As an aquatic engineer who has been designing facilities for many years, one of the more frequent questions I get from owners of indoor pools is “what can we do to get rid of nasty chlorine odors that our customers complain about?” Before I even look at their pool situation, I have a very good idea of the problems they face.

Whenever you have a indoor pool, which inherently has a confined air space surrounding the pool, that utilizes a chlorine based water treatment system, and you add people to the water who tend to sweat and produce other bodily fluids of various types (we will just leave it at that for now), you are going to generate odors from the water. When these odors include a strong chlorine smell, the culprit is the formation of chloramines in the pool water that are “off gassing” as water is stirred up during swimming activities or as water evaporates from the pool.

Swimming pools that use a chlorine-based disinfectant (most all pools fall into this category, even salt water chlorine generation pools) often experience a “chlorine odor” problem. When this problem gets bad enough, pool users will begin to complain of an offensive odor, skin and eye irritation, and even a burning sensation in the lungs. These issues can be present in any pool, but are particularly a problem with indoor pools because of the confined air space in the room.

Most people who encounter a pool with heavy “chlorine odors” immediately assume that the pool has too much chlorine in the water. To the contrary, the pool needs more chlorine and fewer chemical compounds known as chloramines. A pool in this condition needs to be super-chlorinated by the addition of much more chlorine in order for the pool water to return to proper chemical balance.

Since chloramines are not chemicals that we add to a pool for treatment of the water, we need to understand where they come from so that we can properly control their formation in the water.

Chloramines are chemical compounds that are formed when chlorine comes in contact with ammonia-nitrogen in various forms. This ammonia is introduced into the swimming pool through various sources, but the 3 most common sources in a pool are oils, sweat, and urine. Yes, you can now honestly now tell your kids that peeing in the pool does create a harmful chemical in the water, even if it doesn’t really turn blue around them per the myth they often hear. It may be wise however, to keep the blue water myth alive for your kids. I do.

To be more specific as to what causes the formation of chloramines we need to look briefly at the chemistry. When chlorine is added to water, two desirable chemicals are unleashed to disinfect the pool water:  hypochlorous acid, HOCl, and hypochlorite ion, OCl-. A measure of the chlorine in these two beneficial chemicals is known as “free available chlorine” or FAC. This is what we want in the pool to destroy germs and oxidize bad things in the water. When ammonia-nitrogen is added to the pool however, there are 3 possible chloramine compounds generated:

  1. When one of ammonia’s hydrogen ions is replaced with chlorine, monochloramine is formed.
  2. Replacing two hydrogen ions with chlorine produces dichloramine.
  3. Finally, it is possible to replace all three of ammonia’s hydrogen ions with chlorine to formtrichloramine.

Dichloramine and trichloramine are the two compounds that cause most of the odor and skin irritation problems.

So you may think the answer to solve the issues related to chloramine formation is simply not to use a chlorine-based treatment system for the pool water. However, for most swimming pools there really is no good alternative to chlorine disinfection products for the following reasons:

  1. Chlorine is relatively easy to use and cost effective.
  2. Chlorine is easy to measure in pool water.
  3. Chlorine is relatively stable in pool water.
  4. Chlorine has been used so long it’s use is well known and documented for pool operators.
  5. Chlorine is safe and effective within a fairly easy to maintain concentration range (recommended at around 1 to 4 ppm FAC)

So until there really is a better and more cost effective alternative to chlorine use in pools, we need to understand the things that can be done to control chloramine formation in the water.

Methods for controlling chloramines in pool water fall into 3 categories. The first category includes measures to keep chloramines from forming in the first place. The second category includes technologies for destroying them once they form in the water. The third category involves removing from around the pool the chloramine-contaminated air that causes odor and skin irritation. A combination of technologies from all three of these categories will generally result in the most pleasant to use swimming pools for swimmers.

Keep ammonia-nitrogen sources out of the pool

The first category of stopping the formation of chloramines involves keeping ammonia sources out of the pool as much as possible. Simple measures like requiring showers before entering the pool can help in this category. As most pool managers will attest, this is easier said than done.

It is advisable to post signs that remind patrons of the importance of being clean before entering the pool water. Most people will relate better to a discussion of the health benefits in this regard rather than a discussion of chloramines. A poster similar to the CDC’s “Six Steps for Healthy Swimming” may be a useful model for posted signs.

Two other potential sources of ammonia that are often overlooked are landscaping and deck cleaning. Most design regulations require water from decks to drain away from pools and not allow storm-water runoff or deck cleaning to drain into the pool. Pool operators should be aware of this potential ammonia source and eliminate the possibility of any such contamination of the pool, especially when using solvents to clean decks.

Break-up or destroy chloramines in pool water

The second category of destroying, or “breaking-up” the chloramines is a necessary strategy for any pool. For outdoor pools, where chloramine odors are not as much of a problem, you can simply super-chlorinate the pool when chloramine levels rise too high. This is a fairly straightforward procedure and all pool operators should be familiar with this process.

Other effective methods for destroying chloramines in water include introduction of ozone gas or ultraviolet (UV) light.

Ozone gas is a very reactive oxidizer, which will destroy chloramines and usually leave a residual amount of ozone to re-enter the pool. The trick with ozone is to apply just enough to deal with the chloramines, without reacting negatively with anything else around the pool, such as pool plaster.

UV light, applied in proper dosages through specialized equipment, is a very effective way to break down chloramine compounds as well.

Remove chloramine laced air from the pool area

The third category includes methods for removing chloramine compounds from the room atmosphere once they are airborne.  Methods in this category are almost exclusively utilized on indoor pools because with outdoor pools the air drifts away naturally so chloramine odors are not as noticeable.  Although air removal for an outdoor pool does not make sense, it must be noted that chloramine concentrations must also be controlled in outdoor pools so that effective disinfection of the pool continues and other water chemistry parameters of the pool stay in proper balance.

For an indoor pool however, chloramine buildup can reach intolerable levels quickly. The more a pool is utilized, the quicker the build-up of chloramines will occur.

Two methods for removing these airborne particles include fresh air exchanges in the room in a wholesale manner through the HVAC system, and newly developed methods to remove chloramine laced air from more localized areas at the pool surface.

The first method of using the HVAC system to supply fresh outdoor air exchanges to the entire room is expensive because of having to constantly heat and adjust humidity of the resulting large volume of new air coming from outside the room.

Since chloramine laced air is heavier than normal air in the swimming pool room, it tends to stay close to the pool surface where it was created in the first place. Because of this, it is difficult, if not impossible, to design a “whole room” air handling system that effectively removes the chloramine saturated air in a heavily used pool. Often times the actual result of taking the “whole room air exchange” approach alone is that the nasty, heavier air is mixed with the cleaner lighter air above, and the offensive air is then spread throughout the room, making the issue even worse.

A newer method of removing smaller amounts of air from just above the water surface before chloramines spread throughout the room is proving to be more effective. Replacing a smaller volume of air just at the water surface has the added benefit of costing less to heat and condition the replacement air since the volume of air being exchanged is much lower compared to a whole room exchange.

One piece of equipment that has hit the market lately to remove air from just above the water surface, where chloramines first become airborne and tend to hang because of their weight relative to cleaner air, is called the “Evacuator“. For more information on this innovative approach to indoor pool air quality, please read our article on the subject.

Conclusions on controlling chlorine odors in swimming pools

In conclusion, outdoor pool chloramine control is often best achieved by super-chlorination when levels get too high. Since the chlorine smell and skin/eye irritation will be less an issue with outdoor air circulation, other costlier methods to control chloramine formation or chloramine destruction are hard to justify from a cost standpoint.

Conversely, for indoor pools it is advisable to employ methods to control chloramine concentrations before they reach the point of requiring super-chlorination. When it gets to the point of requiring super-chlorination it is likely that there are many unhappy pool users, and the pool operating staff will hear plenty about the chlorine smell. UV, or possibly a tightly controlled ozone feed, are good technologies to consider. Proper design of air handling through room HVAC and installation of a system to remove chloramines closer to the pool water surface are also imperative to achieving great air quality for your patrons.

The worst possible outcome is to build a fancy new indoor pool complex that looks great but smells terrible. Close attention to design of systems for the control of chloramine concentrations is money well spent. From a veteran aquatic facility design engineer, I recommend use of proven systems and not necessarily the ones with the best and flashiest marketing materials.  Don’t be the guinea pig for anyone’s new wiz-bang gizmo. Your end users don’t care about nice marketing materials, they only care about how pleasant your facility is to use.

Designed and constructed properly, your new aquatic facility will be praised by everyone, especially your end users!