I responded to similar questions of high FC safety on other forums so the question comes up a lot. As far as harshness on the eyes, it is only the active chlorine (hypochlorous acid) that is the strong oxidizer that would affect the eyes so you can't look at FC alone. Also, pH is a much bigger factor than chlorine level in terms of eye irritation unless the active chlorine level gets very high (i.e. no CYA such as in indoor pools and an FC that is higher).
The EPA limit for chlorine is 4 ppm FC, but this is based on drinking water (6-8 quarts per day every day) and does not take into account skin absorption nor the fact that most of the chlorine is not active (hypochlorous acid) but is instead bound to Cyanuric Acid (CYA) in chlorinated cyanurates as described in the technical paper in this link. The manufacturers of chlorine products are required to follow FIFRA guidelines for labels and unfortunately this includes this EPA limit. I've contacted the EPA and found that the group that manages disinfectants (including that for use in swimming pools) was not even aware of the chlorine/CYA relationship (even though it was indirectly implied in section 7a of
this EPA PDF file).
There is not any skin absorption data for the chlorinated cyanurates themselves, but as noted in
this PDF file, skin absorption of CYA is minimal (5 µg/kg/day) and it's likely that the same is true for the chlorinated cyanurates (i.e. chlorine bound to CYA), especially since the primary species are negatively charged.
With the FC kept at around 10% of the CYA level, the active chlorine (hypochlorous acid concentration) is roughly the same as in a pool with 0.1 ppm FC and no CYA.
There are quite a few pool services in desert regions that raise the FC to 14 ppm with 100 ppm CYA which then drops to around an FC of 4 ppm the following week when they add more chlorine again. People don't even notice these high FC levels since the reaction rates and outgassing of chlorine are all determined by the active chlorine concentration and not the FC level.
FC alone is really a chlorine capacity or the amount in reserve and is not an indication of chlorine "activity" or power in the water.
My wife swims in an indoor community center pool over the winter that has 1-2 ppm FC with no CYA and she has to replace her swimsuits after just one winter season of use because the elasticity wears out. Also, her skin gets flaky and hair frizzy so she uses chlorine neutralizing shampoo and soap. None of these problems occur in our own outdoor pool during a longer 7-month summer swim season where our pool has 3-4 ppm FC with 30 ppm CYA. Swimsuits have lasted for years with minimal degradation and her skin and hair aren't as negatively affected. I believe that the difference in her experience is the 10-20 times lower active chlorine concentration in our pool. There are also implications to the rate of production of disinfection by-products in a pool without CYA, but I won't get into that here.
Richard
