational, waterborne disease occurs in chlorinated pools, however. Difficulty in maintaining proper chlorine levels and the presence of chlorine-resistant parasites, including Cryptosporidum, are primary causes of disease prevention failures. Cryptosporidium can survive in water at CDC-recommended chlorine levels (one to three mg/L) and pH (7.2 to 7.8) for > 10 days.5

According to the CDC, from 2014 to 2016, documented outbreaks of Cryptosporidium doubled in pools, from 16 to 30 cases. Although resistant to chlorine, the parasite can be removed via proper filtration, UV light and ozone. In the summer of 2014, the CDC released recommendations for states and local jurisdictions on how to reduce illness risks from swimming pools with a focus on microbial pathogens. Although these are voluntary guidelines, the Model Aquatic Health Code (MAHC) provides consistent, science-based standards of practice that did not exist previously. The MAHC recommends additional treatment for Cryptosporidium and is expected to dramatically reduce outbreaks associated with treated recreational water venues.

Disinfection byproduct risks
Although we can easily make a case for the need to continue use of chemical disinfectants to reduce microbial risks associated with pool water exposures, there are additional concerns relative to isolated chlorine disinfectant use, including DBPs. Most of us have heard about the risks in drinking water and the possible carcinogenic effects from drinking chlorinated supplies, but less is known about pool-water exposures. DBPs are not naturally present in water but form after halogenic disinfectants (like chlorine and bromine) combine and react chemically with organic matter in water to form potentially toxic compounds. Trihalomethanes (THM) and haloacetic acids (HAA) are two types of DBPs linked to adverse human health effects. The epidemiological evidence for DBPs and suspected increased risk of cancer, stillbirths, miscarriages and birth defects has been inconsistent and thus a dose-response relationship has been difficult to establish quantitatively.

Bladder cancer risks are most strongly associated with DBPs in water and prompted US EPA to set a legally enforceable standards for total trihalomethanes (TTHM) and HAA in drinking water at 80 and 60 µg/L, respectively.6 Exposure levels to THM and HAA in indoor swimming pools may be several orders of magnitude higher than recommended levels in drinking water. Estimating exposure levels in pools, however, is more difficult as we do not intentionally drink pool water at the same level and frequency.

Toxic effects of DBP exposures in pools typically manifest as respiratory/allergic symptoms and are most prevalent in babies and elite swimmers. Increased cancer risks have not been shown at low-level exposures estimated from swimming, other than in elite swimmers.4 Pool lifeguards, however, reportedly show a 40-percent increased risk of developing bronchial spasms compared to the general public, as well as a greater risk of chronic cough, sore throat and sinusitis, compared to less exposed employees, such as food-service staff. In general, experts believe the adverse effects of DBP exposures from pools have not been adequately evaluated and current studies are underestimating the risks.

Final thoughts
Exposure to microbes in both drinking and pool water sources remains the greatest risk of illness compared to potential exposures to DBPs. Minimizing DBP exposure, however, remains a secondary goal. This is best achieved by reducing the level of compounds in water that can react to form harmful compounds, targeting organic contaminants first. Pool operators should treat for algal contaminants and remove leaves or other deposited organic matter promptly. Swimmers can help by showering before entering a pool and, of course, not peeing or pooping in the pool. Swim diapers are minimally effective for containing urine and feces and thus should be changed frequently. Young children should be frequently reminded to use the bathroom. Alternative antimicrobial water treatment technologies, such as UV light, ozone, copper/silver and filtration, may also be considered. Whether used in combination with chlorine or alone, pool managers must not compromise efficient disinfection. Combined treatment trains can help to reduce levels of organic matter, remove chlorine-resistant Cryptosporidium cysts and minimize DBP exposures.


  1. Castor ML, Beach MJ. Reducing illness transmission from disinfected recreational water venues: swimming, diarrhea and the emergence of a new public health concern. Pediatr Infect Dis J. 2004;23(9):866-870. Accessed July 21, 2017.
  2. Gerba CP. Assessment of Enteric Pathogen Shedding by Bathers during Recreational Activity and its Impact on Water Quality. Quant Microbiol. 2000;2(1):55-68. doi:10.1023/A:1010000230103.
  3. McGinty JC. Is That Pool Really Sanitary? New Chemical Approach Has Answers. Wall Street Journal. Published July 21, 2017. Accessed July 21, 2017.
  4. Florentin A, Hautemanière A, Hartemann P. Health effects of disinfection by-products in chlorinated swimming pools. Int J Hyg Environ Health. 2011;214(6):461-469. doi:10.1016/j.ijheh.2011.07.012.
  5. Hlavsa, M, Roberts, VA, Kahler Am et al. Outbreaks of Illness Associated with Recreational Water–United States, 2011-2012. Morb Mortal Wkly Rep. 2015;64(24):668-672. Accessed July 21, 2017.
  6. Centers for Disease Prevention and Control. Disinfection By-Products | The Safe Water System. 2016. Accessed July 22, 2017.

About the author
Dr. Kelly A. Reynolds is an Associate Professor at the University of Arizona College of Public Health. She holds a Master of Science Degree in public health (MSPH) from the University of South Florida and a doctorate in microbiology from the University of Arizona. Reynolds is WC&P’s Public Health Editor and a former member of the Technical Review Committee. She can be reached via email at [email protected]


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