Cyanuric Acid: Why Is CYA Important To Consider For Pool Operations?

Cyanuric acid (CYA) is an effective chlorine stabilizer in swimming pools.

CYA restricts free chlorine loss due to the sun’s ultraviolet rays, thereby saving on chlorine usage and cost. It also helps buffer pH. CYA is the only stabilizer commonly used in the recreational aquatics market for stabilizing chlorine. So, what’s not to like?

CYA Reduces Chlorine's Effectiveness

CYA slows down chlorine’s reaction rate with waterborne pathogens and this can increase the risk for disease transmission.1 The higher concentration of CYA, the slower the reaction rate of chlorine. CYA is not destroyed by sunlight or chlorine levels, so it continues to accumulate in pool water with each addition, further reducing chlorine’s effectiveness in controlling microorganisms such as Cryptosporidium that can cause waterborne diseases.2 Having diminished active sanitizer levels due to high CYA in the water can also leave a pool unprotected from algae3, which in significant amounts can become breeding grounds for harmful bacteria. High CYA levels also can result in damage to a pool’s plaster. This process can compromise the integrity of the plaster, leading to costly repairs.

CYA Reduces Chlorine's Effectiveness

As chlorine is added to water, many residual compounds are formed (chloramines), and some are marginally effective as sanitizers and some are not. Hypochlorous acid (HOCl) is the primary disinfection agent of chlorine solutions.5 There is no poolside test for HOCl specifically, and this is unfortunate because CYA diminishes HOCl.

The amount of CYA in the water has a significant effect on HOCl concentrations, as illustrated here. There is about 70 times more active sanitizer at 0 ppm CYA than at 100 ppm CYA. Even with a CYA concentration as low as 10 ppm, the HOCl concentration from 1 ppm free chlorine (FC) is reduced to less than 0.066 ppm (pH 7.5, 85 °F). Many state codes allow up to 100 ppm CYA. This means that at 1 ppm FC, HOCl concentrations can range from 0.47 ppm HOCl (0 ppm CYA) to as low as 0.0067 ppm HOCl (100 ppm CYA) (pH 7.5, 85 °F).6

CDC’s 2015 published data outlining CYA’s impact on chlorination disinfection efficacy for destroying Cryptosporidium should be important to all concerned about swimmer/bather safety. Due to CDC findings and many other studies with bacteria, viruses and other pathogens, CDC’s Model Aquatic Health Code does not allow the use of CYA-stabilized products such as trichlor or dichlor in spas or therapy pools. For swimming pool use, CDC’s Model Aquatic Health Code (MAHC) currently recommends a minimum of 2 ppm free chlorine (FC) when CYA is present, and limits the CYA maximum to 90 ppm.7

Effectively meeting these MAHC limits results in a CYA:FC ratio of 45:1.

A 20:1 Ratio Is Even Better

The ad-hoc committee for CDC’s Council for the Model Aquatic Health Code (CMAHC), tasked with defining the impact of cyanuric acid (CYA) on disinfection, recommends that the current Model Aquatic Health Code (MAHC) ratio of CYA to free chlorine (FC) be reduced from 45:1 to 20:1. By reducing the ratio to 20:1, the risk of becoming ill from Giardia should be cut approximately in half and swimmers should be five times less likely to become ill from E. coli O157:H7.6

“With a CYA:FC ratio of 20:1, the concentration of HOCl stays pretty constant at 0.02 ppm,” Ellen Meyer, Product Safety & Government Affairs Manager for Pulsar® Products says. “Chlorinating a pool with an unstabilized sanitizer such as calcium hypochlorite or sodium hypochlorite is hardly any different when operating off of a ratio. With a 20:1 ratio, cyanuric acid can be added to the pool at concentrations up to 20 ppm, and the pool can operate as usual, with the FC concentration allowed to vary between 1 and 4 ppm,” she says. “However, if a stabilized sanitizer like trichlor is used, chlorination operations will be far more difficult. As CYA concentrations climb, the minimum acceptable FC level also climbs.”

Further reductions in infection risk can be obtained with keeping ratios lower than 20:1. “If a pool wants to be able to follow the diarrheal fecal contamination response the CDC is recommending, then you need to keep CYA under 15 ppm,” Meyer says. “Otherwise, the pool will have to be drained to get CYA levels down to 15 ppm before operators can perform remediation procedures.”

With a 20:1 ratio, cyanuric acid can be added to the pool at concentrations up to 20 ppm, and the pool can operate as usual, with the FC concentration allowed to vary between 1 and 4 ppm

Reducing CYA Concentrations

Indoor pools don’t require any CYA, so it’s better not to use any in order to keep chlorine as active as possible to help protect against any disease transmission. Pools using CYA must replace water to remove it. To date, diluting with fresh water is the only practical way for operators to remove CYA. But with record droughts in many parts of the world, and water rates are continuously rising, this method is fast becoming impractical for many pools, especially when significantly high levels of CYA are being added as simply a “side effect” of trichlor or dichlor chlorination.

CYA stabilized chlorine products such as trichlor and dichlor contain anywhere from 49.2 to 57.2% CYA by weight. For every 10 ppm available chlorine (AvCl) of trichlor added to a pool, 6 ppm CYA is simultaneously added. (see table below).

Chemical ppm CYA for every 10 ppm AvCl
Trichloroisocyanuric acid
6 ppm
Sodium dichloroisocyanuric acid
9 ppm
Sodium dichloroisocyanuric acid dihydrate
9 ppm

Consider this hypothetical 50,000-gallon pool

A moderately busy, 50,000-gallon pool in the summer season uses 5 lbs AvCl (5.5 lbs of trichlor) per day. The chlorine is consumed in the pool over time, but the CYA remains and builds up at the rate of 7.1 ppm per day. At 7.1 ppm CYA added per day, the pool would need to dump more than 3,500 gallons per day just to maintain the currently acceptable maximum, per CDC’s Model Aquatic Health Code (MAHC), level of 90 ppm CYA. Water can be expensive, and here the pool’s owner is, essentially, “paying” to get rid of excess CYA.

Rethinking Chlorination Strategies

The CDC findings show that pool facilities using CYA-stabilized products are potentially more vulnerable to microbial outbreaks than many had previously thought. Pool owners and operators currently using trichlor or dichlor should rethink their chlorination strategies because CYA stabilized chlorine products provide virtually no control of CYA concentrations in the water.

“Two highly effective sanitizers that are not stabilized are calcium hypochlorite and bleach,” Meyer says. “The preferred option is to prevent buildup of CYA by using one of these unstabilized sanitizers, so pool operations can stay in control and maintain the desired CYA:FC ratio.”

When considering alternatives to trichlor/dichlor chlorination, one positive option is switching to another solid sanitizing product – calcium hypochlorite. By switching to Pulsar® calcium hypochlorite, a high available chlorine product with no CYA stabilizer, a pool can maintain consistent chlorine residual in the desired range because CYA dosing is maintained separately.

Pulsar® Calcium Hypochlorite Provides Advantages

When choosing between chlorinating either with calcium hypochlorite or bleach, calcium hypochlorite offers a number of advantages over bleach. Calcium hypochlorite is the only sanitation method that chlorinates, removes organics, boosts hardness and shock treats in one simple process.  Pulsar® calcium hypochlorite chlorination systems generate hypochlorite solution on site, making only as much as the pool needs.

Pulsar® systems provide a very small footprint and eliminate the need for bulk solution storage and liquid containment systems that would be required if a pool switched to using bleach instead of trichlor.  The shelf life of calcium hypochlorite is also much longer than that of bleach.

Pulsar® Plus Calcium Hypochlorite Briquettes, are a special blend of 68% available chlorine calcium hypochlorite that contain an additive that significantly reduces the carbonate scale associated with the alkalinity component of water. The briquettes are specially made for use in the Pulsar® system equipment and comply with established NSF design standards and performance requirements. The briquettes can be easily transported in 50 lb and 100 lb containers.

For many hotels and motels and smaller facilities, pool operators have long appreciated the slow-dissolving characteristic of trichlor.  As an alternative, the Pulsar® Infinity system uses non-stabilized calcium hypochlorite in a slow dissolve tablet form. It provides the ease of use of a slow dissolving tablet like trichlor, but there’s no contributing to cyanuric acid levels. The system is operated by an automated feed system designed with the simplicity of an erosion feeder. The system’s streamlined design makes it easy to install, service and requires minimal operator maintenance.

Better Pool & Spa Protection

Maintaining proper water chemistry is of paramount importance. Water chlorination has to be right all of the time – and so does CYA ppm. Owners, operators of commercial pools, public pools, and waterparks deserve a complete sanitizing solution that they can be fully confident in, without adding excess CYA that can leave the pool more vulnerable to disease-causing microorganisms, algae, and damaged plaster.  Alternatives to trichlor including highly effective chlorination methods based on unstabilized disinfectant and well-engineered feed systems are available to better protect pools and spas from crypto and other waterborne pathogens in pools and spas.


  1. Robinton, E.D., and Mood, E.W., An evaluation of the inhibitory influence of cyanuric acid upon swimming pool disinfection. American Journal of Public Health, 1967, 57(2), 301‐310.
  2. Murphy JL, Arrowood MJ, Lu X, Hlavsa MC, Beach MJ, Hill VR. Effect of cyanuric acid on the inactivation of Cryptosporidium parvum under hyperchlorination conditions. Environ Sci Technol. 2015 Jun 16;49(12):7348-55. doi: 10.1021/acs.est.5b00962. Epub 2015 Jun 4. PMID: 26042636.
  3. Sommerfeld, M. R., Adamson, R. P., Influence of stabilizer concentration on effectiveness of chlorine as an algicide, Applied and Environmental Microbiology, Feb 1982, 43(2), 497‐499.
  4. Wojtowicz, J.A., Swimming pool water balance, Part 1: The effect of cyanuric acid and other interferences on carbonate alkalinity measurement, JSPSI 1995 1(1), 7-13.
  5. Yamashita, T., Sakae, K., Ishihara, Y., Inoue, H., and Isomura, S. 1985. Influence of cyanuric acid on viricidal effect of chlorine and the comparative study in actual swimming pool waters. Kansenshogaku Zasshi, March 3, 1988, 62(3), 200‐205.
  6. Falk, R.A., Blatchley, E. R., Kuechler, T. C., Meyer, E. M., Pickens, S.R., Suppes, L. M., Assessing the Impact of Cyanuric Acid on Bather’s Risk of Gastrointestinal Illness at Swimming Pools, Water 2019, 11(6), 1314; doi:10.3390/w11061314.
  7. CDC. Model Aquatic Health Code, 3rd Edition. Available online: