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The Care & Feeding of Pool Waters
Chlorine Stabilization & Cyanuric Acid

Cyanuric Acid (CYA) is the compound used to stabilize chlorine in water. It can be added to water either as CYA by itself or as stabilized chlorine products called Dichlor or Trichlor. Dichlor is granular and Trichlor is a slow dissolving tablet. When Dichlor or Trichlor is added to the water, both CYA and chlorine are released. When CYA is added by itself, free chlorine is added separately either as a liquid (sodium hypochlorite) or a solid, usually a powder, calcium or lithium hypochlorite. To our knowledge, chlorine gas is no longer used in New Jersey for swimming pool disinfection.

New Jersey State Department of Health Regulations allow for the use of stabilized chlorine in OUTDOOR pools, hot tubs and spas. Chlorine stabilization is PROHIBITED in INDOOR pools, hot tubs and spas. (N.J.A.C 8:26-7.8 & 7.12)

CYA reacts with free chlorine to prevent the ultra-violet rays of the sun from dissipating the chlorine in the water. This is why it is allowed for outdoor but not indoor usage. CYA does work. It does help chlorine remain in the water during sunny days. HOWEVER, it also decreases the killing effectiveness of chlorine against microorganisms such as bacteria, algae and protozoans. This means that if you use CYA your chlorine levels should be kept higher.


In June 2015, the Centers for Disease Control published a scientific article on the effect of chlorine stabilizer (Cyanuric Acid - CYA) on the inactivation by chlorine of the microorganism, Cryptosporidium.  The link to this article can be found at the bottom of this page. Based upon this article Garden State Labs, Inc. is recommending that if CYA is utilized, CYA be used at 10 parts per million (mg/l) and not exceed 20 ppm.  If used, CYA should be tested at least weekly.

The New Jersey State Regulations prohibits Cyanuric Acid levels above 100 ppm.

When stabilized products such as Dichlor or Trichlor are used, people often do not recognize that the CYA level will go up over time, even though the chlorine is dissipated. If CYA is added to the water as a separate compound, the level will go down over time as fresh water is added to the pool.

When the CYA level is high you may still get acceptable chlorine readings on your DPD chlorine test kit but, even though the chlorine is in the water, it is not effective in killing the bacteria and other microorganisms.

The higher the CYA level the less effective the chlorine is in killing microorganisms including bacteria, algae and the protozoans, Giardia and Cryptosporidium.

When stabilized chlorine products are used over several months and the pool operator does not check the CYA level, problems can occur. When Garden State Labs detects high bacterial counts (Heterotrophic Plate Count) or positive Coliform results in pools with good free chlorine levels we often get calls from our clients asking how that is possible. Our first question is to ask the client what type of chlorine they are using, and the response we usually get is "I don't know".  We then ask the client to refer to the active ingredient label, and usually it is a stabilized chlorine product.  When we ask what the CYA level is, the response is, again, "I don't know". Garden State Labs recommends that the client either check the CYA level themselves with a test kit or have it tested. Often the CYA levels are high, above 20 ppm, which significantly lessens the chlorine's killing effect. This is potentially why the sample is getting unsatisfactory bacteriological results.

Garden State Laboratories, Inc. feels that pool operators should test their CYA level on, at least, a weekly basis using their CYA test kit. Whether using CYA alone or stabilized chlorine products, the data should be recorded in the same log book as chlorine and pH.

If the CYA level is above 20 ppm we recommend that fresh water be added to dilute the CYA and increase the effectiveness of chlorine in disinfecting the water.

The Professional Pool Operators of America has an interesting technical article on chlorine stabilization and cyanuric acid. While the article has valuable information, Garden State Labs does not necessarily agree with all of the language used.

Effect of Cyanuric Acid on the Inactivation of Cryptosporidium parvum under Hyperchlorination Conditions:  Jennifer L. Murphy,  Michael J. Arrowood,  Xin Lu,  Michele C. Hlavsa,  Michael J. Beach,  and Vincent R. Hill ;  Waterborne Disease Prevention Branch, Division of Foodborne, Waterborne, and Environmental Diseases, Centers for Disease Control and Prevention, Atlanta, Georgia 30333, United States: (c) American Chemical Society, Environmental Science and Technology 2015, 49, 7348−7355

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