Is a Pool Chlorine Level Above 10 PPM Unsafe?

[PBLsQuad450]

Over the past 3 summers I have a tended my pool with over 300ppm CYA and under 30ppm of CYA. I loved reading this thread. I think the pool was much easier and more affordable to operate when the CYA was in the 70-80 range. Obviously I was keeping the Cl at a level appropriate to the CYA according to your table. I had zero problems and I suspect that high doses of Cl (40ppm) combined with 70-80 CYA was a fabulous pool closing combo. My lame (I am a social worker not a chemist) theory was that the CYA would maintain a higher Cl level longer making the early Fall, where it is more likely warmer, more protected from Algae. The colder months should have under the cover temps below 50. So, open the pool early and have it be blue? I think it worked... Granted, I have a 12 year old liner I'm not much concerned about. But the same theory (40ppm) to close worked with my CYA at 38 last year? Is the cover the reason, does a covered pool lose Cl THAT much slower because you shield it from the sun? I have a solid cover by the way. And there is still algae to kill on opening but it is pretty darn good compared to the old days of the closing company broadcasting 2lbs. Of Cal Hypo and covering.

I think, regarding the daily operation of my pool, that the 70-80 CYA level made my life the easiest. I see no difference in Cl smell or damage to suits or hair. So, if my observations are reasonable, it leaves behind the question of whether higher level CYA exposure is damaging us in any way?

[chem geek]

An opaque cover will block the UV in sunlight and have the chlorine last a lot longer. The colder water temperatures also slow down all chemical reactions including those that use up chlorine. In my own pool that has a mostly opaque electric safety cover, my chlorine loss rate with no bather load is around 0.7 ppm FC per day at 88ºF (with bather load and opening the pool every day for 1-2 hours the loss is closer to 1 ppm FC), but at 50ºF over the winter this drops to somewhat less than 0.1 ppm FC per day (about 1 ppm FC every couple of weeks). This is with the FC at around 10% of the CYA level and the CYA at 30 or 40 ppm (depending on season) though it gets diluted from winter rain overflow that I intentionally do (pool pump on cover pumps water into the pool and overflows from under the cover to an overflow drain.

If you truly have an opaque cover, then you should not need to have your chlorine start out that high if the water temp is cold when you close in winter and when you open again in spring. Usually, people only need to do that if their cover is not opaque because sunlight will continue to break down chlorine. Note that sunlight breakdown of chlorine is independent of temperature so having an opaque cover is important if one isn't going to be adding chlorine over the winter and wants to open up the pool algae-free.