Chlorine is a great sanitizer for pools. It's been the backbone of our effort to keep pool water clean, and will remain so for the foreseeable future. But a controversy has emerged in recent years about the process of superchlorinating — what we call "shocking" — pools to reduce the buildup of chloramines.

The standard procedure for dealing with chloramines is laid out in black and white in the CPO manual: You blast (shock) it with a high dose of chlorine — 10 times the measured amount of chloramines — and that takes care of that.

This has been the accepted, customary practice for decades, notes Ed Lightcap, senior account manager for DuPont. "If you take the CPO class and become CPO certified, you will learn that the remedy for a buildup of combined chlorine is superchlorination. Everybody is being taught that. It has been passed down from generation to generation in the pool industry, without close examination."

Without getting too technical, traditional breakpoint chlorination is a process for dealing with chloramines or combined chlorine. You find the total chloramine content in a pool and hit it with 10 times that amount of free chlorine, which is supposed destroy the chloramines.

Of course, that plan only works if the added free chlorine really does, in fact, destroy the chloramines. Turns out it only destroys some of them.

It's important to understand that there are different kinds of contaminants in pool water, and superchlorination treatments affect them differently.

Study #1

graph showing chlorination of creatinine

"In this test (right), we dosed the water with creatinine, an organic component in human sweat. Here we do not see the classic breakpoint curve like we see with ammonia and inorganics. We see the buildup of persistent organic chloramines that remain no matter how much chlorine is added. The added chlorine did nothing but produce a persistent chloramine where there was none." — Tom Tufano

Study #2

graph showing chlorination of human sweat

"This test (on right) shows the chlorination of a sample of human sweat dissolved in water. With a mixture of inorganic (red line) and organic (black line) chloramines, the inorganics react while the organics persist." — Tom Tufano

Breakpoint chlorination techniques are really only effective against inorganic, ammonia-based chloramines. They do not impact the organic chloramines present in all pool water. In fact, they create new worrisome issues by forming rather nasty disinfection byproducts (DBPs), some of which go airborne and cause well-documented problems in the lungs of frequent swimmers and children. (There is a rising chorus of concern in the health community about this issue. There are studies that correlate DBPs with the onset of asthma in children.)

This statement is supported by lab research at DuPont performed by Dr. Tom Tufano, senior R&D chemist, and by work carried out by Professor E. R. Blatchley and Dr. Jing Li at the Purdue University School of Engineering.

In simplest terms, Tufano's argument boils down to two major tenets:

1) Complex organic chloramines do not undergo breakpoint chlorination to form N2 gas, but instead, build up over time in pool water.

2) Large doses of free chlorine react with organic contaminants to form a variety of DBPs which are hazardous to swimmer health.

"The problem is that since you are adding chlorine based on the total combined chlorine instead of just the amount of ammonia-based, inorganic chloramines with which it will react, this leads to a gross over-application of chlorine," Tufano says. "And that over-application leads to all kinds of problems, mainly the formation of chlorinated DBPs, which become airborne."

Perhaps most pernicious of these DBPs is trichloramine (NCl3), a known and confirmed bad actor in the pool environment.

"Large doses of chlorine, as recommended by the traditional superchlorination approach, instead of getting rid of chloramines, result in the production of trichloramine, a noxious lachrymating gas that will volitilize out of the water," says Tufano, "and that's what's responsible when you go into a pool area and your eyes start watering — you're probably feeling the effects of trichloramine in the air. You only see it at very high chlorine to nitrogen doses, but as we know, the current practice of superchlorination puts you in that regime."

Every time somebody runs a traditional breakpoint reaction for the purposes of trying to get rid of combined chlorine, Tufano says, they are getting rid of the inorganic chloramine — not as nitrogen gas, N2, which is inert, but as trichloramine or some other DBP harmful to swimmers.

This particular point is confirmed by Blatchley and Li's research at Purdue, which simulated superchlorination procedures and measured resulting DBPs. This research concluded that even inorganic chloramine, which superchlorination does oxidize (remove), is dispatched in such a way as to cause harmful gases to be released.

In summary, Tufano says: "We should stop the practice of superchlorination. That just exacerbates the problem of forming disinfection byproducts that get into the airspace and degrade air quality especially indoors. We should restrict the amount of chlorine we apply to what is needed to sanitize the pool, as opposed to sanitation plus oxidation."

"The concept of breakpoint chlorination, when applied to the real-world mixture of organic and inorganic chloramines in pool water does not work. High doses of chlorine result in high levels of chloramines which, in the end, give you large amounts of chlorinated DBPs."

The Wider Community

That's the view from DuPont, which, in the interest of full disclosure, makes a non-chlorine oxidizer. DuPont has been promoting chlorine-free oxidation for many years and believes it offers significant advantages versus chlorine shock treatments.

The assertion that superchlorination is ineffective against organic chloramines is generally accepted among the chemists and industry personnel contacted for this story. After that broad accord, opinion becomes more nuanced.

SeaKlear's Terry Arko notes the problems inherent with superchlorination and recommends the use of potassium monopersulfate (MPS) shock, which he says achieves the same result as breakpoint chlorination. But either approach, he notes, only reduces the inorganic form.

"Organic chloramines," he says, "are only reduced by draining and dilution. One practical way to know if you have a majority of organic-bound chloramines is to test and follow the procedure for determining breakpoint chlorination. If superchlorination or chlorine-free shocking does not reduce the level of chloramines, that would be an indication that there are organically bound chloramines present."

Discovering what kind of chloramine is in the pool is important, notes Jeff Smith, product manager of sanitizers and water chemistry, Pentair Water. "The issue is that no one knows whether they have organic or inorganic combined chlorine until they shock and see what residual combined remains. In that situation, I would recommend a monopersulfate shock, but even this method is not proven to be completely effective against all combined chlorine.

"I still promote superchlorination when needed to maximize your free available chlorine," he adds.

Smith brings up two important points in the discussion. The first is that airborne DBPs are much more of an issue for indoor pools than outdoor ones, for obvious reasons. He notes that proper ventilation will minimize swimmer contact with these gases in a closed environment.

He also points out that ozone is an excellent alternative, as it is much more effective as an oxidizer than chlorine shock.

Stay Tuned

It's actually a very exciting time to be a chemist in the pool industry (or even to be writing about it) as old doctrines and traditional methods are under increasing scrutiny, in part because of new technology.

"We're still learning the details of the shock process," says Mary Costanzo, manager, technical services and process excellence for technology, BioLab, Marietta, Ga.

"There are things we're learning now because the instrumentation didn't exist even five years ago. We're able to measure some things that we just weren't able to measure before. And so our understanding of pool chemistry is expanding."

As far as the question of the best methods of shocking pools, Costanzo says the jury is still out. "In terms of understanding what the real answer is, I don't think we're quite there yet."

Comments or thoughts on this article? Please e-mail

Scott Webb is Executive Editor of AQUA Magazine.
Dear Scott, I read your June Making Waves editorial and your article, "Shock & Gnawing Doubts" (clever title) and am compelled to be one of the many who have responded to this article. I will focus on clearing up several technical inaccuracies about the CPO® (Certified Pool/Spa Operator®) certification materials and program. Here is a short list of the article inaccuracies: There is no such thing as a "CPO manual." People who call it this are using jargon and are inaccurate. The National Swimming Pool Foundation publishes the NSPF Pool & Spa Operator Handbook (Handbook) that is used by instructors who teach the CPO® certification course and by thousands of students and industry professionals. The article states that "The standard procedure for dealing with chloramines is laid out in black and white in the CPO [certification] manual: You blast (shock) it with high doses of chlorine - 10 times the measured amount of chloramines - and take care of that." The article also offered surprising comments from my friend and a man I greatly respect, Ed Lightcap's stating that "If you take the CPO [certification] class and become CPO certified you will learn that the remedy for a build up of combined chlorine is superchlorination." For the last seven years the Handbook has deemphasized the vague and ambiguous term "shock." The Handbook, "in black and white", does describe the two types of "shocks" according to EPA guidelines. Some "shocks" also deliver disinfectant to the water. Others are "oxidizing shocks" that do not provide disinfectants, but oxidize contaminants in the water. I am pretty sure we don't use the word "blast" anywhere in the Handbook. The first five people who read the Handbook and find "blast" anywhere in the Handbook, I will send a free NSPF book of your choice. The 2007 World Aquatic Health Conference featured a day-long symposium on Disinfection By-Products (DBPs). This symposium in part reported the NSPF-funded research with Dr. Blatchley and Dr. Li at Purdue University (to which you refer in the article). The 2009 edition of the NSPF Pool & Spa Operator Handbook included a rewrite of the "Pool & Spa Water Problems" chapter to address the research findings of Dr. Blatchley and his team. The revised chapter listed and explained five methods to remove combined chlorine from water: 1) Water Replacement, 2) Breakpoint Chlorination, 3) UV Systems, 4) Ozone, and 5) Indoor Air Handling. I believe the NSPF Pool & Spa Operator Handbook was the first industry educational manual that describes the difference between organic and inorganic chloramines. By the way, NSPF has spotlighted Dr. Blatchley's research and other scientists who have been investigating DBPs at a number of the World Aquatic Health Conferences. Those seminars are available to be viewed online for anyone who wishes to learn more about their research. Three years ago, National Swimming Pool Foundation changed and updated information about the "breakpoint" method described in the 2008 Handbook - and other NSPF educational materials - to reduce the dosage of chlorine in the 2010 Handbook. The ratio is still 10:1 of Free to Combined Chlorine to give slight excess as required for the inorganic chloramines to completely react. We reduced the chlorine dosage by subtracting the existing free chlorine from the amount that is added to minimize the risk of disinfection-by-product formation and using excess chlorine. Also the slightly lower level is needed due to some organic chloramines in the water. Unfortunately, inexpensive practical ways to measure organic chloramines is not readily available. Entrepreneurs - there is money in them there hills to the prospector who finds an accurate and precise "organic amine/amide" method. AQUA is an exceptional trade periodical. We feel very fortunate that the editorial team has great passion for improving our standards and elevating our industry. The hundreds of National Swimming Pool Foundation Instructors and staff share that commitment. We are so much in line with protecting people and thus helping more people live healthier lives, we put our money where our mouth is. A portion of the green that comes from the "black and white from the Handbook" has funded over $ 3.7 million dollars in research to resolve many key issues in our field. So if industry members are thinking of helping shape our industry's future, enroll in a CPO certification course, learn from the most cutting edge science based information in our field at the World Aquatic Health Conference, and be a part of a brighter future.
Daniel M Tucker Friday, 03 June 2011
I have been treating pools for over forty years. I have never had a problem wih chloramines because of the Chlorine Genie. On pools that previously had chloramine problems, the use of the Chlorine Genie has eliminated the chloramine problems. The Chlorine Genie does not require salt to be put in the pool, instead the salt is put into a brine tank. When the chlorine gas is separated from the brine water it is mixed with tap water inside the unit and dispensed directly into the pool water. As a result a portion of the pool water is over the break point before the chlorine is diluted throughout the rest of the pool. The unit also can control the pH of the water without adding acid or soda ash. By keeping the water in balance with this method we have never had a problem with chloramines. On our commercial pools, we have installed the model of Chlorine Geine that also manufactures copper/silver ions and controls the output to keep the ion levels at around 1 to 1.5 parts per billion. At that level we have never had any staining but at the same time have enjoyed the benefits of the synergistic effect of chlorine in combination with copper/silver ions. We had installed thousands of units on residentual and commercial pools in Northern Canifornia so I really can't speak for the rest of the country. We will be opening national distrubution of the Chlorine Genie's generators in 2012. It is truly the next generation of automated on-site pool sanitatio and has been thoroughly field tested.
Karen Spandikow Friday, 03 June 2011
I run two indoor pools and have lowered my CC considerably by simply adding enzymes to the surge tanks. I highly recommend trying this approach, we have not had to shock our pools in quite some time.
Thanks for bringing up this issue and posting it for discussion. Two thoughts from the real world experience of a large pool service company in the Florida Market: First, anyone who superchlorinates with swimmers in the water is irresponsible so the off gassing issue you discuss should never come into play when swimmers are present. Further on this point superchlorination in our experience removes this off gassing which is a direct contradiction to your premise "...says Tufano, "and that's what's responsible when you go into a pool area and your eyes start watering - you're probably feeling the effects of trichloramine in the air. You only see it at very high chlorine to nitrogen doses, but as we know, the current practice of superchlorination puts you in that regime."" If your premise were true we could never remove that eye stinging issue without draining a pool, and that is not the case, Superchlorination corrects that issue in our experience in the field every time. Second, if your premise that "The concept of breakpoint chlorination, when applied to the real-world mixture of organic and inorganic chloramines in pool water does not work. High doses of chlorine result in high levels of chloramines which, in the end, give you large amounts of chlorinated DBPs." If this were the case and as your graph implies creates a persistent nonremovable chloramine then in the field we could never get free chlorine to be near equal to total chlorine, which is not the case. In fact every time a pool is chlorinated under your premise we should be creating a larger gap between free and total chlorine which is again not what is happening in pools, so I have to question your premise based upon what is actually happening in pools.
I applaud Scott Webb for this well written, well-researched, open-minded article. As I understand it, the organic contaminants in the pool/spa water are the main culprits that help create organic chloramines. It is obvious to me that the addition of a safe and strong oxidizer would help to eliminate these organic contaminants before they have a chance to produce organic chloramines in the first place. Ozone has been in use (doing just that) for about 100 years globally in multiple applications including pools and spas. Ozone is recognized by the EPA as an antimicrobial oxidizer, which is significantly stronger than chlorine (even in chlorine's free state [FAC]). The pending MAHC is recommending the use of ozone for this reason as well as for its proven ability to effectively kill Cryptosporidium parvum. There are exhaustive data available on ozone's efficacy and safety for indoor pools and spas, as well as considerable cost savings reports for its use in recreational water. Ozone is simple to install and its management system is designed to seamlessly integrate into an existing pool or spa plumbing system (residential or commercial). If we are looking for scientifically-proven (and documented) solutions to chlorine's disinfection byproducts in recreational water, we are remiss if we ignore the multiple benefits ozone's strong oxidation and antimicrobial properties offer to support chlorine.
I've read that medium pressure ultraviolet light and corona discharge ozone are two supplementary treatment methods that also reduce chloramines. From what I've read, they are so successful that they reduce or eliminate the need for superchlorination. These systems also reduce the required levels of free chlorine residual in the pool. Maybe this is the answer to this article? The only other alternative I see is to upsize the pool HVAC to provide even more fresh outside air to dilute the DBP pool environment. (This is the solution presented in CDC's model aquatic health code's (MAHC) recently released ventilation module.) BUT, this extra equipment AND energy cost is almost sure to be much more than the cost of a properly sized UV or ozone unit. Good article, Mr. Webb. Very interested to learn more on this topic.
Adding metals such as silver and copper can only augment chlorine, not reduce chlorine demand. The kill times of these metals is long and a build up can cause staining, especially if the pool owner or their designate is less than skilled at testing and seeing to the needs of the pool. MPS is still too expensive and doesn't offer much more than oxidation, no sanitation. Great for indoor facilities though.
Thank you for this article. The 10x rule was a mis-application of a chemical rule for chlorine oxidation of ammonia that should never have been used for Combined Chlorine (CC) in the first place. The 10x originally came from needing at least 3 chlorine for every 2 ammonia (so 1.5 to 1) in molecular (molar) units where multiplying by the unit of measurement difference between ppm Cl2 and ppm N (which is 5.06) gives 7.6. Somewhat higher amounts are needed to account for secondary reactions and getting over the dichloramine "hump" so 8-10 is usually quoted, hence the 10x rule. However, that rule only rightly applies to chlorine oxidation of ammonia each measured in their own differing units of measurement. In other words, the 10x rule used for CC is a mis-application because the units of CC are already in chlorine units, ppm Cl2, while the units of ammonia are in nitrogen units, ppm N, which are a factor of 5 different. Also, as described in earlier articles, one of the required 1.5 chlorine per ammonia is already part of monochloramine. So for inorganic chloramine, 10x was never the needed amount. Besides, nothing would ever get "stuck" if you didn't use enough chlorine since you could just add more to continue the reactions where they left off. As for organic chloramines, the largest nitrogenous component of sweat and urine is urea, not creatinine, and urea is slow to get oxidized by chlorine as demonstrated by Professor Blatchley's experiments. Much of the persistent CC may be chlorourea since urea builds up to higher levels in pools unless removed through dilution, supplemental oxidation, or coagulation/filtration. Nevertheless, using creatinine is a reasonable example demonstrating the problem of persistent chloramines. On the other hand, it is incorrect to imply that these organic chloramines are necessarily a problem -- some may be, some may not. Chlorourea has a much higher odor threshold than the inorganic chloramines and does not appear to have the same health concerns. So what ends up happening is that some CC measurements may be showing chlorourea and not really being a serious problem, yet pool codes focus on keeping CC at 0.2 ppm or less. It is true that there is more nitrogen trichloride produced at higher chlorine levels, but what is also not discussed in this article is how the relevant chlorine level is that of hypochlorous acid and that in the presence of chlorine stabilizer (Cyanuric Acid, CYA) that level is much lower. So the rate of production of disinfection by-products as well as the amount of nitrogen trichloride should be lower in stabilized pools. Ironically, such pools if not exposed to sunlight could build up more urea as well as a steady-state of chlorourea so would register higher CC even though the far more serious nitrogen trichloride level would be lower. More research is clearly needed in this area and in particular investigating what happens in stabilized pools (i.e. when CYA is present) and in pools exposed to sunlight where the amount of CC is usually lower possibly due to the greater oxidation of organics from hydroxyl radicals created from chlorine breakdown from sunlight.
One way to mitigate the amount of chlorine needed is with metal ionization, which has worked well for us for decades. The author writes "as old doctrines and traditional methods are under increasing scrutiny", and at the top of the list, right next to break point chlorination is ozone. There's a good reason that people in metropolitan areas are warned on sunny summer days when ozone levels are high to stay indoors and refrain from exertion. Ozone is at least as bad a pollutant as DBPs, in regard to what it does to the air quality in the pool or hot tub area. Not a week goes by that we don't hear from a hapless hot tub owner who needs to disconnect the ozonater on his brand new spa because it's irritating his child's asthma. When it comes to oxidization, MPS is still the best that we have. Maybe the chemists can come up with an oxidizer that breaks down all forms of chloramines. That seems to be the so-called "Holy Grail"!