For 32 years, the voice of the pool and spa service professional.

CYA’s role in chlorine’s efficacy questioned

By Marcelle Dibrell

Although cyanuric acid has been used alongside chlorine in swimming pools since 1956, a great deal of confusion remains about how it works. One of the biggest problems is a surprising lack of peer-reviewed studies concerning cyanuric acid’s effect on chlorine’s ability to kill algae. 

The pool industry is only lately becoming aware that cyanuric acid slows chlorine’s ability to kill bacteria and some microorganisms. For example, it is only in the last few years that the CDC has recommended lowering cyanuric acid to less than 15 ppm prior to superchlorinating in the event of a fecal incident. 

And as pool techs, we want chlorine to kill basically every invisible organism within the pool.

But the organisms most service pros are focused on are algae. 

Plenty of studies exist that show other effects cyanuric acid has on chlorine. 

For example, there are a lot studies that document cyanuric acid’s effect on chlorine as a bactericide; cyanuric acid’s effect on chlorine as a virucide; and cyanuric acid’s effect on chlorine’s ability to kill protozoa. These studies show a significant reduction in chlorine’s kill times when cyanuric acid is present. 

There is also published material that shows that cyanuric acid lowers chlorine’s oxidation potential, which is an indicator that chlorine has lost some of its ability to oxidize organic material and inactivate microorganisms. 

There is a 1981 paper concerning algae, Entitled “Influence of Stabilizer Concentration on Effectiveness of Chlorine as an Algicide,” by Sommerfield and Adamson.

This rare study examined cyanuric acid’s effect on chlorine as an algicide. The authors concluded that while 25-ppm cyanuric acid slightly reduced chlorines algicidal properties, higher stabilizer concentrations resulted in no further reduction in chlorine’s algicidal efficiency. 

This conclusion is not only inconsistent with all of the previous studies already mentioned, but it also goes against what most service techs learn, after just a short time maintaining swimming pools: at a certain point, high cyanuric acid levels make normal chlorine use ineffective against algae.

The pool and spa industry could really benefit from better peer-reviewed studies about cyanuric acid. 

What is true is that chlorine’s ability to kill algae, and pretty much everything else, is determined by the concentration of hypochlorous acid. That concentration varies as a function of both the pH and the cyanuric acid levels. 

This is a truth that is only beginning to be embraced by the pool and spa industry. 

The effect that pH has on chlorine’s ability to disinfect is well understood. For decades, the pool industry has been presented with graphs and tables that show diminished hypochlorous acid as the pH rises. Such well-known charts have informed industry standards that require that pools operate within a set of pH parameters, usually 7.2 to 7.8. We maintain such pH values because they lead to water balance, but just as importantly, we are told, operating at higher pH means chlorine is less effective. 

But in those pools that use cyanuric acid (which is most) the effect that cyanuric acid has on chlorine’s ability to work is far more dramatic than the pH. 

Again, the industry needs more studies.

While peer-reviewed studies are lacking, the hypochlorous acid concentration that is effective against algae is a level that has been determined empirically, based on the experience of both pool owners and service professionals, in the habit of using chlorine with cyanuric acid. 

This concept began with Ben Powell, a service professional and the creator of Pool Solutions, an information site, and The PoolForum. 

Powell had the idea that swimming pool chlorine levels should be adjusted according to stabilizer levels. This was based on years of his own experience servicing commercial pools, information from people who used his forum, a technical paper by John Wojtowicz, and some math.

Powell had noticed that ideal chlorine levels, generally between 2 to 4 ppm, were not always effective against algae if the cyanuric acid levels were high. 

So he set about trying to determine how much chlorine should be present at different cyanuric acid levels. The result, printed sometime in the late 1990s, is “Ben’s Best Guess Guide to Swimming Pool Chlorination.”

In the years that followed, a pool owner with a talent for math and chemistry was able to refine Powell’s work, working out a more precise relationship between chlorine’s efficacy as a function of cyanuric acid levels. 

Richard Falk, known as Chem Geek at the online forum Trouble Free Pool, was able to derive a simple formula for the minimum free chlorine needed to prevent algae based on the amount of cyanuric acid. That number corresponds to a hypochlorous acid concentration of about 0.03. 

Falk began with Powell’s best guesses on free chlorine values that are effective for a given cyanuric acid concentration. He then determined that one should have a minimum free chlorine to cyanuric acid ratio of 7.5 percent to prevent algae in traditionally chlorinated pools. 

Now known as Falk’s Ratio, the idea is that hypochlorous acid concentrations are proportional to free chlorine divided by cyanuric acid:


Falk converted Powell’s Best guesses into the concentration of hypochlorous acid that had been found necessary to prevent green algae. That minimum hypochlorous acid concentration corresponds to a free chlorine level that is 7.5 percent of the cyanuric acid level.

And while that may seem complicated, the math to prevent algae is easy.

FC = 7.5% x CYA

For example, if the measured cyanuric acid in a swimming pool is 50, then a pool operator should maintain a minimum free chlorine level of 3 ppm. 

3.75 ppm FC = 7.5% x 40 ppm CYA

If the cyanuric acid is at 90 ppm, the free chlorine should be maintained at a minimum of 6 ppm. 

6.75 ppm FC = 7.5% x 90 ppm CYA

Falk’s ratio shows that industry standards fall short of where they should be to control and prevent algae, particularly when swimming pools have high cyanuric acid levels. It shows why absolute recommended chlorine levels (1-4 ppm) will not necessarily work. 

That is because if chlorine is less than 7.5 percent of the cyanuric acid, the pool could easily develop algae. For example, a pool could develop algae with 80-ppm cyanuric and 2 ppm chlorine. (And those values are within industry guidelines.) It simply does not have enough chlorine.

This is especially true if the pool has certain minerals known to foster algae growth, such as phosphates and nitrates. 

Falk’s work is well known to the millions of online visitors of Trouble Free Pool, a swimming pool help and information exchange forum. Due to the high traffic at that site, his ratio is increasingly being used by pool operators to maintain an algae-free pool. 

Paid subscribers have full access to all Service Industry News content. Read the entire story — and every feature and news story affecting our industry —  by ordering your subscription today online. Become a subscriber by following this link. Or, contact our circulation department at 949-735-7281 or email for details.

SCE rebate policy shuts out service pros

In Memoriam: Janice Dickman — 1946-2019

In Memoriam: Janice Dickman — 1946-2019