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