1. What is cyanuric acid? – Cyanuric acid is a weak acid that is marketed as a chlorine “stabilizer” for swimming pools.  Other terms used by the pool supply industry are “isocyanurates”, “conditioner” or “CYA”.  When exposed to the ultraviolet rays of the sun, the free chlorine in the pool water will break down and escape.  Cyanuric acid is intended to reduce this loss of chlorine.

2. What are dichlor and trichlor? – Dichlor and trichlor are two solid chlorine compounds that are widely used as disinfectants in swimming pools.  Both are often marketed as “stabilized” chlorine.  Dichlor usually comes in a granular form and is marketed for the residential swimming pool market.  It is not often used in commercial pools because it is unsuitable for commercial disinfectant feeders.  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.  Both dichlor and trichlor release cyanuric acid into the pool water, so it is not necessary to add cyanuric acid to a pool that uses either of these compounds as the primary disinfectant.

3. So what does cyanuric acid do? – Cyanuric acid forms a weak “bond” with the free chlorine in the pool water, effectively trapping it from escaping and protecting it from the sun’s UV rays.  Properly managed, cyanuric acid has been shown to reduce the amount of chlorine that needs to be added in order to maintain the minimum residual in an outdoor pool.  In a small pool with a moderate bather load, cyanuric acid can significantly reduce the costs spent on chemical disinfectants.

4. Is there a trade-off? – Yes, by forming temporary bonds with the free chlorine, cyanuric acid will reduce the overall effectiveness of the chlorine.  The amount of time it takes to kill bacteria can be significant increased with the use of cyanuric acid.  For this reason, the Pennsylvania Department of Health recommends that all outdoor pools that use cyanuric acid as a stabilizer maintain a minimum free chlorine residual of 2 parts per million (abbreviated ppm or 2 mg/L).

5. I have an indoor pool, should I use cyanuric acid? – No.  Remember that cyanuric acid is intended to reduce the loss of free chlorine that is caused by the sun’s UV rays.  Indoor pools are not exposed to direct sunlight and therefore, there is no benefit in using cyanuric acid or trichlor or diclor in indoor pools.

6. 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 two or three parts per million.  Other authorities recommend about 20 ppm for a good cost-to-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.

7. How much is too much cyanuric acid? – While Pennsylvania has no upper limit in the Public Swimming and Bathing Code, other states have set maximum levels in the range of 80 to 100 ppm.  The Pennsylvania Department of Health recommends that cyanuric acid levels should never exceed 80 ppm.

8. What are the effects of higher levels of cyanuric acid? – As the level of cyanuric acid rises, the “killing power” of the free chlorine residual weakens.  At above 50 ppm of cyanuric acid, the time it takes to kill bacteria in the water is much longer compared to swimming pool water without cyanuric acid.  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 in the pool water, high bacterial test results, and even algae growth can occur.

9. 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 Pennsylvania Department of Health does not recommend the use of cyanuric acid or stabilized chlorine in any hot tubs or spas.

10. How does one test for cyanuric acid? – Any pool operator who is interested in using cyanuric acid or stabilized chlorine should purchase a cyanuric acid test kit from their pool chemical supplier and be sure that they understand how to use it properly.  The best way to manage the problems that come with too much cyanuric acid is to avoid them by testing the cyanuric acid level regularly and ensure that they never exceed 80 ppm.  Cyanuric acid levels should be tested at least once a week.  Never add more cyanuric acid before checking the current level.

11. My pool as cyanuric acid levels above 80 ppm, how can I reduce them? – Unlike chlorine, cyanuric acid is never “used up”.  Once you have added it to the pool water, it will remain in the water.  Adding more cyanuric acid will continue to increase the levels.  The best way to reduce cyanuric acid is to partially drain the pool and add fresh water.  Note that some cyanuric acid can cling to the plumbing and filtration system, so even after completely draining and refilling the pool, there will probably still be detectable levels of cyanuric acid in the water.

12. My pool uses bromine as a disinfectant, should I use cyanuric acid? – No, bromine does not experience the same breakdown when exposed to sunlight that chlorine does.  There is no benefit to adding cyanuric acid to a pool that uses bromine as the primary disinfectant.

Summary of Pennsylvania Department of Health Recommendations

1. Cyanuric acid and stabilized chlorine (diclor or trichlor) should be used in outdoor swimming pools only.  It should never be used in indoor swimming pools or spas and hot tubs.
2. Both dichlor and trichlor release cyanuric acid to the pool water and it is never necessary to put additional cyanuric acid into a pool that uses dichlor or trichlor. 
3. Pools that use cyanuric acid should maintain a free chlorine residual of 2 parts per million (ppm)
4. Cyanuric acid levels should be tested at least once a week and before any additional cyanuric acid is added..
5. Cyanuric acid levels should never exceed 80 ppm.

References

1-     Williams, Kent. “Cyanurics – Benefit or bomb?”. Professional Pool Operators of America. Newcastle, California, 1997.

2-     Williams, Kent. Aquatic Facility Operator Manual, 3^rd edition.  National Recreation and Park Society. 1999.