Results of a New Study May Change Guidance on the Use of Chlorine Stabilizers in Swimming Pools

SOURCE: Council for the Model Aquatic Health Code

Research suggests cyanuric acid and free chlorine concentrations in swimming pools should be regulated using a maximum ppm ratio of 20 to 1.

Decatur, Ga., July 30, 2019 ­– Swimming pool owners and operators should be using a maximum cyanuric acid to free chlorine ppm ratio of 20 to 1 to maintain stable chlorine levels and safely disinfect the water for use by swimmers, according to the results of a study published in the June 25, 2019 edition of the journal Water.

Cyanuric acid helps stabilize chlorine levels in swimming pool water by protecting the chlorine from being broken down by the sun’s ultraviolet rays. Without cyanuric acid, free chlorine levels can fall quickly, leaving the water vulnerable to harmful pathogens. However, cyanuric acid also reduces chlorine’s germ-killing power by lowering the concentration of hypochlorous acid. Hypochlorous acid is the primary active form of chlorine responsible for killing parasites and bacteria.

For decades researchers, operators, and regulators have debated the maximum level of cyanuric acid recommended for swimming pools to achieve a balance between chlorine stabilization and disinfection power. This study was the first to evaluate the maximum concentration of cyanuric acid relative to free chlorine based on the associated risk of gastrointestinal illnesses often transmitted at public swimming pools, including those caused by the parasites Cryptosporidiumand Giardia.

The study results demonstrate that while hypochlorous acid cannot be measured directly, its concentration in swimming pools varies as the cyanuric acid to free chlorine ratio varies. This led study authors to recommend that cyanuric acid and free chlorine levels be managed together as a ratio to minimize the risk of infection. 

Current pool codes, including the Centers for Disease Control and Prevention’s (CDC) Model Aquatic Health Code (MAHC), have independent limits for cyanuric acid and free chlorine concentrations. Moving pool operation to the use of a ratio is a different concept that will require pool operators to adjust the free chlorine target based on cyanuric acid readings. The following table from the study is a user-friendly tool to apply the cyanuric acid to free chlorine ratio of

20 to 1. 

Required minimum free chlorine for given cyanuric acid concentration.

CYA (ppm)

Min. FC (ppm)

















“The 50-year controversy over the cyanuric acid limit focused on the wrong question,” said lead study author Richard Falk.  â€śScientifically, we should be looking at regulating the cyanuric acid to free chlorine ratio.” 

The research originated from the Council for the Model Aquatic Health Code’s (CMAHC) Chlorine Stabilizers Ad Hoc committee, which was organized to develop guidance on appropriate stabilizer levels and use. The committee will use the results of the study to propose changes to the MAHC. The analysis will be used to create a Change Request for submission to the CMAHC for consideration at its October 2020 “Vote on the Code” conference. 

About the CMAHC

The CMAHC was created to manage updates to, and promote voluntary adoption of, CDC’s MAHC, the only all-inclusive national model pool code. The MAHC is kept sustainable, current, and complete because the people who use it also research, develop, discuss, and vote on proposed changes through membership in the CMAHC. The CMAHC submits these proposed changes to CDC as advice on how the MAHC needs to change to keep up with the latest science and best practices. Throughout the year, CMAHC staff educate local and state authorities, aquatic facility owners and operators, designers and builders, manufacturers, and suppliers about the MAHC and its potential to ensure a healthy and safe swimming experience for everyone. Learn more at

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