Re: What's an Oxidizer do?
PoolOwnerNumber9,
Look at the post by mas985 later on in
this thread to see the experiment that he did that showed that higher CYA levels do protect chlorine breakdown from sunlight in a non-linear way such that even proportionately higher FC levels are still protected (i.e. you lose less absolute FC at higher CYA levels in the 60-80 ppm range). This was an unexpected result and my current guess about it involves the non-linear absorption of UV by CYA with depth combined with imperfect mixing near the surface (
this post goes into some details). The Taylor book is wrong as is PPOA and several other sources. We've had too many cases (in addition to Mark's experiment) of non-linear beneficial effects of lower chlorine drops at higher CYA levels (even at the same FC/CYA ratio) for this not to be true. This would not be the first time that industry "wisdom" was not true.
As for toxicity studies with CYA, the most comprehensive summary is shown
here for Cyanuric Chloride which in water hydrolyses to Cyanuric Acid. The oral LD50 (lethal dose for 50% of people) is about 320 mg/kg of body weight while the dermal LD50 is > 2000 mg/kg. The LOAEL (Lowest Observed Adverse Affect Level) is 50 mg/kg. These are huge amounts compared to what one is exposed to in a pool where 100 ppm CYA is 100 mg in a liter of pool water (which you don't normally drink and you are normally far more than 1 kg or 2.2 pounds in weight). There is also
this EPA link in section 13.1b that shows oral LD50 > 5000 mg/kg. Section 13.3c shows similar results for dermal LD50. You can read the other data as well. Boron (shown
here) is more toxic in terms of oral dose at pool water levels which is why we recommend that dogs not regularly drink pool water with Borates in it (note that 50 ppm is measured as Boron so is equivalent to 286 ppm Boric Acid).
The bottom line is that CYA at pool levels is VERY far from toxic effects and you'd have to drink lots and lots of pool water at very low body weight before even getting to first symptoms of any kind. So having a limit of 100 ppm is reasonable given children or babies that might gulp water, but it's a rather safe limit. Mostly, as you say, very high CYA levels have other more serious side effects of reducing chlorine effectiveness, but it's roughly proportional to the FC/CYA ratio and not a rapid dropoff (except from no CYA to a small amount of CYA).
This post goes into details about how the FC/CYA ratio comes from the equilibrium chemistry.
Richard