Indoor air quality in natatoriums has many variables and moving pieces, but at the end of the day, it all boils down to one thing: how well can the natatorium's system manage chloramine vapor? More specifically, how effectively can the building's air handling system remove vaporized pool air that is laden with disinfection byproducts (DBPs) like trichloramine and trihalomethane?
What is Chloramine Vapor?
Chloramine vapor is the airborne form of trichloramine, which off-gasses from swimming pools or other treated water sources. The term chloramine vapor is also used more broadly to include other airborne disinfection byproducts, including the harmful trihalomethane gas chloroform (also called methyl trichloride, or trichloromethane, CHCl3). Basically any DBPs that off-gas from a swimming pool are commonly included when talking about chloramines, even though it's not technically accurate.
Chloramine vapor occurs when nitrogen compounds like ammonia and urea are oxidized by chlorine. In water treated with bromine, the predominant DBPs are called bromamines, or bromamine vapor. Since the overwhelming majority of swimming pools are treated with chlorine, this article will remain focused on chloramine vapor.
Ammonia gets oxidized by chlorine–specifically the strong form of aqueous chlorine, hypochlorous acid (HOCl)–and a reaction occurs. Rather than just destroying the ammonia, a chloride replaces one of ammonia's three Hydrogens. This takes a 5:1 weight ratio of HOCl:NH3, which is a 1:1 molar ratio. The new molecule is called monochloramine (NH2Cl). More chlorine (10:1 weight ratio) will eventually replace another Hydrogen, creating dichloramine (NHCl2). And still more chlorine (15:1 weight ratio)will eventually replace the final Hydrogen, creating nitrogen trichloride, more commonly known as trichloramine (NCl3).
These reactions are the crux of what is called combined chlorine. Chlorine combines with nitrogen compounds besides ammonia too. It's just part of the oxidation process. The final stage of the reaction is off-gassing, which is where chloramine vapor comes from.
Vapor, Humidity and Expansion
Let's go over some glossary terms, which can be helpful for this topic. Water vapor is evaporated water in the air. Humidity is a measurement of the amount of water vapor in the air. Absolute humidity is the amount currently in the air, and relative humidity (RH) is the amount of water vapor compared to how much vapor the air could potentially hold. Both heat and humidity rise and expand, so it is common for natatoriums to have ceiling exhausts. While they make sense from a dehumidification standpoint, ceiling exhausts are a common mistake in natatorium design. Sure, they exhaust the hottest, most humid air in the room, which can reduce the moisture load on the dehumidifier. But the problem with natatorium air quality is not water vapor, it's chloramine vapor.
Unlike water vapor, chloramine vapor does not rise. Chloramines and other DBPs are heavier than oxygen. They stay low in the natatorium, just above the surface of the pool in what we call the breathing zone. Another term for this is the chloramine bubble, coined by Randy Baxter in this article. The more chloramines and DBPs off-gassed, the denser the bubble, and the more displacement of fresh air for swimmers to breathe. It's not a good thing.
Left alone, chloramine vapor will eventually fill the natatorium from the bottom up. With poor HVAC design and a high return intake, this can lead to a stratification problem, where chloramines just build and build. Notice, chloramines do not rise and expand like humidity does.
A low return intake can compound a chloramine problem too, by recirculating them. In both scenarios, the issue is that neither return location captures or removes chloramines effectively. Normally the exhaust blower is within the return air path, and it's just a fan; a fan without the ability to separate good air from chloramine vapor. Stratification and recirculation are different, but they are both problems. Which leads us to the solution...
How to Control Chloramine Vapor
Eventually, the concentration of trichloramine in the water will want to match the concentration in the air, per Henry's Law. But natatoriums are large buildings with plenty of space for chloramine vapor to spread. Henry's Law is not much of a concern in the real world, because trichloramine concentrations would need to be significant to overpower the vapor pressure pushing outward from the pool. But we're getting into the weeds now.
It takes a multi-pronged approach to handle chloramine vapor.
In the water...
From the water chemistry side, we want to both minimize bather waste (including nitrogen compounds like urea) and maximize the removal of the subsequent byproducts (like chloramines). Encouraging swimmers to take bathroom breaks and to shower before entering the pool can help, but are also hard to enforce and therefore unrealistic. Maximizing the removal of DBPs while they are still in the water, however, can be accomplished by supplementing chlorine. Enzymes can digest the lion's share of carbon-based organic waste, while secondary sanitization systems like UV and Ozone can help eliminate formed chloramines and other combined compounds. Ozone can also oxidize ammonia and urea directly, which is very helpful too.
There are also systems, like hyper-dissolved oxygen (HDO), that have recently come onto the market. HDO adds an incredible concentration of purified oxygen into the water, which accelerates chlorine and enzyme performance, and also has the added benefit of releasing pure oxygen into the swimmers' breathing zone. If nothing else, HDO helps dilute the chloramine bubble.
In the air...
Once DBPs like chloramines go airborne, water chemistry is in the past. They are now an air problem, and must be handled accordingly. We strongly recommend designing natatoriums with air quality as the foremost priority. Design with end users (swimmers, lifeguards, coaches, etc.) in mind! It is essential to have an air system that is designed to match the needs of the swimming pool, and takes into account the shape of the room too. With a good airflow layout on the supply side, locating the return midway up the wall, and perhaps another return up high is a smart way to go. The higher returns can effectively grab the humid, hot air and condition it.
Equally important is using source-capture exhaust to capture and remove chloramine vapor. Source-capture exhaust allows the system to exhaust the highest concentration of chloramine vapor, while the rest of the room can be recirculated and conditioned by the pool dehumidifier. Capturing heavy chloramines and trihalomethanes at their source keeps them from spreading around, and helps a pool maintain excellent air quality 24/7.
Chloramine vapor is an inevitable contaminant in indoor swimming pools. Outdoor pools have it too, but because it is outdoor, the abundance of fresh air means we hardly notice. We can try to mitigate chloramine vapor production via water chemistry strategies, but there will always be some that makes it out into the air. And while water vapor rises and expands, chloramine vapor does not, because its atomic weight is heavier than oxygen. This makes chloramines predictable and easy to capture and remove with a targeted source-capture exhaust system.