Chlorine levels - is it ever unsafe if it is in range of your cya?

OscarB

Bronze Supporter
Aug 30, 2018
68
Mantua/nj
I totally understand the relationship of cya and free chlorine. My pool has been sparkling for two years now all due to this site. However, I have a question. When people seek help from this site, their cya levels are usually way high - like over 100. With levels like that you need to have very high chlorine levels to slam and then moderately high levels to keep clear.

Here is something I never fully understood... Is a chlorine level ever too high and unsafe to swim in IF it is within range for your cya? For example, with a cya of 110 you need between and 8-13 ppm of chlorine to stay clear- I get that - but is swimming in a pool with that much chlorine ever dangerous? Does the cya have some kind of buffering affect where it is not as potent? So is an 8 chlorine with a cya level of 110 the same concentration of a 3 chlorine level with a cya of 30?
Thank you!
R
 

mknauss

Mod Squad
TFP Expert
Bronze Supporter
May 3, 2014
33,966
Laughlin, NV
Active chlorine levels are different than Free Chlorine levels. As long as your FC/CYA ratio is between 5 and 40%, the water is safe to swim in from a chlorine perspective.
 
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JoyfulNoise

TFP Expert
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May 23, 2015
17,456
Tucson, AZ
Pool Size
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Chlorine
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Free chlorine is really an aggregate value of three distinct forms of chlorine -

FC = [HCy-Cl] + [HOCl] + [OCl-]
Free chlorine = "chlorine bound to CYA" + Hypochlorous acid + Hypochlorite anion

When CYA is present in pool water above 30ppm or so, greater than 95% of the chlorine in the water is bound to the CYA molecule where it is not reacting with anything. The rest of the chlorine splits up into hypochlorous acid (HOCl) and hypochlorite anion (OCl-). Those two chlorine species are often called "active chlorine" as they are the chemical agents responsible for oxidation and disinfection. HOCl is what kills algae & bacteria and inactivates viruses, and it is also responsible for the oxidation of organic compounds. Hypochlorite anion is a weak oxidizer and disinfectant but sit strongly reacts with UV light and is reduced to oxygen gas and chloride ion (Cl-). As the HOCl and OCl- get used up, more chlorine is released from the CYA to maintain an equilibrium balance. So chlorine bound to CYA is often referred to as "reserve chlorine". So in this way, CYA acts as a buffer for chlorine's reactive species.

Because of the chemistry involved, the DPD dye reacts to all three forms of "active chlorine" and so you are measuring an aggregate value. There is no way to measure the three species separately outside of a very sophisticated chemistry lab, so the simpler value of FC is what is used. The problem is, the "harshness of chlorine" depends on the HOCl level mostly and so basing recommendations on FC without considering CYA is illogical and why the industry has such a poor track record at helping people maintain clean and clear swimming pools.
 

OscarB

Bronze Supporter
Aug 30, 2018
68
Mantua/nj
Thank you both for your reply. So do you think that the local pool stores do not understand the relationship between free chlorine and cya or do they understand and not inform people so they can make money?

How about pools for places like swim clubs or hotels? Do they follow this method? Do they understand about cya?

still learning,

Oscar B
 
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tim5055

Mod Squad
TFP Expert
LifeTime Supporter
May 11, 2014
11,067
Franklin, NC
Thank you both for your reply. So do you think that the local pool stores do not understand the relationship between free chlorine and cya or do they understand and not inform people so they can make money?

How about pools for places like swim clubs or hotels. So they follow this method? Do they understand about cya?

still learning,

Oscar B
The majority of pool stores have no idea

Commercial operators (in most states swim clubs or hotels fall in to this category) are generally required to follow a strict set of guidelines set down by the authority having jurisdiction. Most of those guidelines do not take this science into account sadly.
 

OscarB

Bronze Supporter
Aug 30, 2018
68
Mantua/nj
So do those commercial pools ever go green? I can’t imagine the pools in Vegas ever having an issue!


The majority of pool stores have no idea

Commercial operators (in most states swim clubs or hotels fall in to this category) are generally required to follow a strict set of guidelines set down by the authority having jurisdiction. Most of those guidelines do not take this science into account sadly.
 

mknauss

Mod Squad
TFP Expert
Bronze Supporter
May 3, 2014
33,966
Laughlin, NV
There are some organizations that are starting to understand the relationship between CYA and chlorine. But, it is much easier, and safer from a corporate standpoint, to go by the 1-4 ppm FC based on drinking water standards.

Public and/or municipal pools must follow the guidelines from their local health districts. Most will not allow any CYA and chlorine is kept at 1 ppm or so. The water is very harsh compared even to a SLAM level FC. But they focus on person to person transmission of disease. And high active chlorine is needed for that.

Do they go green? Absolutely. And then then dump the water and refill.
 
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OscarB

Bronze Supporter
Aug 30, 2018
68
Mantua/nj
Wow. So the 1 ppm of chlorine in a pool with no cya is very potent compared to a pool with an 12 ppm of chlorine with a cya of 100?
 

mknauss

Mod Squad
TFP Expert
Bronze Supporter
May 3, 2014
33,966
Laughlin, NV
Oh yes.
 
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Palpatine

Bronze Supporter
Sep 3, 2019
180
West Hills, CA
After maintaining my own pool and reading the information on this site I unfortunately don't want to swim in any kind of public/commercial pool again. Heck, my buddy is a pool man and after I see how they operate, compared to the methods on this site, I don't even want to swim in my friends pools any more.
 
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mgtfp

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Mar 5, 2020
594
Melbourne, Australia
Pool Size
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Chlorine
Salt Water Generator
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Astral VX 7T
There is an interesting article that has been linked to in TFP's "Further Reading", that explains how the FC/CYA relationship has been recognized by the EPA now for public pools, and at least partially implemented.

Reference 2 at the end of this article:


The regulations described there acknowledge that higher CYA levels require higher FC levels, a CYA:FC ratio of 20:1 is required to guarantee sufficient sanitation, leading to an HOCl concentration of about 0.02ppm (at pH of about 7.4). But they are still sticking to the old opinion that above FC=4ppm your skin will burn off. They just increased the lower limit, depending on the actual CYA level. The upper limit always remains at 4ppm (that seems to be a federal limit for public pools). Once FC higher than 4ppm is required to achieve a CYA:FC ratio of 20:1, the pool needs to be drained to lower the CYA.

What I find absurd is that these regulations allow an upper limit of FC=4ppm also for CYA=0ppm. At pH=7.4, this is equivalent to an HOCl concentration of 2ppm. This is just absurd - no one at TFP would recommend to swim at these HOCl levels. The highest "swimmable" FC level in TFP's CYA/FC table is 39ppm for CYA=100ppm. This sounds like a lot, being 10 times bigger than the 4ppm that the EPA tolerates. But the HOCl concentration at FC=39/CYA=100 is about 0.3ppm, that is more than 6 times smaller than what the EPA allows for CYA=0. Or to demonstrate it the other way round: FC=4 at CYA=0 is equivalent to FC=81 at CYA=100.

So, the EPA doesn't allow a public pool to be operated at FC=4.1 and CYA=82 (equivalent to HOCl=0.02ppm), but FC=4 at CYA=0 (equivalent to HOCl=2ppm - a hundred times more!) is OK.

I assume the reason behind this is, that to guarantee the minimum FC=1 at the end of the day, they have to allow FC=4 to create a manageable working window for the whole day. The consequent solution would have been to make CYA mandatory, this is the only way to have a sufficient chlorine reservoir without having astronomical HOCl levels. I guess there are still people in the industry who consider CYA to be the source of all evil, so that they rather allowed FC=4 at CYA=0 than to make CYA mandatory.

I hope I got the message of that article right. Please correct me if I'm wrong.
 
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