Most people associate a strong chlorine smell with "too much chlorine." The opposite is true. The smell comes from chloramines (combined chlorine) — the by-products of chlorine reacting with nitrogen compounds. The fix is more chlorine, not less.
Key Facts
- The irritating pool smell comes from chloramines (dichloramine and nitrogen trichloride), not free chlorine.
- A well-maintained pool with adequate free chlorine has very little odour.
- Combined chlorine above 0.5 ppm is the action threshold — breakpoint shock is needed.
- Eye irritation from pool water is caused by chloramines, not free chlorine.
The Chemistry of the Smell
When free chlorine reacts with nitrogen compounds in pool water (from sweat, urine, body waste, and sunscreen), it forms compounds called chloramines. The three types are monochloramine (NH2Cl), dichloramine (NHCl2), and nitrogen trichloride (NCl3). Monochloramine has a mild odour. Dichloramine and nitrogen trichloride are volatile and responsible for the strong, eye-irritating pool smell. These compounds are also much weaker disinfectants than free chlorine. The pool smells worst when free chlorine is too low to convert all nitrogen compounds, and chloramines accumulate instead.
Measuring Combined Chlorine
To confirm a chloramine problem, test both free chlorine (DPD-1) and total chlorine (DPD-3). Combined chlorine = Total - Free. If the result is above 0.5 ppm, chloramines are present at an action level. A reading of 1.0 ppm or above combined chlorine corresponds to a strongly smelling pool. A result of 0.0 ppm combined chlorine in a pool that still has an odour suggests the smell may be from other sources — check that chemicals are stored away from the pool area, as chlorine storage odour can be mistaken for pool smell.
Eliminating the Smell
The only effective treatment for chloramine odour is breakpoint chlorination — adding enough free chlorine to reach 10 times the combined chlorine level. At breakpoint, chloramines are chemically destroyed. Calculate the dose using the shock calculator (combined chlorine x 10 x pool volume factor), add the full dose after dark, run the pump continuously overnight, and test free chlorine the following morning. The smell will be gone once breakpoint has been reached and FC returns to normal range. Adding algaecides, fragrance products, or pH adjustments will not address the odour.
Examples
After a pool party, the water smells strongly and guests are complaining of red eyes. Test: FC 1.5 ppm, TC 2.8 ppm, CC 1.3 ppm — well above threshold. Breakpoint requires adding 13 ppm of FC (10 x 1.3). The pool volume is 20,000 gallons. The shock calculator shows approximately 8 lbs of cal-hypo is needed. Added after sunset, pump running overnight. Next morning: no smell, no eye complaints, FC 4.5 ppm (still slightly elevated), CC 0.0 ppm. Water is clear and fresh.
Common Mistakes to Avoid
- Reducing chlorine when the pool smells — this makes the combined chlorine problem worse by providing less free chlorine to compete with the chloramines.
- Adding fragrance or deodorising products to a smelling pool — these mask the odour temporarily but do not address the chloramine chemistry.
- Testing only free chlorine and concluding the pool is "over-chlorinated" because it smells — without measuring total chlorine and calculating CC, the diagnosis is incomplete.
- Pool & Hot Tub Alliance — Pool & Spa Operator Handbook, 2022
- Taylor Technologies — Pool/Spa Water Chemistry Reference