Cyanuric acid: Friend or Foe of swimming pools?

A common household name in the pool service industry.
Cyanuric acid is now in the toolbox of most residential and commercial pool service professionals who are servicing outdoor pools.

Since it’s invention in 1829, cyanuric acid has been revered as magical, invaluable and essential to keeping chlorinated pools clean and sanitary. At the same time it is labeled as misused, misrepresented, controversial and dangerous. To illustrate this contradiction, some health inspectors insist on the use of cyanuric acid and some health inspectors restrict its presence in public pools.

How can this be the case among so many experts? Let’s take a deeper dive on the benefits and misconceptions of cyanuric acid.

Cyanuric acid, as it’s name suggests, is an acid. Albeit a weak acid, it is commonly sold as white crystalline powder, which is able to dissolve in water and has little overall effect on a pool’s pH. It is commonly referred to as a chlorine stabilizer or conditioner, forming a weak and temporary bond with chlorine. This means it will chemically hold onto free chlorine in the water and protect it from the sun’s UV rays until the chlorine is consumed for oxidation or disinfection of material in the pool.

The primary benefit of cyanuric acid in pools:
A chlorinated pool with cyanuric acid will remain chlorinated in direct sunlight, whereas without a stabilizer, chlorine will dissipate and leave the pool unprotected without free chlorine in a matter of hours. Most important, cyanuric acid will not be consumed in this helpful work and will, over time, start to build up in pool water.

Cyanuric acid is a miracle worker in many ways.
It has allowed the typical homeowner, HOA and pool service professional to manage a pool’s water chemistry with less effort and with higher consistency. Allowing for weekly water maintenance instead of daily has created an opportunity for service companies to take care of more pools with less hassle and emergency house calls.

Overall, the market for cyanuric has exploded creating a $300 million a year business.
  Unfortunately, the misconception of “if some is good, more is better” has perpetuated throughout the industry and could not be farther from the truth. Let’s investigate the practical science of why this is the case and why the importance of cyanuric measurement is critical in keeping our pools clean and safe.  

Consequences of a high cyanuric acid level:
Chlorine’s potential to oxidize, often measured as oxidation reduction potential (ORP), is a good indicator of the effectiveness of chlorine to oxidize and sanitize swimming pool water.

Over the years, numerous independent and academic research has come to show that when cyanuric levels build up in the pool, ORP levels consistently drop indicating that the effectiveness of chlorine diminishes. In addition, there is a clear correlation to increased levels of cyanuric acid and the time it takes to effectively kill bacteria present in pool water. The chart below demonstrates the amount of time to kill 99 percent of bacteria at various levels of cyanuric acid and different levels of chlorine. Taking one example from the data, at cyanuric acid levels of 100 ppm, it takes 20 to 50 times the amount of time to kill 99 percent than at 0 ppm of cyanuric acid.  

Cyanuric Acid Graph

Furthermore, at high levels of cyanuric acid, chlorine is rendered ineffective in killing the most dangerous microorganisms in the water: Cryptosporidium parvum.

Effect on Cryptosporidium:

Cryptosporidium parvum, or “Crypto” as it is commonly referred to, is a chlorine resistant microorganism that causes gastrointestinal illness, similar to that of Giardia, that reproduces in the gut of humans. Crypto has a strong and durable outer shell that allows it to also survive outside of the human body, is spread through drinking water or swimming pool water, and is tolerant to a wide range of chlorine concentrations.  

Every year, thousands of documented cases of cryptosporidiosis occur in public swimming pools, mostly originating from fecal matter. When a suspected case of Crypto occurs, the Centers for Disease Control (CDC) recommends that the free chlorine concentration to 20 ppm for 32 hours to inactivate 99.9 percent Crypto in the pool (CT=15,300 mg*min/L). However, the CDC recommendation does not include the level of cyanuric acid that can, and in most cases is, present in outdoor pools.  

Recent CDC research presented at the October 2015 World Aquatic Health Conference demonstrates that even at cyanuric acid levels as low as 10 to 20 ppm, the current recommended remediation protocol is not adequate to inactivate the necessary 99.9 percent of Crypto in pool water. It also concludes that additional methods and secondary sanitation systems to protect swimmers from fecal accidents are desired.  

Moving forward:
Responsible training and education on the benefits and misuse of cyanuric acid is critical. Poolside kits that can accurately test for cyanuric acid are available commercially although are underused or not used at all. Although more people are becoming aware of the consequences of high cyanuric levels, it is still not measured to the degree of other water constituents, like free chlorine.

What happens when your cyanuric acid level is too high? It is simple — the only reliable method to remove cyanuric acid from swimming pools is through draining/dilution or removal through reverse osmosis.  


The above article points out the reasons why Watermaid Europe, S.L. recommends residential pools be equipped with a Saltwater Chlorination system and the installation of  Automatic ORP systems for Communal  and Commercial Pools 


The chart below (figure 1) shows the "staying power" of chlorine in swimming pool water at different cyanuric acid levels. The percentages represent the amount of chlorine remaining in pool water after one hour. It is easy to see that any level beyond 40 parts per million has very little additional benefit. Not only are excessively high levels of cyanuric acid economically unwise, but levels in excess of 70 ppm begin to cause "chlorine lock" making algae growth more prevalant
Swimming pools that are sanitized using slow dissolving, 'stabilized' tabs run the risk of exceeding these levels. This is because the binding agent used in this product is cyanuric acid. Watermaid Europe, S.L. discourages the use of too much 'stabilized' product. 
To obtain the correct stabilizer levels, cyanuric acid is hand fed when needed to achieve the optimum 20 to 40 parts per million. On residential swimming pools, we use a Watermaid Saltwater Chlorination system and on communal and commercial facilities an Automatic dosing system to feed sodium hypochlorite as the sanitizing agent.

More information on 'overstabilization' can be found at, the leading manufacturers of stabilized chlorine.


High Cyanuric Acid Levels & Plaster Degradation In Swimming Pools
by Ellen M. Meyer, Ph.D

Ellen M. Meyer, Ph.D., is technology manager of Arch Water Products in Smyrna, Georgia,
a leading global supplier of swimming pool and spa sanitizers and related products. 


It is well known that the build-up of cyanuric acid stabilizer in swimming pool water will cause overstabilization, which usually results from the extended use of stabilized sanitizers in conjunction with stabilized shock treatments. 

It is also known that overstabilization decreases the effectiveness of chlorine in killing bacteria and algae and will reduce the oxidation reduction potential or the oxidizing power of chlorine. 

Low levels of cyanuric acid do serve a purpose in protecting chlorine from sunlight degradation, however, too much cyanuric acid will negate any benefit and cause problems. 
When used properly, the recommended level of cyanuric acid is between 20 and 50 parts per million (ppm). Cyanuric acid use is not recommended for indoor pools. 

To determine the effect of cyanuric acid on sections of white pool plaster, Arch Chemicals conducted laboratory studies in 2004. The levels of cyanuric acid tested were 200 ppm and 500 ppm. After five weeks, the cyanuric acid in the water with the plaster 'coupons' - which are individual rectangles of plaster made using the same plaster composition that is used for pools - had dropped considerably and surface analysis showed the accumulation of cyanuric acid on the plaster. Surface reaction was observed at 250 ppm and 500 ppm, with the reaction of cyanuric acid being much faster in the 500 ppm sample, as shown in the graph entitled 'Cyanuric Acid Reacts with Plaster'.

Based on these initial results, a six month tank test was conducted to better understand the effect of cyanuric acid on plaster. The water in the tank tests was was adjusted to try and maintain pH between 7.2 and 7.8 and alkalinity between 60 ppm to 100 ppm. Because the plaster coupons were new, the pH rose continuously and therefore needed to be adjusted daily. The free available chlorine was maintained between 1 ppm and 4 ppm and cyanuric acid levels of 0 ppm, 25 ppm, 50 ppm, 100 ppm, 250 ppm, and 500 ppm were tested. Images taken with a scanning electron microscope at a magnification level of 250X show degradation of the plaster surface at increasing levels of cyanuric acid (see photo at bottom of page)

There was little plaster degradation at 100 ppm cyanuric acid. Due to the results of the tank tests, additional tests were initiated in larger bodies of water where the water balance parameters could be maintained more easily. 

Pool testing with plaster coupons was started in May, 2005, with the pools being operated at cyanuric acid levels of 0 ppm - 50 ppm - 110 ppm - 125 ppm, and 200 ppm - 250 ppm. The water parameters were maintained at a pH of 7.2 - 7.6, alkalinity at 80 ppm - 120 ppm, calcium hardness at 180 ppm - 250 ppm, and available chlorine at 1 ppm - 4 ppm. 

After four months of operation, photographs show surfave degradation on the plaster coupons in high, 200 ppm - 250 ppm cyanuric acid pools (see photos above). 

Results reported in a July, 2004 study conducted by the National Pool Industry Research Center (NPIRC) and the National Plasterer's Council (NPC) indicate that low pH trichlor products can affect plaster surfaces. However, in the NPIRC study, it was difficult to discern whether the effect on the plaster was a result of the low pH or the trichlor sanitizer, or from some other chemical factor. 

In the tank studies described above, the pH and alkalinity of the tanks ran on the high side (pH~8, TA~90 ppm), while the pool were being maintained as indicated above. Despite the high pH and alkalinity in the tank test, plaster degradation was still seen. The pool tests further confirmed the effect of cyanuric acid on plaster. 

These results indicate that the presence of cyanuric acid can affect plaster surfaces and that a minimum amount should be used only if chlorine stabalization is necessary.


Magnification of plaster coupon in pool with 0 ppm Cyanuric acid. Right: 14X magnification of plaster coupon in pool with 200-250 ppm cyanuric acid.  

                                                        Reprinted from "Pool and Spa Marketing" January - Atlantic City Show Issue - 2006 

We are often quoting references to chlorine stabiliser in our press advertising and articles
The extract below will help to explain the reasoning behind our attempts to make pool owners aware of the problems
associated with Chlorine Stabiliser and the requirement for it in outdoor pools
Pool Chemical Calculator
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