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Cyanuric Acid: A Discussion on Chlorine Stabilizer

You may know CYA as pool stabilizer or conditioner. This curious chemical seeks out and attaches itself to the chlorine molecule. In doing so, it acts as a sun shield, absorbing sunlight, and reducing the sun's degradation to your chlorine.

The chemical was discovered in 1829, by Friedrich Wöhler a German chemist. It was not until the 1950s, however, that a class of disinfectants known as chlorinated iso-cyanurates were developed. Soon after, savvy chlorine manufacturers  began adding it to "stabilize" chlorine tablets.

Cyanuric acid that is sold as pool stabilizer or conditioner is a white, flaky powder, similar in appearance to calcium chloride. It is a weak acid, and is not corrosive or considered hazardous.

How is Cyanuric Acid Used to Swimming Pools?


Stabilizer should be added to refilled pools to raise the level to the range of 30-50 ppm. After this foundation, the small amount of CYA pressed into your stabilized tablets should help replace the CYA lost during backwash, splash-out or winterization.

The TriChlor tablets and the DiChlor shock that we sell are examples of chlorine products that are combined crystallized with the salts of cyanuric acid. These are known as stabilized chlorine and are meant to help maintain the residual of cyanuric acid in your pool.

Cyanuric Acid Reduces Chlorine Degradation

I like to think of the CYA molecule holding a sun parasol, shielding the chlorine. While it absorbs up to 50 percent of the sun, you can expect to see a reduction of up to 50 percent of your chlorine usage. After adding CYA to the pool, you'll immediately notice the increase in your chlorine's lifespan, and your wallet size.

A pool without stabilizer can lose 90 percent of its free chlorine with just a few hours of bright sun. When stabilized, your chlorine level will last longer, and you'll use about half of the tablets to maintain your chlorine residual.

Too Much Cyanuric Acid

A 2007 study by the CDC showed that levels of Cyanuric acid above 50 ppm significantly diminished chlorine’s ability to kill bacteria. Chlorine molecules that are overprotected by too much CYA can lead to excess chloramine formation as nitogen and ammonia are easily able to attach themselves to these "slow and sluggish" chlorine molecules.

Many users of chlorinated isocyanurates report problems with a buildup of CYA over time to levels beyond 50 ppm. To lower the levels of CYA, there is no magic potion. Lowering CYA levels is accomplished by dilution; that is, draining and refilling a portion of the pool water.

If you are using stabilized chlorine (Trichlor or Dichlor), and you have problems with too much CYA, without the ability to drain and refill, you can switch chlorine types. Cal Hypo tablets and shock can be used that do not contain cyanurates. Trichlor and Dichlor are a nearly 50 percent cyanurate compound.

Another solution is to switch to a salt water chlorinator. You'll still want to add CYA to the pool to help your salt cell from working too hard, but you won't be adding additional CYA from stabilized tablets or shock.

Still another solution is to add a mineral sanitizer like Nature2 or Frog, or add an ozone purifier from Del Ozone to reduce your chlorine demand by half, which will reduce CYA buildup by also exactly half.

To lower the CYA level by dilution, here's the math. If your cyanuric acid level was at 100 ppm (way too high), and you want to lower it to 50 ppm, you will need to replace half the pool water (assuming your fill water has 0 ppm of CYA).

Cyanuric Acid Reduces Kill Rates

A study performed by the Pinellas County, Florida, Health Department found that when pools with 1-5 ppm of chlorine were treated with 100 ppm of cyanuric acid, nearly 80 percent of the pools would be deemed unfit for swimming by the sanitarian.

Experts on poolforum and TroubleFreePool such as Richard Falk (chemgeek) and Ben Powell have led lengthy discussions on the topic of the suppressive effect of cyanuric acid on the efficacy of free chlorine levels. Charts have been produced to help pool owners find the best chlorine level, based on their level of cyanuric acid.

In most cases, and with the exception of Crypto or Giardia contaminations, these higher chlorine levels will compensate for the effects of cyanuric acid on chlorine activity. For heavily used commercial pools, it has been suggested that double this amount be used. This would roughly equate to a maintaining chlorine levels at 20 percent of the cyanuric acid level.

Notice also in the chart below that it's not only the daily chlorine residual that needs to be boosted to compensate for the CYA, but also when shocking the pool. CYA bonds almost instantly to chlorine in the water and makes it difficult to reach breakpoint chlorination.

So, what's the problem? When the chlorine isn't as powerful as you think it is, your pool is not as sanitary, and possibly not fully disinfected. The validity of the standard minimum 1.0 ppm free chlorine has come into question.

OK, what's the answer? If you add pool conditioner or stabilizer, or use Trichlor tablets or Dichlor shock (both stabilized chlorine types), you should make some adjustments.

Ben's Best Guess Chart

1. Run your chlorine level higher. As compensation for the suppressive effect of CYA, use more chlorine tablets until you test a consistent 2 or 3 ppm.

See chart, adapted from Ben's Best Guess Chart at Pool Solutions, that has some recommendations for chlorine level, at different levels of CYA.

In short, the higher your level of cyanuric acid, the higher your chlorine level.

2. Monitor your cyanuric acid Level. Test your level at the start and end of every season. Don't add any cyanuric acid to the pool if you already have 20 ppm. Remember that chlorine tablets add cyanuric acid, and the level will increase naturally. If your cyanuric acid level gets too high, dilute it by draining a portion of water and refilling or using Bio-Active CYA reducer. Test your level with a good test kit, like my favorite Taylor K2005, or use Test Strips that check all levels, like LaMotte 6-way test strips.

Not Enough Cyanuric Acid

Outdoor pools without a residual of cyanuric acid in the water have trouble maintaining a chlorine residual during a sunny day. Even small levels of CYA in the water will have a pronounced protective effect.

Indoor pools can also benefit from a very low residual (5-10 ppm) of CYA as it helps to prevent off-gassing of chlorine and reduces chloramine formation.

If you need to raise your cyanuric acid level, you simply pour the granular white powder directly into the pool, at a rate of about 1 pound per 10,000 gallons to raise it about 10 ppm.

The Best Level of Cyanuric Acid

Aquachek-pro-II test strips for Cyanuric Acid, Hardness and TDS

Testing the water for a cyanuric acid level is the first step. The test for cyanuric acid is interesting; it's called a turbidity test. The presence of CYA will turn the water sample cloudy, or turbid.

If you don't have a turbidity test for CYA, you can use these test strips to check your cyanuric acid, hardness and total dissolved solids (TDS) levels. Since you only need to check these levels monthly, it makes sense to have separate test strips for these tests.

Maintaining the level around 30-50 ppm is recommended. There is little advantage in exceeding the level, and as shown above, the higher the level, the higher the kill time for your chlorine becomes.

Too Much Cyanuric Acid

Other effects of high cyanuric acid levels include problems with cloudy water, and a difficulty in maintaining a free chlorine level in the pool, with a corresponding increase of chloramines.


Lower your level of cyanuric acid by dilution (draining and refilling with fresh water), or by using Bio-Active cyanuric acid reducer. You can reduce the amount of cyanuric acid added from chlorine tablets by using a mineral purifier or an ozonator to reduce your chlorine demand by 50 percent.

So students, here ends another lesson in swimming pool chemistry. In summary: It's prudent and necessary to use some level of cyanuric acid. Test monthly to be sure that it doesn't rise much above 50 ppm over time, and if it does, lower by dilution, until we come up with a better solution.

Is Cyanuric Acid Dangerous or Unhealthy?

Cyanuric acid by itself is not immediately dangerous or hazardous to one's health. It can cause eye or skin irritation if direct contact is made. It is not toxic or carcinogenic. Based on data from toxicological investigations, cyanuric acid does not damage organs. Damage to the kidneys and bladder has been observed in rats tested with a saturated solution (5375 ppm) of cyanuric acid for their drinking water. CYA should not pose a risk to humans during  use in swimming pools. Cyanuric acid is quickly excreted unchanged from the kidneys.

Can I Use Cyanuric Acid to Lower my pH?

Technically, CYA is an acid and is in the family of sulfuric acid, with a pH level of around 4.0. It will slightly reduce your pH level when added to the pool in measureable quantities but may be hard to notice. So, no, cyanuric acid is best used for shielding your expensive chlorine from the sun, but for lowering your pH, better to use sodium bisulfate.

In The Swim makes every effort to provide accurate recommendations based upon current ANSI/APSP/ICC-5 2011 (R2022) standards, but codes and regulations change, and In The Swim assumes no liability for any omissions or errors in this article or the outcome of any project. You must always exercise reasonable caution, carefully read the label on all products, follow all product directions, follow any current codes and regulations that may apply, and consult with a licensed professional if in doubt about any procedures. In The Swim assumes no legal responsibility for your reliance or interpretation of the data contained herein, and makes no representations or warranties of any kind concerning the quality, safety, or suitability of the information, whether express or implied, including, without limitation, any implied warranties of merchantability or fitness for a particular purpose.