The Great Cyanurate Debate

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Cyanuric acid plays a vital role in pool sanitizing because of its role in protecting chlorine residuals from degradation in sunlight. The enduring question, however, is how much is too much?

Some chemistry experts argue that anything over 50 ppm is too high because CYA not only protects chlorine from the sun, it also ties it up and renders most of it ineffective as CYA levels increase. The long-recognized standard for CYA levels is 100 ppm (or 90 ppm, depending who you ask) — a level some say is much too high, while others believe it is artificially low.

Proponents of products that contain CYA, also known as isocyanurates, argue that there is no evidence to suggest that levels above 100 ppm result in any type of water quality problems, and therefore, changing the standard is not only unnecessary but possibly harmful. Those who think the recommended level should be lowered say it’s a long overdue adjustment based on the fundamental chemistry behind chlorine and CYA.


While there is never a shortage of people with strong opinions on both sides of the debate, the recent surge in the discussion has largely been driven by veteran chemist, author and educator, Robert Lowry.

“The method I’m now teaching is based on in-depth research into cyanuric acid, and specifically chlorine’s effectiveness in the presence of CYA,” Lowry explains. “My reexamination of the CYA/ chlorine relationship started when I was writing the Independent Pool and Spa Service Association’s two chemical training manuals, which I wrote in 2006 and 2009 respectively. During that time, I received a correspondence from theoretical chemist Richard Falk, who told me that some of the information in the manual was incorrect, especially where CYA and chlorine are concerned.”

RELATED: Keeping Pool Water Balanced

Lowry recalls that Falk supported his feedback with a number of citations. “One of those came from an out-of-print book titled ‘The Chemistry of Water Supply, Treatment and Distribution’ by Jay O’Brien.”

Falk pointed Lowry to a chapter about the equilibria of isocyanurates. According to O’Brien, there was more chlorine tied up by CYA than Lowry had previously believed. “I subsequently spoke with some of the chemists at Monsanto, which makes cyanuric acid, and also some of the companies that manufacture trichlor,” he recalls. “These conversations confirmed the view that there is indeed an equilibrium relationship between chlorine, and I came to realize that at any given time, you only have 3 - 4% chlorine available to do anything. That led me to look at the level of chlorine we need to kill algae and bacteria.”


The upshot of Lowry’s calculations indicated that the free chlorine required for any pool is 7.5% of the CYA level in the water as a minimum or as a target.

“If CYA increases,” he explains, “this requires a compensating rise in free chlorine. My method calls for a maximum CYA of 50 ppm, for which the free chlorine required would be 3.75 ppm (50 ppm x 0.075). This falls within the APSP guideline of 2.0 to 4.0 ppm, but 2.0 would not be enough to control algae.”

The mechanism driving that recommendation is not new. It’s based on the idea that when chlorine and CYA are in the water, the chlorine binds to CYA and that is determined by chemical equilibrium.

“It is up to you to know how much of the chlorine that is in the water is in the killing form, free chlorine,” he says. “This requires knowing the pH and CYA level in addition to FC. Chlorine combines with CYA to form new chemicals — chlorinated isocyanurates. These new chemicals are not significant disinfectants or oxidizers. They are at least 150 times less effective. In other words, they don’t kill or oxidize much of anything. CYA has a strong affinity for chlorine so most of the chlorine in the water is bonded to CYA.”

While that may lead some to believe that Lowry and those who follow his recommendations are opposed to the use of CYA, in fact, he is adamant that a low residual is necessary for chlorinated pools.

RELATED: New Thinking: Chlorine/Cyanuric Acid In Balance

“You might be asking yourself, ‘What if I don’t use CYA?’” he says. “The answer is you could, but it would be expensive and require that you add chlorine multiple times per day. The real world UV degradation of chlorine in a swimming pool is about 75% loss in two hours. The next two hours the loss is 75% of that. The chlorine that you added will be almost zero in four hours.”

Perhaps the most provocative implication of Lowry’s approach is what it means for the use of stabilized chlorine products such as trichlor tablets, one of the most popular and enduring forms of chlorine treatment.

“If you’re using trichlor as a regular chlorinating source, the CYA is going to be increasing rapidly,” Lowry says. “For each 10 ppm of FC there will be an increase in CYA of 6.0 ppm. The average pool in summertime sun loses about 1.5 to 2.0 ppm FC per day just due to sunlight. So in a week, the loss is 10 to 14 ppm of FC. This will mean that if trichlor is used, the CYA will increase by 6 to 8.5 ppm per week. This also means that the FC level would need to be increased by about 0.5-0.6 ppm each week to prevent algae (more than 0.05 ppm HOCl). If the CYA level goes up and the FC level is not raised to compensate there may not be enough FC to kill algae.”


Given that the current conventional wisdom on CYA levels has been in place for decades, it’s not surprising that Lowry’s position has drawn criticism. Floating dispensers and erosion chemical feeders using trichlor tablets, which release chlorine and CYA together into millions of pools, are staples of the pool industry. Changing the status quo would be a historic sea change for many industry professionals and consumers alike.

Zach Hansen, manager of new product development and technical services for Biolab, a company that manufacturers both CYA- and non- CYA-stabilized chlorine products, points to the proven success of the established CYA regimen.

“People are suggesting changes that will affect the way operators, service technicians and homeowners manage their swimming pools, which will limit their choice. There’s nothing new that’s happened, no study or problem from the field suggesting that we should change how pools are being operated. We see absolutely no basis for making a change to recommendations, industry standards, best practices or even regulations limiting the use of CYA. At this point it is a solution that is looking for a problem.”

It is true, he acknowledges, that cyanuric acid is added as part of the delivery system with products like trichlor or sodium dichlor, “but there has been nothing to suggest that there are water quality problems or adverse bacteriological conditions that are caused by CYA levels at 50 ppm, or 100 or even higher. Those are all arbitrary numbers at this point. That’s simply not supported by any data that comes from real pool conditions.”

RELATED: CYA, pH, and Pool Water Treatment Realities

In response to the assertion that chlorine should be calculated based on CYA concentrations, specifically 7.5%, Hansen says the equilibriums and models involved are extremely complex and do not necessarily support using that calculation.

“It is our position that if you start limiting the use of cyanuric acid based on a multiplier that comes from a theoretical model without any real pool data to back it up, you may have some possibly serious unintended consequences. If there was some empirical field data that indicates problems with cyanuric acid levels higher than 100 ppm, then sure, we’d want to look at that and consider the practical implications, but that’s just not the case right now. There’s no evidence to show there’s a problem.”


Still, there is an upper limit to CYA concentration, Hansen says. “We do make a recommendation to drain and replace at 200 ppm,” he explains. “Basically it’s an indicator of the water being old and is often associated with elevated total dissolved solids. So, we do think that getting fresh water into the pool from a general maintenance perspective is a good practice.”

He also acknowledges that CYA concentrations have an impact on total alkalinity testing. “CYA is counted in a total alkalinity test,” he explains. “So you wind up with test results that are higher than the true TA. By reducing CYA levels when they get up to 200 ppm, you help ensure more accurate TA readings, which are essential to maintaining water balance. A third of your CYA level is contributing to the test result that is not true alkalinity, so if you don’t make that adjustment, you might have significantly less total alkalinity than you think based on your test results. We believe you have to base your total alkalinity using the correction factor, so that pools can be managed in the proper saturation index range.”

All things considered, the bottom line remains the same, he says. “We see no difference in pools that are managed with less than 100 ppm cyanuric acid when compared to pools up to 200 ppm or even above. Our customers have a high level of success either way.”

With well-qualified pool chemists on both sides of the controversy, and a lack of empirical data to settle the matter, this question is sure to continue to spur discussion in the pool water care community. Clearly, the great cyanurate debate is not likely to abate anytime soon.

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