CYA Chlorine Relationship - Further Reading

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

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.

Which means that 20 ppm CYA and 1 ppm FC(Free Chlorine) is virtually the same as 200 ppm CYA and 10 ppm FC. With 3 ppm FC and zero CYA (perfectly safe and ideal according to the CDC) a body of water with pH of 7.5 at 80 degrees F would have an HOCl of 1.451. A pool with 2.5:1 CYA:FC concentration (SLAM Level) would have roughly 0.3 ppm HOCl.

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.

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

How Was The Chlorine / CYA Chart Developed?

The relationship between chlorine and cyanuric acid (CYA) was described in the paper published in 1974 in this link that determined the equilibrium relationship between the "active" chlorine, hypochlorous acid, and the chlorine combined with CYA (chlorinated cyanurates). Though the paper is technical, you can read the initial "Introduction" in the first 2 pages as well as the "Summary" in the last 2 pages. 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.

Ben Powell, who created The PoolForum and PoolSolutions, noticed the relationship of FC and CYA in terms of a pool's tendency to produce algae. He found that if he kept the FC level at a certain level relative to CYA, that algae growth could be prevented with chlorine alone, even in pools that were high in algae nutrients (e.g. nitrates, phosphates). He came up with a Best Guess CYA chart. We've since modified that chart 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), to produce the Chlorine / CYA Chart.

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 algae. We say "may" because algae growth also depends on other factors including nutrients, sunlight, and amount of circulation determining local chlorine levels.