Chloramines (the "pool smell")
Chloramines are compounds formed when chlorine reacts with sweat, urine, and skin oils in pool water. They're "spent chlorine" — no longer sanitizing — and they're what causes the strong "pool smell," red eyes, and irritation commonly blamed on chlorine itself.
Common myth
"That strong chlorine smell means too much chlorine." Actually the opposite: it means too many chloramines, which usually means not enough free chlorine. A properly chlorinated pool has almost no smell.
Target ranges (Combined Chlorine, CC)
Chloramines are measured as "combined chlorine" (CC), calculated by subtracting free chlorine from total chlorine:
| CC Level | Status | Action |
|---|---|---|
| 0 – 0.2 ppm | Excellent | None — chloramines are negligible |
| 0.2 – 0.5 ppm | Normal | Acceptable, monitor on next test |
| 0.5 – 1.0 ppm | Elevated | Consider shocking soon if it persists |
| > 1.0 ppm | High | Shock the pool now — chloramines are building |
How chloramines form
Free chlorine sanitizes by oxidizing organic contaminants. When the contaminant contains nitrogen — like ammonia from sweat, urine, sunscreen, or skin oils — the reaction creates chlorine-nitrogen compounds called chloramines. They come in three forms, each progressively worse:
- Monochloramine (NH2Cl) — mild, mostly harmless, still has some sanitizing power
- Dichloramine (NHCl2) — produces noticeable odor, causes some irritation
- Trichloramine / nitrogen trichloride (NCl3) — the actual "pool smell." Volatile gas, severely irritates eyes, lungs, and skin
The "pool smell" you notice walking into an indoor pool isn't chlorine — it's trichloramine off-gassing from the water.
Why "less chlorine" makes it worse
When pool owners notice the smell and reduce chlorine to "give people a break," chloramines continue forming faster than they can break down. Free chlorine drops, more contaminants accumulate, more chloramines form, the smell gets worse.
The fix is counterintuitive: add more chlorine, not less. Specifically, you need enough free chlorine to push past breakpoint chlorination — the threshold at which existing chloramines get destroyed instead of perpetuated.
Breakpoint chlorination (the fix)
To destroy chloramines, raise free chlorine to roughly 10x your current combined chlorine level. This is what "shocking" does. Once you cross the breakpoint, chloramines decompose into nitrogen gas and harmless byproducts. CC drops to zero. Smell disappears.
Example
If CC reads 1.5 ppm and FC reads 2 ppm, you need to raise FC to about 15 ppm (10 × 1.5) to hit breakpoint. Maintain that level until CC drops below 0.5 ppm — usually 12-24 hours with the pump running.
How to prevent chloramines
Chloramines are unavoidable in a used pool. But you can slow them down:
- Maintain FC above your CYA-based minimum. Chlorine that's too low can't keep up with contaminant inflow.
- Encourage pre-swim showers. A 30-second rinse removes most sweat, sunscreen, and skin oils before they hit the pool.
- Discourage urination in the pool. Urine is the single biggest source of nitrogen in residential pools.
- Run the pump during and after heavy bather load. Filtration plus circulation distributes chlorine where it's needed.
- Cover the pool when not in use. Sunscreen residue and dust contribute to chloramine formation.
Health effects
Brief chloramine exposure causes the classic "pool eyes" — red, stinging, watery eyes. Heavier exposure (long indoor swim sessions, swim instructors, lifeguards) can cause coughing, asthma-like symptoms, and "swim-coach lung." Trichloramines are also corrosive to indoor pool buildings; well-run aquatic facilities use breakpoint chlorination and aggressive ventilation to minimize them.
For home pools, occasional CC above 0.5 ppm is normal and not dangerous. Chronic high CC is uncomfortable and signals you're under-chlorinating.
Want to...
Catch chloramines before the smell hits
PoolChem Tracker calculates combined chlorine automatically from your FC and TC readings and warns you the moment CC trends past 0.5 ppm.
