Is Fc Level Of 5 Really Dangerous?

[Seahunt]

My CYA of my inground concrete pool is 50. Therefore, I keep my FC around 5 (10% of CYA). After adding liquid chlorine I read the back of the bottle. It said to keep FC between 1.5 and 3 and DO NOT swim if it is above 4. It then went on to say how chlorine is dangerous to humans. Of course, I know you souldn't drink the stuff but don't swim if FC is above 4? Is this stuff the lawyers made them put or is it really dangerous?

[Dan S]

Think about your eyes... If you do it at least wear goggles. I would talk to your local pool store. That much chlorine could be harsh on your skin. Part of the reason why they recommend such a low Fc level is in case someone, especially a child, accidentally swallows in some water.

[chem geek]

I responded to this same question in this post but repeat it below. 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 point about a child swallowing the water is valid, but unless they are regularly drinking from the pool, then that's not the same as an accidental gulp.

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 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 chorinated cyanurates, 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

[Seahunt]

Thanks for the info Richard, very helpful. I'm not very techical so forgive me if I butcher your info by trying to understand it but...are you saying the combo of CYA and FC is really what this is about. In your example about the indoor pool, no CYA and low chlorine is actually more damaging then high CYA and high chlorine.

Indoor pool with no CYA and 1-2 ppm FC is worse then my pool with CYA 50 and 5 ppm FC. That is, the CYA kind of minimizes the harmful (and sanitizing) effects of the FC?

Thanks again!

[chem geek]

You've got this exactly right.

[waterbear]

To add to what Richard said many state health departments allow much higher levels in commercial pools and spas. Here in Florida anything under 10 ppm is considered OK to swim in by Florida statues.