The science behind the chlorine / CYA relationship has been known definitively since at least 1974 when in this paper the equilibrium constants between chlorine bound to CYA and chlorine that is unbound were definitively determined. What makes the chemistry complex is that there are many different chemical equilibria involved, but one can understand the principle by just looking at the most dominant chemical species at pool pH which are the following:
HClCY
- + H
2O <<<---> HOCl + HCY
-
"Chlorine bound to CYA" + Water <<<---> "Active Chlorine" + Cyanurate Ion (a form of CYA)
When CYA is present, most of the chlorine is bound to it -- that is, the chemical equilibrium shown above is way to the left. Chlorine bound to CYA is barely reactive and for practical purposes can be ignored except as a reserve or reservoir of chlorine. It is hypochlorous acid, shown on the right, that is the active chlorine that disinfects, kills algae, and oxidizes bather waste (see the "Chlorine/CYA Relationship" section in
this post for more details).
So the first thing you need to get out of your mind is the idea that CYA acts like a "sun shield" to protect chlorine. That's what many in the industry call it, but it is terribly misleading since it makes it sound like CYA and chlorine are kept separate which is not true. CYA binds to the chlorine to produce a new compound that, for practical purposes, does not react to disinfect, prevent algae, or oxidize bather waste.
When CYA is present, most of the FC is inactive.
In order to prevent algae growth, one needs a minimum active chlorine level. So the question is how does one determine that level given that all one knows are the pool water chemistry parameters we measure (FC, CC, pH, TA, CH, CYA, temp)? The answer is derived technically in
this post where due to the chemical equilibrium I showed above, the active chlorine level is proportional to the FC/CYA ratio. So 3 ppm FC with 30 ppm CYA has the same active chlorine level as 6 ppm FC with 60 ppm CYA and as 10 ppm FC with 100 ppm CYA.
As for the actual active chlorine level itself, it is very low since most of the chlorine is bound to CYA. At a pH of 7.5 and 77ºF (to be conservative), an FC that is at the minimum FC we recommend for manually dosed pools which is around 7.5% of the CYA level has the same active chlorine level as a pool with only 0.06 ppm FC and no CYA. That is not a typo. Fortunately, it takes a low active chlorine level to prevent algae growth.
However, to kill off already established algae, one needs a significantly higher chlorine level since it gets used up locally quickly and must penetrate algae clumps which takes longer to do so in order to get ahead of algae growth (reproduction), one needs a higher chlorine level. Though technically something like 20% would probably be enough to kill algae faster than it grows even in a bloom, it would take longer to kill off so we use an FC that is 40% of the CYA level as the shock level for clearing a pool of existing algae. This FC/CYA ratio has the same active chlorine as an FC of 0.6 ppm with no CYA.
So a pool with 32 ppm FC and 80 ppm CYA has an active chlorine level the same as only 0.6 ppm FC with no CYA. So these high numbers are just that, high numbers, and the only thing actually "high" is the amount of chlorine in reserve.
Another way to think about this is considering soldiers fighting a war. The front-line soldiers have weapons for killing the enemy (though more technically a closer analogy would be that they sacrifice themselves in hand-to-hand combat with an enemy and only kill an enemy soldier when they themselves get killed). Whenever a front-line soldier gets killed, a replacement is made from soldiers in reserve. You can see that the rate of killing the enemy is related only to the number of soldiers on the front-line and that the number of soldiers in reserve is irrelevant in terms of the rate of kill. The reserve just tells you how long you can continue to fight.
Does that make sense? If not, let me know what is still not clear and I'll see if there's another explanation that might help clarify.
As for your worry about your kids swimming in a pool with high FC, again
the FC number by itself is irrelevant towards chlorine's actual effects when CYA is present. My wife has personally experienced this difference when she has swum in an indoor commercial pool with 1-2 ppm FC and no CYA and her swimsuits would degrade (elasticity gets shot) in just one winter season of swimming and her skin was flakier and hair frizzier. In our own outdoor residential pool with 3-6 ppm FC and 40 ppm CYA, her swimsuits would last for 7 years and the effects on her skin and hair were substantially less noticeable.
