Despite the settled standards for swimming pool water chemistry, there's a powerful argument to be made for rethinking one of the most fundamental aspects of water treatment: the relationship between chlorine and cyanuric acid. Here, chemical consultant and author Robert Lowry makes his case.

The swimming pool industry has long embraced the recommended standard for chlorine concentrations of 1 to 3 ppm or 2 to 4 ppm, depending on who's doing the recommending. The industry has also observed a recommended cyanuric acid range of 10 to 100 ppm, a range that is viewed as necessary to adequately prevent degradation of chlorine residuals by sunlight.

There are, however, those who now challenge the basic presumptions that chlorine, when used as the sole sanitizer, should be applied within a set concentration range. In a nutshell, the idea is that the proper chlorine concentration should be based on the level of cyanuric acid present in the water — the higher the CYA concentration, the higher the concentration of free available chlorine.


It's an idea that has received little exposure or discussion. Yet according to chemical consultant and author Robert Lowry, one the industry's most prominent authorities on water chemistry, the time has come to rethink the way the industry manages water chemistry.

Specifically, Lowry is recommending that FAC should be maintained at 7.5 percent of the CYA concentration, which ideally should not exceed 50 ppm.


There's no doubt this idea is a radical departure from pool professionals' basic understanding of water chemistry, but it comes from a qualified source.

Since joining the industry back in 1973, Lowry has started two chemical companies (Robarb and Leisure Time Chemicals), authored 12 books, co-founded Service Industry News and developed 111 products. He has worked as an independent consultant since 1995.

Lowry's seminars on water chemistry in pools and spas are widely considered the gold standard for water treatment education within the industry.

"I've always tried to share information with the industry that I believe to be true," he says. "Sometimes that means challenging our basic assumptions. The relationship between CYA and chlorine is one of those assumptions."

Lowry's re-examination of the CYA/chlorine relationship began as a result of his authorship of the Independent Pool and Spa Service Association's two chemical training manuals, which he wrote in 2006 and 2009 respectively.

"I received a correspondence from theoretical chemist Richard Falk," he recalls. "He told me that some of the information in the manual was incorrect, especially where CYA and chlorine are concerned."

Falk shared with Lowry a number of citations to support his critique. One of those came from an out-of-print book titled "The Chemistry of Water Supply, Treatment and Distribution" by Jay O'Brien (Ann Arbor Publishing, 1973).


Falk pointed Lowry to a chapter about the equilibria of isocyanurates. "According to O'Brien, there was more chlorine tied up by CYA than I had previously believed," Lowry recalls. "I subsequently spoke with some of the chemists at Monsanto, which makes cyanuric acid, and also some of the companies that manufacture trichlor. These conversations confirmed the view that there is indeed an equilibrium relationship between chlorine and CYA."

(Lowry also cites chemist Ben Powell for first developing the 7.5 percent concept back in the '70s.)

"Based on existing information," Lowry explains, "I came to realize that at any given time, you only have 3 to 4 percent chlorine available to do anything. That led me to look at the level of chlorine we need to kill algae and bacteria."

Given that there are potentially hundreds of organism species in a swimming pool, Lowry began exploring which of those should be used as a standard for the necessary levels of FAC at a given concentration of CYA.

"Should we use as a standard E. Coli, Pseudomonas, Giardia, Cryptosporidium or some type of algae? When you look at the required chlorine concentration for various organisms, the differences are huge," he explains. "The basic idea is to determine how much chlorine we need to prevent growth. We don't need to kill everything's that in there, but instead prevent growth to the point that these organisms have no negative effect on bathers and water quality. Simply put, we need to kill algae faster than it can reproduce."

With that fundamental question in mind, Lowry lit on the simple answer: Because algae is tougher to kill than most forms of bacteria, maintaining enough chlorine available to prevent algae growth will ensure there is plenty to take care of bacteria in a residential pool. Public or commercial pools require enough chlorine to oxidize organics and accommodate larger numbers of people.

"I discovered that in order to stop the growth of algae, you need only .05 ppm of HOCl, which certainly doesn't sound like much," he says. "The problem is that in the presence of cyanuric acid at a concentration of 30 ppm, as an example, only 3 percent of chlorine is available to do anything. Under those conditions, a concentration of 3 ppm chlorine means that you only have .1 ppm available, but we also have to consider that at a pH of 7.5, only 50 percent of that concentration is available as HOCl, which brings us to .05."

(The amount of HOCl and OCl– is determined by the pH of the water and at a pH of 7.5 there is 53 percent in the HOCl form.)


The problem, he explains, is that when CYA levels increase, the percentage of available chlorine decreases; at 60 ppm CYA, 98 percent of chlorine is bound. [The bound chlorine never gets above 98.2 percent.] For pools treated with trichlor, the level of CYA increases rapidly and concentrations over 100 ppm are typical.

"For every 10 ppm of trichlor that you add, your increasing CYA by 6 ppm, so obviously the level is going to rise very quickly, which in turns binds larger percentages of chlorine," he explains. "If your chlorine demand is 1 ppm chlorine, that means every 10 days, your CYA concentration is going up by 6 ppm, and that's if the chlorine demand is only 1 ppm. If the demand is 2 ppm, which is very common, CYA levels increase at twice that rate."

In effect, he says, a homeowner or service technician may keep chlorine residuals at recommended concentrations but unbeknownst to them have inadequate FAC to be effective. All of which explains why water quality will decline even though the chlorine levels are kept in what we've been taught is the proper range.

(This phenomenon has often been loosely referred to as "chlorine lock" a largely inadequate term that betrays the fact that chlorine and CYA exist in a state of equilibrium.)

"At first, when the concentration of CYA is 20, 30 or zero, chlorine at a normal concentration will get the job done," Lowry explains. "But once you get in ranges above 50 ppm, which can happen within a few weeks, or less depending on the chlorine demand, you come to the point where you start to see algae and you're left wondering what's going wrong.

"While algae itself doesn't present a health hazard per se, common sense tells us that if you're not killing the algae, you also might not be killing the bacteria."

While at first blush, this concept of the CYA/chlorine equilibrium might seem to demonize CYA and chlorine products that contain it, Lowry is quick to point out that without CYA in the mix, it's nigh onto impossible to maintain chlorine residuals because of how quickly chlorine is destroyed by UV light.

"There's no question that in a chlorinated body of water exposed to sunlight, you need cyanuric acid," he says. "What the equilibrium equation tells us is simply that we need to think in terms of maintaining lower levels of CYA."

To that point, Lowry recommends rejiggering the standard to limit CYA concentrations to 50 ppm, meaning that when that level is reached it's helpful to switch to chlorine that doesn't contain CYA, sodium hypochlorite or calcium hypochlorite being the most commonly used.

And, at whatever the CYA concentration, FAC should be maintained at 7.5 percent of that concentration, i.e. at 100 ppm CYA, you need 7.5 ppm FAC.

"It turns out that's the only way we can be sure we have enough FAC to do the job," he concludes.

For a detailed discussion of this issue, check out Lowry's paper here.


Takeaway Points

The implications of the adjoining discussion are far-reaching. To help make sense of the issue, Lowry offers the following takeaways:

• The free chlorine level should be 7.5 percent of CYA. The recommended level of 2 to 4 ppm doesn't always work.

This is not hard to maintain or calculate. If you add CYA separately and then use liquid chlorine as the chlorinating source, the CYA will not change and you will always know what the free chlorine level should be (target). As an example, you add 30 ppm of CYA and you will always need about 2 ppm of free chlorine (30 ppm × .075 = 2.25 ppm). Or if you have 50 ppm of CYA, you will need about 4 ppm of free chlorine (50 × .075 = 3.75 ppm).

Unless you add more CYA, the free chlorine required will not change. This gives you a target for free chlorine and makes it easy to know if you have enough free chlorine to provide safe water and prevent algae from growing.

• CYA maximum is 50 ppm. At that level, switch from using trichlor as main chlorinating source.

As stated in the article, the buildup of CYA from trichlor use is huge. For each 10 ppm of chlorine added to the water from trichlor, 6 ppm of CYA is added. In a typical residential pool, the chlorine loss to sunlight even with CYA and low swimmer loads is about 1.5 ppm per day or about 10 ppm of chlorine per week. This means that if you started with a low CYA of only 30 ppm, in about three weeks you would be at 50 ppm. This is the maximum CYA that we recommend. Even using the APSP guideline of 100 ppm maximum, in 11 weeks you would have more than 100 ppm. Of course using the CL/CYA percentage of 7.5 percent, this would mean that you would need 7.5 ppm of free chlorine.

It's better to use trichlor until CYA reaches 50 ppm and then switch to liquid chlorine, liquid bleach or calcium hypochlorite.

• Using liquid chlorine or bleach will not raise the pH of the water.

Most people believe that using liquid chlorine or bleach will raise the pH of the pool water. This is not so. It is true that the pH of liquid chlorine is about 13 and bleach is about 11. So adding these will initially raise the pH. However, when chlorine is degraded by sunlight or used in disinfection, the result is that HOCl that becomes HCl and this amount of acid is almost equal to the amount of base (sodium hydroxide) in the liquid chlorine. So the result is almost a net zero change in pH. I have said that because in some liquid chlorine there is a slight excess of sodium hydroxide and this small amount may have a minor effect on pH.

• A benefit of using CYA is less exposure to high levels of chlorine.

Using the free chlorine level of 7.5 percent of CYA can result in using a FAC level that is higher than EPA recommends — currently 4.0 ppm. However, EPA's maximum is based on free chlorine in water without CYA. Based on the O'Brien work, which was converted into a formula by Richard Falk, we know that the amount of chlorine bound to CYA is between 96 and 98 percent.

This of course means that at 4 ppm of free chlorine with 30 ppm of CYA at a pH of 7.5 that swimmers are exposed only to 0.112 free chlorine (4.0 ppm × 0.028 = 0.112 ppm). In contrast, at even 1.0 ppm free chlorine without CYA results in an exposure of 1 ppm of free chlorine. So having CYA in the water results in an exposure to chlorine 10 times less than without it.

The roughly 3 percent of chlorine that is not bound to CYA and is free to kill organisms and destroy organics is in equilibrium. As some chlorine not bound to CYA is used in oxidation or destroyed by UV, more chlorine is released from the CYA to maintain the 3 percent. So the 97 percent of the chlorine bound to CYA is a reservoir that replenishes the 3 percent as needed.

• Realize that CYA is a double-edged sword.

CYA is needed to protect chlorine from sunlight degradation. It prevents exposure to high levels of chlorine and acts as a buffer to prevent pH from drifting lower but too much in the water can slow down chlorine disinfection. Using CYA keeps the chlorine in the water eight times longer than without it. With no CYA the loss of chlorine to sunlight is 75 percent in two hours, so in four hours you have essentially no chlorine.


Comments or thoughts on this article? Please e-mail

Eric Herman is Senior Editor of AQUA Magazine.
Now I am confused..... How do i systematically reduce my cya level without having to drain the pool? Ias there a cya reducer available?
Robert Lowry 4 weeks ago
First, you do not need to drain the whole pool all at once. Drain some every week until you get the level down. There is a chemical removal kit available from NC Brands. I think it is the CYA Reducer Kit.
thanks will follow up...... got 110,000 gal pool dont want to drain any any water at all.
The common borax product that you can buy at supermarkets is sodium tetraborate decahydrate. But chemical companies also sell sodium tetraborate pentahydrate, which is a stronger strength of borax, and they also sell boric acid. The two formers requires a little acid to adjust the pH back down (neutralize), and the (boric acid) requires a little bicarb or better yet soda ash to adjust the pH back up (neutralize). They all add boron to the water.
I would not consider it a small amount of acid to lower the pH from using sodium tetraborate pentahydrate (sometimes called 5 mole borate) or sodium tetraborate decahydrate (borax), (sometimes called 10 mole borate). These two chemicals will raise the pH to about 9.0 and increase alkalinity by nearly 120 ppm for a 50 ppm dose. The amount of acid needed for 5 mole is 2.12 fl oz of muriatic acid per 1 oz of borate or 1.62 fl oz of muriatic acid per ounce of 10 mole borate. This will mean you will add 675 oz of 5 mole or 885 oz of 10 mole and you will need to add 11 gallons of muriatic acid in a 15,000 gallon pool to get TA back to 90 ppm which will then make the pH too low so you will have to aerate to bring the pH up with no change to alkalinity. If you use boric acid, the only change will be the pH will lower by 0.2 which you can raise by aerating and causing turbulence with no change to alkalinity. Also, the amount of boric acid is 570 oz instead of 675 oz or 885 oz. Boric acid is much easier and cheaper to use.
Thank you Robert for clarifying the amount of acid needed when adding borax. I should have mentioned and clarified that I never add or advise to add all 50+ pounds of borax at one time. I only add about 4 pounds per treatment visit until I achieve the desire level.
Robert, please check your math again. I believe your "11 gallons of muriatic acid" to add is far too much, at least for the typical pool acid of 31.45% strength.
Sorry, the amount of 5 mole borate is 675 oz so divide this by 1.62 to get 416 fl oz of muriatic divided by 128 fl oz per gal equals 3.25 gallons. The amount for 10 mole borate is 885/2.12 = 417.5 fl oz or 3.25 gallons also. I am not sure where the 11 gallons came from Very sorry for the confusion.
While this seems like rock solid advice, I have been following this TFP protocol for 12 months in an 18 month old fiberglass pool with a SWG and I am getting what I think is damage to my gelcoat according to a FB Pool expert that has no monetary interest in saying either way what he believes is the issue. A few others on the TFP forum have had fading or what they call oxidation as well although they believe for different reasons. The expert is steadfast that chlorine above 5ppm for even as little as 18 months with perfect balance or as little as a few weeks if PH is out of range ( his opinion and experience only) can etch a gelcoat. Of course fiberglass manufacturers say anything >3ppm with all other chemicals compliant. TFP also recommends for a SWG that you maintain CYA of 70-80. Even with the lower ratio 4.5% chem-geek provided for SWG, this makes a very narrow range between safety for swimmers vs. safety for FB gelcoat. For example at CYA 70 the MINIMUM is 3.15ppm FC. This is already puts you out of manufacturers warranty. At CYA 80 the target is 5 ppm. Would the CYA tie up the damage that FC can do to FB gelcoat as well theoretically? Any thoeries on why you would want CYA higher with a SWG?
I agree with Chem-Geek. One of the reasons for higher CYA with an SWG is because the concentration of chlorine at the injection point is sufficient to use up all of the available places that chlorine can attach to CYA. This leaves some chlorine not protected and as it enters the pool on a sunny day, the UV rays of the sun degrade chlorine very quickly. The UV degradation near the surface is 90% in 2 hours. Therefore, you can see that any unprotected chlorine is degraded quickly. Also, using this higher level of CYA means the chlorine is around longer and you will not need to make as much. This extends the life of the SWG. If you add 50 ppm borate to the water and using an SWG,you could successfully have the free chlorine level at 3% of CYA with a minimum of 2.0. So if using 70 ppm CYA, 50 ppm borate, FC minimum could be 2.1 ppm. So you could be in the 2.0-3.0 FC range. The borate is an algaestat (not an algaecide) so it prevents algae. In addition, borate also slows down the rise in pH caused by the SWG. So you will use less acid and/or bicarb after lowering pH and having alkalinity too low. I do not know about an SWG affecting gel-coated fiberglass. I have only anecdotal information just like TFP has. Even at 30 ppm CYA there is only 3% of all the chlorine in the water that is not bound to CYA. So the exposure to HOCl or OCl- is minimal to bathers or the vessel when using CYA. Hope this helps a little.
Thank you Robert! I appreciate it. I have never heard of a Boric Acid component to a pool. I am assuming it's been studied to be safe for swimmers long term? Although I must admit, I have not researched the safety of higher chlorine level effects or even CYA either.
There is a free 12-page tech Bulletin available on this website: Just go there and when it opens you will see a pop-up that asks you for your email. You can download the bulletin for free. It will answer all your questions.
I do believe CA needs to be maintained in the 30-50ppm range! Bio active does not reduce CA, and Ben Powell didn't even get into to pool industry until mid 80's.
Jason B wrote:
Carol wrote:
How do you find a pool service company that thinks this way? My last pool guy was keeping the CYA at 30ppm with a salt generator- pool chlorine level was always in my standard test kit range. The problem is with my PH - it is constantly rising. He was adding muriatic acid every week which would bring the TA done to 30 or 40. Then add the sodium bicarbonate to bring the TA up. A few days later the PH was way too high again. So I either have take over this myself which I don’t want to do or find someone who thinks like this. What should I ask for interviewing? The last guy I interviewed said I needed to bump my cya up to 120ppm! Ahhhh!
. This is a common problem with saltwater chlorine generators they will raise the pH daily I could go into a lot is scientific reasons that might just confuse you. You should try and add 30-50ppm of borates which will buffer your ph from increasing you want tetraborate decahydrate a great product is endure by Ultima
. Sorry typo sodium tetraborate pentahydrate
We are working on it. I have started Pool Chemistry Training Institute and in the last 2 weeks we have had almost 100 people take our 1-Day Pool Chemistry for Residential pools class, pass the exam and become certified. We will be putting the names of residential pool chemistry certified technicians on our website. We have held classes in Concord, CA, Sacramento, CA, Canoga Park, CA, Ontario, CA and Phoenix, AZ. The books used in the course are Pool Chemistry for Service Pros, 28 pages, $8.99 plus shipping and Pool Chemistry for Residential Pools, 226 pages, $49.95. Both are available to anyone on our site.
Carol wrote:
How do you find a pool service company that thinks this way? My last pool guy was keeping the CYA at 30ppm with a salt generator- pool chlorine level was always in my standard test kit range. The problem is with my PH - it is constantly rising. He was adding muriatic acid every week which would bring the TA done to 30 or 40. Then add the sodium bicarbonate to bring the TA up. A few days later the PH was way too high again. So I either have take over this myself which I don’t want to do or find someone who thinks like this. What should I ask for interviewing? The last guy I interviewed said I needed to bump my cya up to 120ppm! Ahhhh!
. This is a common problem with saltwater chlorine generators they will raise the pH daily I could go into a lot is scientific reasons that might just confuse you. You should try and add 30-50ppm of borates which will buffer your ph from increasing you want tetraborate decahydrate a great product is endure by Ultima
How do you find a pool service company that thinks this way? My last pool guy was keeping the CYA at 30ppm with a salt generator- pool chlorine level was always in my standard test kit range. The problem is with my PH - it is constantly rising. He was adding muriatic acid every week which would bring the TA done to 30 or 40. Then add the sodium bicarbonate to bring the TA up. A few days later the PH was way too high again. So I either have take over this myself which I don’t want to do or find someone who thinks like this. What should I ask for interviewing? The last guy I interviewed said I needed to bump my cya up to 120ppm! Ahhhh!
What about UV sanitising and Cl/CYA levels? Any input is appreciated!
I totally support this level of thinking because I've applied it to how I am managing my water chemistry this year and for the first time I have seen crystal clear water with truly balanced levels. I opened the pool in June, which is about the time I do every year in Southern NJ, and this year I noticed a yellow gel like fungus and some black near the skimmers. I knew my CYA was not very high when closing the pool and it was closed for about 7-8 months so perhaps I should have treated the water during the winter. But, this year I went the distance and bought the higher end test kit, bought all the proper chemicals and had them all on hand, and made sure I was very attentive to accuracy for all testing.

I used CLOROX Pool&Spa 23025CLX XtraBlue(rated 94% on the label). Best 3" tabs I've ever used because they actually pushed the CYA levels up the way the should. I was getting an average increase of 1.4ppm of CYA per 3" tablet so once I got my TA, Ph, and CH levels to their ideal recommendations for a plaster pool, I put 3 of the 3" tablets in both skimmers to boost my CYA and also placed 4 more into the floater. I saw total chlorine levels rise to above shock levels (more than 10) and they did not drop dramatically like the HTH trichlor 3" tablets I was using before. And the 1lb bags that I had never boosted chlorine levels for any length of time worth talking about. Maybe this is a plug for clorox but hey, these things worked.

Anyway, I waited a few days, and as the tablets dissolved, I watched the chlorine levels slightly falling and after a week or so, I added 3 more pucks to each skimmer. In 2 weeks I was able to gradually increase the CYA to 30+ and boy does this make a difference when it comes to keeping levels up for longer periods of time. But I also knew that the CYA would increase as the season continued. So I started to shock with liquid Calcium Hypo. I have room to play with my CH as it is usually about 200ppm when I stop adding anything to raise it.

Today I am holding steady with
CYA @ 40ppm
TA @ 90ppm
Ph @ 7.5ppm
CH @ 250ppm
As for chlorine, I am still learning how to use the test kit but for all intents and purposes, I am maintaining the ideal setting of 5/10 which is 5-Cl and 10-Br according to the kit and using a test strip that shows I am hovering around 3ppm or so for FC and I also run a G35 nature 2 cartridge to help sanitize the water without having to maintain such high levels of chlorine. I have the daily chlorine level drop to 1-2ppm with Ph @ 7.2 while using the Calhypo liquid and all other test readings were being maintained with the exception of the Ph spikes when shocking with calhypo liquid. I feel the success of maintaining a good balanced pool with very low cost is related to keeping my CYA levels from exceeding 50ppm and also learning the relationship between CYA and the different chlorine levels that most people never knew about. And all of this was picked up from this read so even if I am not completely educated I am definitely more aware and more educated as a result of what I was able to grasp thus far. And I believe the CYA and chlorine relationship is very important to good water chemistry indeed
That's great. If you want a book, there is a book called " Pool Chemistry for Service Pros" available as an eBook from Amazon for $9.99 that has a how to method for maintaining your pool chemistry. Print version are available for $8.99 from IPSSA and NPC but they may not sell to you. It is a great book. I know because I wrote it. Sorry for the plug.
Saif wrote:
This pool is very creative settings.

Very interesting reading the linked thread from 2007. I especially like what Chem Geek was trying to change about TFP advice regarding TA levels so many years ago. The correction that Chem Geek was trying to address, and that he was still trying to change in 2015 when I faced a troublesome pool caused by a TA level too high was/is that all pools have different out-gassing rates and other factors, such as fill water with different chemistries, and as such, pool owners and operators should be aware that the published TA level recommendations are just averages and should not be considered rigid parameters, as owners would not know this unless it's addressed in the pool school curriculum. Only if someone runs in to a problem with drifting PH and specifically asks why? will the coaches tell members about the TA dilemma.

At or about the summer of 2015, I was a new TFP follower. I had read and studied all of pool school. I had a good test kit to do all my own testing, and I was following all the tenants TFP and keeping all my levels in range as published. I felt I had a good understanding of all the basics, however, I did not have a trouble free pool. At least twice per week, I had to add an acid dose to keep PH in range. The suggested TA range back then for my kind of pool was 70-90 (it's 50-90 nowadays), and so not being a chemistry-minded person and simply following what was being taught by TFP and trying to keep all levels in range, whenever I'd do a full round of tests, after all those acid doses dropping not only PH but also TA, about every two or three weeks, my TA would be out of range on the low side. And being a good student of TFP, I'd add enough baking soda to get my TA back up to at least 70; I'd target 80. And so of course, I was creating my own work, but didn't understand it. After weeks of this, I asked about it on the forum. I asked if I could leave my PH at 8.0, because I never even thought to ask about TA, since I wasn't constantly adjusting that number. I didn't get much help for a few days other than coaches telling me to make sure I keep the PH in range and try to avoid aerating, but then finally Chem Geek came on and asked me for some numbers. He then posted that I could let my TA drop to around 35 before I was at risk of a PH crash, and that I should just keep lowering PH until it quit drifting and then leave TA where it was. It worked. That summer, the TA level that best kept my PH in range was 40. Nowadays, it's 50; and I've not adjusted PH or TA in a season and a half.

Nowadays, TFP coaches address this asked about issue at least twice per week during the season, and nowadays they are quick to give the right answer, but I find it odd that they chose not to change the curriculum in or to keep things simple when it's such a common problem. And I also wonder how many pool owners never asks about this dilemma and choose just to keep adjusting PH or just let PH drift high?