Pool water contains three chlorine forms:
The relationship: if your test kit shows TC = 2.5 ppm and FC = 2.0 ppm, your combined chlorine is 0.5 ppm — above the 0.3 ppm threshold. The pool may read fine on a basic free chlorine test strip, but the 0.5 ppm of combined chlorine is degrading water quality.
The reaction sequence:
| Chloramine Type | Symbol | Irritation Level | Notes |
|---|---|---|---|
| Monochloramine | NH₂Cl | Mild | Weak sanitizer, relatively stable |
| Dichloramine | NHCl₂ | Moderate | More irritating, distinct odor |
| Nitrogen trichloride | NCl₃ | Severe | Highly volatile, causes pool smell, respiratory irritation |
The persistent myth that pools that "smell like chlorine" have too much chlorine is completely backward. A properly maintained pool with high free chlorine and low combined chlorine has essentially no smell. The harsh, eye-watering, nose-burning "pool smell" is nitrogen trichloride — a chloramine — and it indicates the pool is under-sanitized relative to its bather load, not over-chlorinated.
Indoor pools smell worse than outdoor pools for a secondary reason: nitrogen trichloride is volatile and accumulates above the water surface in enclosed spaces. Outdoor pools naturally disperse it; indoor natatoriums can build significant concentrations that cause respiratory symptoms for regular users.
Combined chlorine cannot be measured directly by most home test equipment. Calculate it:
Combined Chlorine = Total Chlorine − Free Chlorine
You need a DPD drop test kit (Taylor K-2005 or similar) that tests both free and total chlorine. The total chlorine test uses the DPD #3 reagent to oxidize both forms. The difference tells you how much combined chlorine is present.
Test strips cannot reliably detect combined chlorine at the 0.2–0.5 ppm level where action is needed. Always use a drop kit for accurate CC measurement.
The only effective solution is breakpoint chlorination:
The most common combined chlorine mistake: adding a small amount of shock without calculating whether it reaches breakpoint. If the dose is below the 10× threshold, adding chlorine temporarily increases chloramine formation before it decreases. You spent money on chemicals and made the problem worse before it slightly improved. Always calculate the breakpoint dose first.
SplashLens logs free chlorine, total chlorine, and calculates combined chlorine from your test results. See exactly when CC is trending above the 0.3 ppm threshold — and get a notification to shock before the pool smell starts.
Open SplashLens Free →Combined Chlorine = Total Chlorine − Free Chlorine. Use a DPD drop test kit (Taylor K-2005) — not test strips, which can't detect CC reliably at the 0.2–0.5 ppm level where action is needed. Target combined chlorine below 0.3 ppm.
High bather load (urine, sweat, body oils), infrequent shocking (chloramines not destroyed), maintaining chlorine at the minimum rather than adequate levels, and organic contamination from leaves and debris. Shock weekly and after heavy use to prevent accumulation.
Breakpoint chlorination: add free chlorine at 10× the combined chlorine reading. Adjust pH to 7.2 first. Use calcium hypochlorite at night. Run the pump 8+ hours. Combined chlorine should be below 0.2 ppm the next morning. Don't underdose — it temporarily makes the problem worse.
Not acutely toxic, but it causes eye, skin, and respiratory irritation. High combined chlorine in indoor pools can cause significant respiratory symptoms in regular users. Outdoor pools dissipate the volatile nitrogen trichloride more easily. Target CC below 0.3 ppm consistently for swimmer comfort and health.
The smell comes from combined chlorine (chloramines), not free chlorine. A pool can read 2 ppm free chlorine on a test and still have 0.5+ ppm combined chlorine causing odor and irritation. Test total chlorine and calculate CC — if CC is above 0.3 ppm, perform breakpoint chlorination to destroy the chloramines.