If combined chlorine levels get too high, the local health department might shut a commercial pool down. We have seen it happen before. But while pool closures are rare, high combined chlorine numbers are disturbingly common.

How is combined chlorine measured?

Without being too redundant on what combined chlorine is (because we have already written an in-depth article about it), let's talk about why it matters. From a testing perspective, combined chlorine is just the difference between the Total Available Chlorine (TAC) and the Free Available Chlorine (FAC). 

TAC - FAC = Combined Chlorine (CC)

It's easy to measure when you have a quality chemical controller with probes for both TAC and FAC. The controller can just spit out the math, giving pool operators a real-time combined chlorine reading.

Without an automation system, a commercial-grade test kit should allow you to test for both TAC and FAC. Then it's just simple subtraction. 

When combined chlorine gets over 0.2 ppm, chloramines are considered a problem, and the health department starts to care.

Why do combined chlorine levels matter?

Having more than 0.2 ppm of combined chlorine indicates that your chlorine is falling behind. Your chlorine is fighting a significant enough oxidant demand–particularly with nitrogen compounds like ammonia–that its ability to sanitize is becoming compromised. This is a potential problem for the health and safety of bathers, since chlorine is the primary sanitizer. It's our first line of defense against Recreational Water Illnesses (RWIs). 

Combined chlorine levels are also a reliable metric that tells us additional information–much like ORP does. We can tell instantly how efficient (or inefficient) our chlorine is.

But as far as indoor air quality is concerned, combined chlorine is the closest measurement we have for knowing the rate and severity of airborne chloramine production. The higher the combined chlorine, the worse the air is going to smell. So it behooves pool operators to take combined chlorine seriously; try to mitigate it in the first place, and reduce it when it climbs.

How to reduce combined chlorine

There are two general ways to reduce combined chlorine that we will discuss here: chemically and physically.

Chemically

Superchlorination is by far and away the most widely used and accepted practice for addressing combined chlorine. Superchlorination is dumping an excess of free chlorine (the industry standard is 10x the level of combined chlorine, though that's more than necessary) into the pool. This surge of oxidizing power destroys the oxidant demand pretty effectively. It also kills just about everything that might be lurking in the water too, which is a plus. The downside is that the pool must be closed for superchlorination, and when it's done, the FAC level needs to be brought back down.

With respect to the breakpoint chlorination process, superchlorination is the rocket boost that finally pushes past the demand, achieving and exceeding the breakpoint, and building a free chlorine residual. In the chart above, when chlorine struggles to get over the hump at point (B), superchlorination is a reliable option.

Non-chlorine shocking involves adding more oxidizer to the water, but in a form that is not chlorine. The chemical most commonly used is potassium monopersulfate. It can help with oxidants, but will not add disinfecting power.

Enzymes do not directly reduce combined chlorine, but they directly supplement chlorine against non-living organic bather waste. Enzymes metabolize carbon-based body products like body oils, deodorants, cosmetics, lotions and sunscreen. That way, chlorine has less to oxidize. Less oxidant demand means more chlorine in the war against nitrogen compounds.

Related: Pool Water Chemistry Resources

See, chlorine is an outstanding sanitizer and disinfectant, but a relatively weak oxidizer. It gets reduced quickly.  Chlorine can remove carbon-based waste (albeit at a severe cost). But unlike carbon-based organics, chlorine cannot simply oxidize and destroy nitrogen compounds. It combines with them...hence the name combined chlorine. Eventually they get destroyed, or in the case of chloramines, off-gassed into the air.

Hydrogens are replaced by Chlorides as more and more chlorine attacks.

There are some other chemicals that have been explored, but are not yet proven to be safe enough for use in swimming pools. One of them is chlorine dioxide, which is a very powerful form of chlorine, capable of destroying chloramines with relative ease. It is not used in commercial pools at this time, however, but it is popular in other industries.

Physically

Secondary sanitization systems like UV and Ozone are quite popular in commercial pools.  Ultra Violet (UV) systems can destroy formed chloramines, but not nitrogen compounds themselves. UV itself is not an oxidizer, it is a sanitizer. That said, it is capable of breaking down monochloramine and dichloramine. Once trichloramine is formed and off-gassed, UV can no longer touch it...for obvious reasons. This means UV has a limited impact on indoor air quality. But we are told that if trichloramine does stay waterborne and passes through a UV chamber, it will be destroyed.

Related: Comparison of the Best Secondary Systems for Swimming Pools

Ozone is also a point-of-contact system, like UV, so ozone also has a limited impact on indoor air quality. One advantage ozone has over UV, however, is that ozone will oxidize and destroy just about anything. Yes, that includes nitrogen compounds, organic bather waste, and waterborne chloramines. Ozone is also a powerful sanitizer that can kill germs and diseases. Ozone is the clear favorite for outdoor pools, mainly because a major contaminant in outdoor pools is sunscreen. UV systems have a hard time shining through a substance that is designed to block UV. Ozone does not have such an issue.

Hyper-Dissolved Oxygen (HDO)  is a relatively recent technology in the pool industry, though it has been around for many years in other industries like food processing and agriculture. It injects hundreds of millions of nanobubbles of purified oxygen into solution; bubbles so small they can stay in solution for days. This means HDO is not just a point-of-contact system. Oxygen helps boost chlorine and enzyme activity. So while the oxygen itself does not reduce combined chlorine, it boosts chlorine, which does. Air quality, however, is where HDO stands out. There is a noticeable improvement to indoor air quality using HDO. This is largely because there is so much oxygen in the water, that when it off-gasses, it introduces pure oxygen above the surface of the water (right in the breathing zone). Fresh air dilutes chloramine gases. We know from personal experience that it dramatically improves indoor air quality...despite not reducing chloramines.

Conclusion

Combined chlorine is a measurement of chlorine that has combined with nitrogen compounds. We generally refer to all of these molecules as chloramines, even though chloramines are just one specific group. There are more disinfection byproducts (DBPs) that we can list on here. The variations happen, in part, because of incomplete oxidation. When chlorine runs out of steam after halfway destroying something, you're left with a half-destroyed mutant byproduct. 

None of these byproducts are desirable, and many of them are known by the CDC to be harmful. 

We are often asked how to reduce combined chlorine, that's why we wrote this article. It is irresponsible to think that indoor air quality can be "perfect" if the water quality is perfect. Even if pool operators have state-of-the-art equipment and best practices, chloramines will still be created, because swimmers use pools. It's an essential set of chemical reactions that have to take place in order to keep water safe. It would be a miracle if there was a safe chemical that could devour urea and nitrogen compounds without harmful byproducts...but such a chemical does not exist today.

Until then, we must rely on chlorine, chemical supplements to it (like enzymes), and secondary systems to reinforce.