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Open AccessArticle

Assessing the Impact of Cyanuric Acid on Bather’s Risk of Gastrointestinal Illness at Swimming Pools

The Council for the Model Aquatic Health Code Ad-Hoc Committee, P.O. Box 3121, Decatur, GA 30031-3121, USA
Lyles School of Civil Engineering and Division of Environmental & Ecological Engineering, Purdue University, 550 Stadium Mall Drive, West Lafayette, IN 47907-2051, USA
Tom Kuechler, LLC, 531 Westernmill Drive, Chesterfield, MO 63017-2736, USA
Innovative Water Care, LLC, P.O. Box 800, 1200 Old Lower River Road NW, Charleston, TN 37310-0800, USA
Swim-Chem Consulting Services, LLC, 1241 Northwestern Drive, Monroeville, PA 15146-4403, USA
Environmental Public Health Program, University of Wisconsin–Eau Claire, Hibbard Humanities Hall 360, 124 Garfield Ave., Eau Claire, WI 54701-4042, USA
Author to whom correspondence should be addressed.
Water 2019, 11(6), 1314;
Received: 31 May 2019 / Revised: 20 June 2019 / Accepted: 20 June 2019 / Published: 25 June 2019
Current regulatory codes for swimming pool disinfection separately regulate free chlorine (FC) and cyanuric acid (CYA). It is well-known that CYA affects disinfection rates by reversibly binding to FC in aqueous solutions. However, limits for these regulated parameters have neither systematically accounted for this chemistry nor been based on the risk of gastrointestinal illness. This study was intended to determine the minimum concentration of FC relative to CYA based on the risk of gastrointestinal illness from normal fecal sloughing of selected pathogens and to find a simple regulatory rule for jointly managing FC and CYA for consistent disinfection. Literature data on CYA’s effect on microbial inactivation rates were reanalyzed based on the equilibria governing hypochlorous acid (HOCl) concentration. A model was developed that considers the rates of pathogen introduction into pool water, disinfection, turbulent diffusive transport, and pathogen uptake by swimmers to calculate the associated risk of illness. Model results were compared to U.S. Environmental Protection Agency (EPA) untreated recreational water acceptable gastrointestinal illness risk. For Cryptosporidium, correlation between log inactivation and Chick–Watson Ct was far better when C refers to HOCl concentration than to FC (r = −0.96 vs. −0.06). The HOCl concentration had a small variation (± 1.8%) at a constant CYA/FC ratio for typical FC and CYA ranges in swimming pools. In 27 U.S. states, the allowed FC and CYA results in HOCl concentrations spanning more than a factor of 500. Using conservative values for a high bather load pool with 2 mg/L FC and 90 mg/L CYA, the model predicted a 0.071 annual probability of infection for Giardia, exceeding the EPA regulatory 0.036 limit for untreated recreational waters. FC and CYA concentrations in swimming pools should be jointly regulated as a ratio. We recommend a maximum CYA/FC ratio of 20. View Full-Text
Keywords: chlorine; cyanuric acid; disinfection; recreational water; risk assessment chlorine; cyanuric acid; disinfection; recreational water; risk assessment
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Falk, R.A.; Blatchley, E.R., III; Kuechler, T.C.; Meyer, E.M.; Pickens, S.R.; Suppes, L.M. Assessing the Impact of Cyanuric Acid on Bather’s Risk of Gastrointestinal Illness at Swimming Pools. Water 2019, 11, 1314.

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