Cyanuric acid: friend or foe of swimming pools?

iStock_000014001512_XXXLargeA common pool service household name, 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 following two tabs change content below.

Nick Rancis

Nick is an accomplished microbiologist with more than 10 years of experience in emerging water technologies, industrial microbiology and commercialization of public/private ventures. In addition, he is the author of multiple patents and led microbial bio prospecting expeditions in extreme environments. A frequent speaker in the pool industry and the university technology and startup accelerator spheres, he is a thought leader in several areas of the water energy nexus and business strategy. Nick holds a BS in Microbiology from the Colorado State University and is fluent in English and Spanish.

Latest posts by Nick Rancis (see all)

Categories: Commercial Pools, Crypto, Dealer, Health, Home Pools, Pool Maintenance, Pool service, Pool technology, Water tech

9 comments on “Cyanuric acid: friend or foe of swimming pools?

  • Roohollah says:

    Dear Mr.Rancis ,

    I would like just to thank you for sharing this informative and instructive lesson with us ,and I do appreciate you taking the time to do technical research on this industry for escalating the importance of chemical elements in pool water .

    I wish you and your colleagues very happy new year ahead ,

    Yours Sincerely,


  • Richard Falk says:

    Cyanuric Acid (CYA) significantly moderates chlorine’s strength by reducing the active chlorine (hypochlorous acid) level. The specific chemical equilibria of CYA with chlorine and various combination species has been well known since at least 1974 as described in the following paper:

    The difference in active chlorine level with 1 ppm FC and 100 ppm CYA compared to 1 ppm FC with no CYA is a factor of 131 difference (this is temperature dependent and is at 77ºF while at 104ºF the difference is a factor of 18). While too much CYA results in too low an active chlorine level with the primary result allowing algae to grow faster than chlorine can kill it (when algae nutrients, phosphates and nitrates, are present), too little or no CYA results in too high an active chlorine level unless the Free Chlorine (FC) level is kept very low.

    This is why the DIN 19643 standard in Europe specifies such low 0.3 to 0.6 ppm FC (0.2 to 0.5 with ozone) because no CYA is used. This is not the case in the U.S. where the minimum FC with no CYA is 1 ppm and many commercial/public pools have higher FC even with no CYA (especially indoors). This results in much faster oxidation rates of swimsuits, skin, and hair as well as a faster creation of disinfection by-products. My wife has had personal experience with this difference where her swimsuits would degrade (elasticity get shot) in just one winter season at an indoor commercial pool with 1-2 ppm FC and no CYA while the same swimsuits in our own pool with 3-6 ppm FC and 40 ppm CYA in the summer season would last around 7 years. The difference in active chlorine level between the pools at 88ºF was a factor of 2.4 to 10.7.

    • Sheppard Bowman says:

      Hi. I recently moved into a home with a 32 x 16 in ground pool. The last owner used a pool service, and I may ultimately do the same, but not before I at least give it a go myself.
      Based on what I thought was good advise, I went to a local pool supply and purchased enough chemicals to care for the pool over the summer. Now the advice that I got was from a friend that lives in a different state (he is a pro) so I didn’t have the luxury of him walking me through any of it. He explained to me what to do with what I had purchased, and to be honest his advice was very different than what I was reading on line, and what my supplier was telling me. That advice was to shock with GLB Super Charge, and to load the feeder completely with GLB stabilized chlorine tabs.
      Long story short, I ended up with Cyanuric acid levels that were off the charts. I drained the pool down by 18″ twice, and they were still crazy high. (I had the testing done at my supplier). I got fed up and drained the pool completely, so I could just start from scratch. As I type , it has just reached the full level, and I certainly don’t want to have to do that again!
      The above article and post have left me a bit confused.If the only solution to high cyanic acid levels are to drain dow or to dilute, should I use it at all? My plan is to shock the pool at dusk, so UV shouldn’t be an issue. (Please correct me if I am wrong, because I am here to learn) My concern is that if I use the stabilized chlorine tablets, then at some point the levels will be above normal again, and I have to repeat the drain down/dilute process.
      Any and all advice would be much appreciated. I really don’t want to have to call a pro, because I am pretty certain that the last company that serviced the pool contributed to those high levels.

      • Flasck says:


        After years of struggling with pool chemistry and fighting algae, here is what I have found:

        1) Advise from pool supply stores, pool pros, and government organizations are worse than useless.

        2) Go to to get good reliable info.

        3) Summary: Run your CYA levels at about 50-60. Run your chlorine levels at 7-10. Use liquid chlorine bleach, NOT hockey pucks or powder, to adjust chlorine levels. CYA reducers DO NOT WORK. Reducing CYA levels requires either a drain and refill OR waiting for an extended period; CYA levels decrease about 3 ppm per month, if you do not add CYA based “stabilizers”. I have seen NO detrimental health effects with CYA levels up to 300 ppm (although I recommend a level of 50-60) or chlorine levels up to 10. I also DO NOT tolerate ANY “potty” accidents in my pool. For very young toilet-trained-challenged kids, I set up a separate little inflatable wading pool in the yard and do not allow them in the pool.

        4) Use a good FAS-DPD pool testing kit to do your own testing..

        Again, is an invaluable resource. Do NOT trust information or testing from pool stores.

        My pool has been clean, crystal clear, and without any health downsides for over 3 years now.

        Hope this helps.

  • Dustin Underwood says:

    Good info, many like myself know this to be true just by gut/experience. We all have seen how much harder it is to clear a green pool with high CYA. Oreq is now putting out a CYA reducer, anybody have any thoughts or experience with it? D1.

  • Maria says:

    Hi there I’m totally new to owning a pool and just had ours put in about three weeks ago. I purchased a TFP 100 Test kit and have been testing my levels almost daily. Upon first starting the pool we of course had zero CYA levels and I kept my Corine in between the 1-3 ppm. At this point we are having to add chlorine constantly. Over the course of the last week or so I didn’t know that I did not have to add chlorine every day and that my CYA test although still not registering at 20 the water was getting cloudier. On Monday being that we were headed into a heat wave and it was the Fourth of July and I knew my kids would be in the pool all day I added a liquid bleach as my free chlorine was registering at one. I have kept a TriCor Pok in the skimmer at all times as a friend had said that is the best way to raise the CYA slowly. I added the amount that the pool calculator on trouble-free pools told me to and let the filter run for approximately two hours before I tested the water again. This time I free chlorine reading was around a 5.5 -six which I felt was too high but my husband disagreed and let the kids go in the pool. but from what I’m reading this is perhaps not a safe level to swim in given that our CYA level is less than 20. But I am starting to wonder if something is wrong with the CYA test since today is Thursday in my free chlorine has been around a five since adding only bleach on Monday and having a puck in skimmer. I just want to be sure that it’s OK that they swim in the high chlorine level, I mean I know there’s nothing I can do about it now and they are older so it’s not like they’re drinking it, but I still worry. They do shower every night after using the pool regardless of the chlorine level. Baiting suits were fine no bleaching noted and no skin irritation other than my 11-year-old being a little red in the chest but I believe that was sunburn and rubbing from the pool float. This is another I told you so moment for my husband!

  • dancome says:

    Hello ~ Awesome blog ~ Thanks

  • Carol Sampson says:

    Can cyanuric acid in a public pool taken care of by a Pool Company cause HIVES??

  • John says:

    Absolute best advice is to find a replacement for cyanuric acid. It can and will give you false readings of free chlorine, resulting in being able to grow algae and bacteria while reading a 3 ppm level of FC. It’s all about weak bonds.

Leave a Reply

Your email address will not be published. Required fields are marked *