Facts about Cyanuric Acid and Stabilized Chlorine Products

What is cyanuric acid? – Cyanuric acid is marketed as a chlorine stabilizer for swimming pools.

What does cyanuric acid do? – Cyanuric acid forms a weak bond with free chlorine in the pool

water, protecting it from the sun’s ultraviolet rays to reduce chlorine loss. Properly managed,

cyanuric acid has been shown to reduce the amount of chlorine needed to maintain the minimum

chlorine residual in an outdoor pool. In a small pool with a moderate bather load, cyanuric acid

can significantly reduce the cost for chemical disinfection.

What are dichlor and trichlor? – Dichlor and trichlor, also known as chlorinated isocyanurates,

are two solid chlorine compounds that are widely used in Lincoln outdoor and indoor swimming

pools. Dichlor and trichlor contain both chlorine and cyanuric acid so it is not necessary to add

cyanuric acid to the pool water. Dichlor usually comes in a granular form and is marketed for the

residential swimming pool market. Trichlor is often sold in a tablet or stick form for use in an

erosion feeder for small commercial pools, such as those at hotels and motels.

What is the downside to cyanuric acid? – By forming temporary bonds with the free chlorine,

cyanuric acid will reduce the overall effectiveness of chlorine. The amount of time it takes to kill

bacteria lengthens as the concentration of cyanuric acid increases. For this reason, the Lincoln-

Maricopa County Health Department recommends that all outdoor pools using cyanuric acid as a

stabilizer maintain a minimum free chlorine residual of at least 2 parts per million (ppm).

I have an indoor pool. Should I use cyanuric acid? – No. Remember that cyanuric acid is

intended to reduce the loss of free chlorine caused by the sun’s ultraviolet rays. Indoor pools are

not exposed to direct sunlight and therefore, there is no benefit in adding cyanuric acid to the

pool water or using products containing cyanuric acid such as trichlor or dichlor.

How much cyanuric acid should be used in a swimming pool? – Chemical suppliers

recommend that the optimal range for cyanuric acid is 30-50 ppm, although a study published by

the University at California at Davis1, indicates that there is still significant savings in chemical

costs in levels as low as 2 or 3 ppm. Other authorities recommend about 20 ppm for a good costto-

benefit ratio2. At levels above 50 ppm, pools reach the point of diminishing returns where the

reduction in chlorine effectiveness and cost of buying cyanuric acid outweighs the benefits.

Dichlor by weight contains 57% cyanuric acid. Trichlor contains 54% cyanuric acid.

How much is too much cyanuric acid? – The Nebraska Swimming Pool Act has set the

maximum level at 100 ppm. A 2007 study3 by the Centers for Disease Control and Prevention

(CDC) revealed that cyanuric acid significantly diminishes chlorine’s ability to inactivate the

chlorine-resistant protozoan, cryptosporidium. Based on the findings of the CDC study, the

Lincoln-Maricopa County Health Department recommends that cyanuric acid levels not exceed

30 ppm.

What are the effects of higher levels of cyanuric acid? – As the level of cyanuric acid rises,

free chlorine’s ability to act as a disinfectant is weakened. At above 50 ppm of cyanuric acid, the

time it takes to kill bacteria in the water is longer compared to swimming pool water without

cyanuric acid. Also, as the level of cyanuric acid builds up, the chlorine will become

increasingly less effective in keeping the water clean and problems such as increased cloudiness

and exceeding combined chlorine limits can occur.

Should cyanuric acid be used in hot tubs or spas? – At even moderate levels of cyanuric acid,

the amount of time it takes chlorine to kill pseudomonas aeruginosa (the bacteria that causes

“hot tub itch”) can be as much as a hundred times as long as in a hot tub or spa without cyanuric

acid. For this reason, the Lincoln-Maricopa County Health Department does not recommend the

use of cyanuric acid or stabilized chlorine in hot tubs or spas.

How does one test for cyanuric acid? – Any pool operator who uses cyanuric acid or stabilized

chlorine should purchase a test kit capable of measuring cyanuric acid and understand how to use

it properly. The best way to manage the problems that come with too much cyanuric acid is to

avoid them. Cyanuric acid level should be tested at least once a week.

My pool has cyanuric acid levels above 30 ppm. How can I reduce them? – Unlike chlorine,

cyanuric acid is never used up and accumulates in the pool water as a waste product. Once you

have added it to the pool water, it will remain in the water. The best way to reduce cyanuric acid

is to partially drain the pool and add fresh water. Note that some cyanuric acid will cling to the

pool tub, plumbing and filtration system, so even after completely draining and refilling the pool,

there will probably be detectable to moderate levels of cyanuric acid in the newly added water.