Changes to the Model Aquatic Health Code (MAHC) are slated to come out sometime during the summer of 2021. Among the most significant alterations to the code may be those involving cyanuric acid. A variety of proposals concerning how to approach cyanuric acid have been considered.

The current MAHC sets a minimum free available chlorine level of 1 ppm for pools not using cyanuric acid, and 2 ppm for pools that do use cyanuric acid with a FAC maximum of 10 ppm and a cyanuric acid maximum of 90.

• Without CYAPools: 1-10 ppm FAC.

• With CYA Pools: 2-10 ppm FAC.

• Max CYA 90 ppm.

Over the course of the past few years, however, there has been a push to set recommended chlorine concentrations as a ratio of cyanuric acid concentrations as it has been recognized that the hypochlorous acid concentration is proportional to the ratio of those two chemicals.

Because hypochlorous acid is the agent responsible for oxidation and disinfection, it makes sense to recommend a set value for an ideal concentration of hypochlorous acid in pools.

The trouble is that pool and spa operators cannot measure hypochlorous acid levels directly. But since hypochlorous acid concentrations are proportional to a ratio of cyanuric acid and free available chlorine – which can be measured directly – it is possible to calculate the resulting hypochlorous acid concentration. This calculation isn’t particularly easy, and involves manipulating 12 equilibrium expressions and associated equilibrium constants.

An excel spreadsheet to calculate HOCl was made available to the public in the supplementary materials of the study “Assessing the Impact of Cyanuric Acid on Bather’s Risk of Gastrointestinal Illness at Swimming Pools,” by Richard Falk et al.

The following HOCl concentrations were calculated using this spreadsheet at pH 7.5, a temperature of 25°, and TDS of 800 ppm.

The minimum allowable HOCl concentration dictated by the current MAHC corresponds to 0.008 ppm HOCl, which results when the free available chlorine is 2 ppm and the cyanuric acid is maxed at 90 ppm. Those limits correspond to a ratio of 45:1 CYA: FC.

However other authorities have different recommendations concerning minimum free available chlorine.

For example, the APSP’s 4th edition Service Tech Manual recommends free chlorine levels between 2 and 4 ppm with a maximum cyanuric acid level of 100 ppm. That translates to a minimum HOCl concentration of 0.0078 when the free chlorine is 2 and the cyanuric acid is 100. Those limits correspond to a ratio of 50:1 CYA:FC.

Meanwhile, the Independent Pool and Spa Service Association, or IPSSA and the Pool Chemistry Training Institute service tech manuals both recommend using a ratio approach where the free chlorine would be maintained at 7.5% of the CYA, which would eliminate upward limits to both cyanuric acid and free chlorine, were it not for the fact that the EPA guidelines say that swimmers should not enter the water when the free chlorine is above 4 ppm.

At 4 ppm free chlorine, the corresponding cyanuric acid should be 53 ppm. When chlorine is 7.5% of cyanuric acid levels, then, the HOCl concentration is about 0.03 ppm. And when chlorine is 7.5% of cyanuric acid, the ratio of cyanuric acid to chlorine is 13.3:1 CYA:Cl. These recommendations are based on preventing algae growth.

Finally, after 4 years of work, the Council for the Model Aquatic Health Code also recommends a ratio approach of cyanuric acid to chlorine of 20:1 CYA:Cl. This is based on the risk of gastrointestinal illness from mostly Giardia. For typical free chlorine and cyanuric acid ranges in pools (CYA between 20 and 90 ppm and free chlorine between 1 and 4.5 ppm), this corresponds to an HOCL concentration of 0.02 ppm HOCl.

These levels are summarized in the accompanying table, Figure 4: Existing standards and recommendations on free chlorine and cyanuric acid. * The EPA allows a maximum 4 ppm free chlorine. If the 7.5% rule is followed, it results in a maximum CYA of 53.2 ppm According to the Council for the Model Aquatic Health Code’s webpage, the MAHC is kept current and sustainable by the people who use it, which includes CMAHC Ad Hoc Committees, who submit change requests to the code that are reviewed by the Technical Review Committee.

The change requests concerning how the pool and spa industry should approach cyanuric acid has included recommendations that the industry use a cyanuric acid to free chlorine ratio approach, and the review committee has considered and agreed to ratios up to 30:1 cyanuric acid to free chlorine.

However, some have expressed concern that a ratio approach might be confusing to pool and spa operators and feel that it might be easier for service techs to remember ranges.

So, it has been proposed to set chlorine levels between 2 and 4 ppm and limit cyanuric acid to no more than 90 ppm, with chlorine adjusted up as the cyanuric acid rises by about 30 ppm increments. (see accompanying table of one range considered) But a problem with such a proposed sliding scale is that it results in varying cyanuric acid to free chlorine ratios.

So, for those who like the ratio approach, the range approach is not as elegant. But for those who like the range approach, the ratio approach may be too difficult.

It will be interesting to see which approach is published this summer.