Recreation Management Rec Report - The Newsletter for Recreation, Sports & Fitness Facility Managers

Feature Story

September 2019

New Study May Change Use of Chlorine Stabilizers in Swimming Pools

By Chandler Garland

According to a new study published in the June edition of the journal Water, swimming pool 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.

Cyanuric acid stabilizes chlorine levels in swimming pool water by protecting the chlorine from the sun’s ultraviolet rays, which oxidize liquid chlorine and causes it to break down. Without cyanuric acid, free chlorine levels 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—the primary active form of chlorine responsible for killing parasites and bacteria. Thus, a well-researched balance of these two chemicals is necessary for healthy water.

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

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 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 50-year controversy over the cyanuric acid limit focused on the wrong question,” said lead study author Richard Falk. “Scientifically, 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.

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