CYA Chlorine Relationship

The Chlorine/CYA Relationship

Maintaining the proper chlorine levels in a swimming pool is essential for the health and safety of swimmers. While high chlorine levels may seem dangerous, understanding the relationship between free chlorine (FC), cyanuric acid (CYA), and hypochlorous acid (HOCl) can provide a better understanding of how to manage chlorine levels effectively. This article will explore the science behind these relationships, their impact on swimmer safety, and best practices for maintaining a safe and healthy swimming environment.

When is a Pool Safe to Swim at High Chlorine Levels?

It's common to question the safety of high chlorine levels, especially when health departments classify levels above 10 ppm as an "immediate hazard." However, the safety of high chlorine levels is directly related to the presence of CYA. When chlorine is bound to CYA, the active chlorine (HOCl) concentration is significantly reduced, making the pool less harsh on swimmers while still maintaining adequate sanitation.

The question often arises about the science behind the safety of high chlorine levels, such as TFP SLAM levels, when the CDC and public health departments publish that any FC levels 10 ppm and above constitute an "immediate hazard" to the health of swimmers.

The safety of high chlorine levels when bound to CYA is explained in Aqua Magazine: CYA and Chlorine in Plain Language

What is the Relationship of CYA with Free Chlorine in a Pool?

The most important quote from the Aqua Magazine article is:

With a CYA:FC ratio of 20:1, the concentration of HOCl stays pretty constant at 0.02 ppm.

The crucial factor in understanding chlorine safety is the CYA:FC ratio. At a 20:1 ratio, the concentration of the active sanitizer HOCl remains relatively constant at 0.02 ppm. This means that a pool with 20 ppm CYA and 1 ppm FC has the same HOCl concentration as a pool with 200 ppm CYA and 10 ppm FC. In fact, a pool with a 2.5:1 CYA:FC concentration (SLAM Level) has only 1/5th the active chlorine (HOCl) compared to a pool within the CDC's guidance for safe and ideal levels.

So a pool at SLAM level has only 1/5th the active chlorine (HOCl) as a pool that is within CDC guidance for safe and ideal. More chlorine can be less harsh, one of the most difficult things for anybody new to TFP to buy in to until it is experienced.

Why Does CYA Make a Pool Safe to Swim at High Free Chlorine Levels?

It really comes down to equilibrium. At any instant, most of the chlorine is combined with cyanuric acid. Only a very small amount is free hypochlorite or hypochlorous acid. This greatly reduces the activity of the chlorine yet leaves sufficient hypochlorous acid (HOCL) available as the active sanitizer in the pool water.

The key is the equilibrium that was figured out by O'Brien. J. E. O'Brien, J. C. Morris and J. N. Butler, “Equilibria in Aqueous Solutions of Chlorinated Isocyanurate”, Chapter 14 in Alan J. Rubin, ed., Chemistry of Water Supply, Treatment and Distribution, 1973 Symposium, (published 1974), Ann Arbor Science Publishers, Ann Arbor, MI, pp. 333-358. ISBN 0-250-40036-7

The Chlorine/CYA Chart

The relationship of chlorine to CYA can be expressed as ratios or percentages.

A reasonable approximation to the table below is the following:

• "Min FC" is 7.5% of the CYA level
• "Target FC" is 11.5% of the CYA level
• "Yel/Mstrd Min" is 15% of the CYA level
• "Shock FC" is 40% of the CYA level
• "Yel/MstrdShock" is 60% of the CYA level.

For many it is easier to visualize it in the table below.

Chlorine/CYA Chart

CYA Level	Minimum FC Level	Target FC Range	SLAM Level	Minimum HOCL	Target HOCL
20	2	3-5	8	0.02	0.031-0.051
30	2	4-6	12	0.02	0.038-0.063
40	3	5-7	16	0.02	0.044-0.076
50	4	6-8	20	0.02	0.050-0.089
60	5	7-9	24	0.02	0.056-0.102
70	5	8-10	28	0.02	0.062-0.114
80	6	9-11	31	0.02	0.068-0.127
90	7	10-12	35	0.02	0.074-0.140
100	8	11-13	39	0.02	0.080-0.153

• CYA and FC is ppm. HOCL is mg/L Ci.
• This table is for tablet or liquid chlorination at a 7.5% minimum FC/CYA ratio
• SWG chlorination can be maintained at a 5% minimum FC/CYA ratio

The Origin of the Chlorine/CYA Relationship

The relationship between chlorine and cyanuric acid (CYA) was first described in a paper published in 1974 by O'Brien, Morris, and Butler. This paper, titled “Equilibria in Aqueous Solutions of Chlorinated Isocyanurate,” determined the equilibrium relationship between the "active" chlorine (hypochlorous acid) and the chlorine combined with CYA (chlorinated cyanurates). The paper's introduction and summary provide valuable insights into the foundational understanding of this relationship.

Additional scientific studies in the late 70's and into the 80's showed that CYA significantly reduced chlorine's effectiveness, mostly showing that hypochlorous acid was the primary disinfectant and that hypochlorite ion and the chlorinated cyanurates had little disinfecting capability. Oxidation-Reduction Potential (ORP) values also show that it is hypochlorous acid that has the primary oxidizing potential.

The Creation of the Chlorine/CYA Chart

Ben Powell, the creator of The PoolForum and PoolSolutions, developed a Best Guess CYA chart. Powell noticed the importance of understanding the relationship between FC and CYA in maintaining a safe and healthy swimming pool environment.

We've since modified Powell's Best Guess Chart to simplify the process for pool operators and owners taking into account the chemistry determined in 1974, as well as additional analysis of what users have reported in newer real pools, including SWG pools (and yellow/mustard algae), into the TFP's Recommended Free Chlorine Levels tool.

The Chlorine/CYA Chart, which helps pool operators determine the appropriate levels of FC and CYA, combines the findings from O'Brien's papers and subsequent research, allowing pool operators to determine the appropriate FC levels based on the CYA concentration in their pools. By adhering to the recommended ratios on the chart, operators can ensure that the concentration of hypochlorous acid (HOCl), the active sanitizer, remains consistent and effective in combating harmful microorganisms.

You can certainly have a pool with a high CYA level, but if you do not proportionately raise the FC level to be, in a manually dosed pool, at least 7.5% of the CYA level, then the pool may develop bacteria and algae. We say "may" because algae growth also depends on other factors including nutrients, sunlight, and amount of circulation determining local chlorine levels.

What About the Studies That High CYA is Bad?

With regard to the Conventional Wisdom™ of that Scientific-study-that-proves-cya-is-evil® one has to actually go and dig deep to understand what that "study" is and is not showing and not just believe the conclusions. What you find in those studies (there are European studies and one that was done by the Wisconsin public health department ... who the heck swims in a pool in Wisconsin???) is that they look at the CT kill rate of chlorine on various pathogens of interest and then they vary the CYA and free chlorine (FC) concentrations to see what effect it has on the kill rates. And, low and behold, oh-my-gosh, the kill rates get worse when you increase CYA ..... quick, drain the pool, pour gasoline on it and burn it to the ground!!!! (But wait, it's already in the ground) ...

What all of those studies fail to do is report the CT kill rates while controlling for HYPOCHLOROUS ACID (HOCl) concentration, NOT FC. It is HOCl that is the primary disinfectant and oxidizer when you add chlorine to water. Free chlorine is a measure of HOCl, OCl- and, if CYA is present, the chlorine that is bound to the CYA. So FC in and of itself is a useless quantity to use an independent variable. When one takes their data sets and actually calculates HOCl levels and then replots the data, guess what happens?? All the CYA is eeeeevvvviiiiilllll evidence completely evaporates. Why?? Because all it takes is about 50-100ppb (that's parts per billion) of HOCl to completely sanitize water and make disease transmission very difficult. Using an FC/CYA ratio of ~ 7.5% (note - it's a ratio!) yields HOCl levels that are completely sanitizing and, because CYA is present to moderate the oxidizing power, DBP's and THM's are never really an issue.

But, sadly, all that information and hard thinking takes too much effort for people in the pool industry to care about ... they have products that they need to sell to make money so the narrative needs to remain ... can't be messing up their great profit margins with all that fancy-sciencey talk.

Implementing the Chlorine/CYA Ratio in Pool Maintenance

Understanding and applying the Chlorine/CYA ratio is vital for maintaining a healthy and safe swimming pool. Pool operators and owners must be diligent in monitoring and adjusting the FC and CYA levels in their pools. Here are some tips on how to effectively implement the Chlorine/CYA ratio in pool maintenance:

Regularly test FC and CYA levels

Frequent testing with a test kit is necessary to ensure that the pool maintains the recommended Chlorine/CYA ratio. This will help in keeping a consistent HOCl concentration, which is crucial for effective sanitation.

Use the Chlorine/CYA Chart

Refer to the Chlorine/CYA Chart and TFP's Recommended Free Chlorine Levels tool to determine the appropriate FC levels based on the current CYA concentration in the pool. Adjust the chlorine levels accordingly to maintain the recommended ratio.

Avoid High CYA Levels

High CYA levels are often called over-stabilization by Pool Stores. What is a high CYA level depends on the way you chorinate your pool and your local environment. In general pools chlorinated with tablets or liquid chlorine should have CYA up to 40ppm. Pools chlorinated using a SWG can have CYA up to 70-80ppm.

Above those CYA levels maintaining adequate free chlorine and HOCL sanitizer can become difficult for the method of chlorination. This is often called "chlorine lock" by Pool Stores.

If CYA levels become excessive, partial draining and refilling of the pool may be necessary to lower the concentration.

Adapt To Varying Bather Loads

Be prepared to adjust chlorine levels as bather load increases. TFP Chlorine/CYA recommendations are for a residential pool with family usage. A higher bather load can deplete the FC levels more rapidly, making it essential to monitor and adjust the chlorine concentration accordingly.

A pool party with a lot of people in the pool for a while can have a high chlorine loss and need to start with a higher FC level to not have the FC fall below the minimum level for the CYA before the party is over.

One person-hour in a 20,000 gallon pool consumes 0.05 ppm FC. One cup of urine from someone urinating in the pool is roughly equivalent to 5 person-hours in a pool, a loss of 0.25ppm FC in a 20,000 gallon pool.

FC and bather loads can be more of a consideration in a spa which is a small body of water for the number of people in it and the high water temperatures. The sanitation of a spa can drop below safe levels with many people in it for long times.