How much is too much?

[pool_girl]

After I shock it takes awhile for the chlorine level to return to "normal", or 2-3 ppm. So at what level is it considered safe for swimmers? I've heard that chlorine levels actaully have to be quite high to harm swimmers, but have never heard a number on it.

Thanks if you know!

[chem geek]

You are correct that with CYA in the water it takes VERY high chlorine levels to be a problem. At typical shock levels of 40% of the CYA level, this is equivalent to swimming in 0.6 ppm FC with no CYA as in some indoor pools (many have 1-2 ppm FC with no CYA so are actually even higher in disinfecting chlorine level).

This is a very non-traditional stance, but it is true with respect to chlorine's reaction rates (oxidation power against skin, etc.). Now drinking pool water is another matter, but even there you have to drink a LOT for it to be a problem.

General consensus is that 10 ppm FC or less is fine, but chemistry dictates that shock levels of 40% of the CYA level are also fine even if higher than 10 ppm FC.