Walk into an indoor pool and smell that strong “chlorine” odor, and most people think the pool has too much chlorine in it. It’s usually the opposite. That smell is chloramines — and it means the pool doesn’t have enough free chlorine doing its job.
If you run a pool, especially an indoor one, chloramines are the single most important water-quality problem to understand. They make swimmers miserable, they make lifeguards sick, and they eat your building.
What chloramines are
Chloramines form when free chlorine binds with ammonia. The ammonia comes from swimmers: sweat, urine, skin cells, sunscreen, body oils, lotions. Every bather adds it, and chlorine’s first job is to react with it.
That reaction produces three compounds, in sequence:
- Monochloramine — the first and mildest form.
- Dichloramine — forms as more ammonia is added.
- Trichloramine (nitrogen trichloride) — the last and worst form. It’s volatile, which means it leaves the water and enters the air — right at the surface, right where swimmers are breathing.
This is also why showering before entering the pool matters so much. Every bather who rinses off first is ammonia that never enters the water — and chloramine that never forms. (We wrote a whole post on the pre-swim shower rule and what actually gets people to follow it.)
Why they’re bad for swimmers
Trichloramine is an irritant, and it concentrates exactly where people breathe
- Eyes, nose, and throat — the burning eyes and scratchy throat swimmers blame on “too much chlorine” are almost always chloramines.
- Skin irritation — itchy, dry skin after a swim, especially with repeated exposure.
- Lungs — trichloramine aggravates asthma, and it causes an occupational condition in indoor pool workers often called “lifeguard lung.” Lifeguards and instructors who spend hours at water level, breathing the air just above the surface, are the most exposed people in the building.
- Sanitation — and this is the part that matters most: chlorine tied up in chloramines is chlorine that isn’t killing germs. A pool with high combined chlorine is a pool with less disinfection capacity than its test readings suggest.
Why they’re bad for the building
Chloramines don’t just stay in the water. Trichloramine in the air is corrosive, and an indoor natatorium gives it plenty to attack:
- HVAC and ductwork — the air handling system moves chloramine-laden air through the whole building.
- Structural steel and metal components — corrosion on beams, fasteners, and supports in the pool hall.
- Electrical systems — corrosion on panels, contacts, and fixtures in the natatorium environment.
- Dehumidification equipment — the systems that manage indoor pool humidity are working in the most corrosive air in the building.
We’ve seen the corrosion side of this firsthand — pool environments are hard on buildings, and chloramines are a big part of why. An indoor pool that smells strongly of “chlorine” and shows rust or corrosion on metal in the pool hall is telling you the same thing two ways.
How to measure them
Combined chlorine is the number to watch. It’s the difference between your total chlorine and your free chlorine readings:
Combined chlorine = total chlorine − free chlorine
The targets:
- 0.2 ppm or less — where you want to be.
- Above 0.5 ppm — time to act. At this level swimmers are noticing it, and disinfection is compromised.
Test for it regularly. If you’re only testing free chlorine, you’re missing half the picture.
How to fix them — and prevent them
Chloramines are a symptom of organic load overwhelming your chlorine. The fixes work in order, from cheapest to most involved:
1. Source control — get swimmers to shower. It’s the single most effective step, and the cheapest. Every bather who rinses first is ammonia that never becomes chloramine.
2. Maintain free chlorine. A pool running at the low end of its free chlorine range has no margin — the first bather load of the day eats it, and combined chlorine climbs.
3. Breakpoint chlorination. Shocking the pool — raising free chlorine to roughly ten times the combined chlorine level — breaks chloramines apart. It’s the standard corrective treatment, and it works, but it’s a response, not a prevention.
4. Dilution and water replacement. Fresh water dilutes everything chloramines are made from. Regular partial water replacement is part of long-term control.
5. UV or ozone secondary disinfection. Both destroy chloramines directly, and they’re increasingly common in indoor facilities for exactly this reason.
6. Ventilation. An indoor pool needs fresh air, exhaust at water level where trichloramine concentrates, and negative pressure so the air doesn’t migrate into the rest of the building. Recirculating chloramine-laden air just spreads the problem.
The bottom line
If your indoor pool smells like chlorine and your metal is corroding, that’s not a chlorine problem. It’s a chloramine problem — and it means the water needs attention, not more of the same. Test your combined chlorine, get your bathers showering, and if the numbers won’t come down, that’s when to call us.
Matthew Selman is the owner of Southern Aquatic Services, a commercial pool service company in North Georgia, and a CPO instructor who has trained more than 1,000 pool operators.