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That sharp "chlorine" smell is not chlorine. It is chloramines — irritating compounds formed when chlorine reacts with sweat, urine and cosmetics brought in by swimmers. A strong smell means a large share of the pool's chlorine has already been consumed by that load, leaving too little free chlorine doing the real work. The fix is the opposite of what instinct says: measure combined chlorine, then raise free chlorine to about ten times the combined reading — breakpoint chlorination — to destroy the chloramines, and for indoor pools, ventilate. A clean, well-chlorinated pool has almost no smell at all.
Walk into a pool hall that reeks, eyes stinging before you touch the water, and everyone reaches the same conclusion: they have put in too much chlorine. It is one of the most confidently held beliefs in swimming — and it is almost exactly backwards.
Chlorine dissolved in clean water has only a faint odour. What you are smelling is chloramines: the by-products left over after chlorine has reacted with the nitrogen compounds swimmers bring into the water — sweat, urine, body oils, cosmetics and sunscreen. Chloramines are chlorine that has already been spent. So a heavy smell is telling you two things at once: a lot of contamination has gone into this water, and a lot of the chlorine has been used up fighting it. The pool is not over-chlorinated. It is over-loaded, and quite possibly under-protected.
The CDC's guidance for aquatic professionals makes the same point plainly: a well-managed pool has little odour, and the "chlorine smell" is chloramines off-gassing from the water.
To follow the fix, you need the vocabulary your test kit uses.
| Reading | What it is | What you want |
|---|---|---|
| Free chlorine | Chlorine still available to kill germs | In range — at least 1 mg/L, at least 2 mg/L if stabiliser is used |
| Combined chlorine | Chlorine already reacted into chloramines — spent, irritating, smelly | As close to zero as possible |
| Total chlorine | The two added together — what many cheap kits actually measure | — |
The arithmetic matters: combined chlorine = total chlorine − free chlorine. A kit that only reads total chlorine can show a comfortable number while most of it is spent chloramine. This is how a pool ends up smelly, irritating and under-disinfected at the same time, with a test result that looks fine. If your kit cannot distinguish free from total, that is the first thing to change (see pool test kits & reagents) — and for a formal answer, laboratory testing reports the split properly.
Chlorine reacts with ammonia and other nitrogen compounds in stages, producing monochloramine, then dichloramine, then nitrogen trichloride (trichloramine). That last one is the villain of the pool hall: it is volatile, so it leaves the water and collects in the air just above the surface — precisely the layer swimmers breathe. It is what stings your eyes and scratches your throat at a badly run indoor pool, and the WHO's recreational water guidelines flag it as the main air-quality problem in indoor pools.
Splashing and water features make it worse, because agitation drives the gas out of the water faster. A busy training session in a poorly ventilated hall is the perfect chloramine machine: heavy bather load feeding the chemistry, and churning water pumping the product into the air.
Here is the counterintuitive fix. To get rid of the smell, you add more chlorine — enough, briefly, to burn the chloramines out completely.
When free chlorine is pushed high enough relative to the contamination, it stops making chloramines and starts destroying them, oxidising them through to nitrogen gas, which simply leaves the water. The threshold where this happens is called the breakpoint. The working rule, as the CDC gives it: raise free chlorine to about ten times the combined chlorine reading, hold it there while circulation does its work, then let the level fall back into the normal operating range before swimmers return.
The trap: dosing "a bit extra" instead of a measured breakpoint dose. Undershooting the breakpoint leaves you in the zone where added chlorine makes more chloramines, not fewer — the pool smells worse, someone concludes chlorine is the problem, the dose gets cut, and the cycle repeats. Breakpoint is a calculation, not a splash. Persistent high combined chlorine also responds to partial dilution with fresh water.
One practical note for tablet-fed pools: shock dosing is a job for unstabilised chlorine such as liquid sodium hypochlorite, because a large trichlor dose adds stabiliser you will be living with long after the chloramines are gone — see cyanuric acid and chlorine lock.
Outdoors, wind carries trichloramine away. Indoors, whatever off-gasses stays until the ventilation removes it. The CDC's guidance for pool operators identifies the drivers of chloramine build-up in air: agitated water releasing the gas faster, limited fresh air moving across the water surface, and air-handling systems set to recirculate rather than exhaust — a setting often chosen to save energy, at the cost of keeping the pollution indoors.
So for an indoor pool that smells, the checklist has two halves. In the water: keep free chlorine in range, keep pH and balance correct so the chlorine actually works, and break chloramines before they accumulate. In the air: fresh air supply and exhaust across the water surface, run properly during and after bathing load — not just when someone complains. Supplementary systems such as UV or ozone, fitted to the circulation, also destroy chloramines in the water and are commonly used on heavily loaded indoor pools.
Usually the opposite. The smell comes from chloramines — compounds formed when chlorine reacts with sweat, urine and cosmetics brought in by swimmers. A heavy smell means a lot of chlorine has already been used up fighting that load, and there may not be enough free chlorine left doing the work. A clean, properly chlorinated pool has very little smell.
Chloramines and unbalanced water, not chlorine dose. Chloramines irritate eyes, skin and airways, and a pH far from the eye's natural level of about 7.4 makes the stinging worse. If a pool leaves everyone red-eyed, the fix is to test combined chlorine and pH — not to cut the chlorine.
A deliberate, temporary overdose of chlorine that destroys chloramines. The operator raises free chlorine to about ten times the combined chlorine reading, holds it while the chloramines are oxidised away, then lets the level fall back to the normal range before swimmers return. Dosing too little makes the problem worse rather than better, so it needs a measured dose, not a guess.
Ventilation. The most irritating chloramine, nitrogen trichloride, leaves the water as a gas and sits in the air just above the surface. Outdoors the wind removes it. Indoors it accumulates unless the air handling system brings in enough fresh air and exhausts the polluted air — which is why an indoor pool needs both water treatment and ventilation to smell right.
Measure free and total chlorine; the difference is combined chlorine. Then raise free chlorine to about ten times the combined reading — breakpoint chlorination — with the pool closed, circulate until combined chlorine collapses towards zero, and let the level fall back into range before reopening. For an indoor pool, also bring fresh air in and exhaust it across the water surface. Longer term, pre-swim showers, holding free chlorine in range continuously, routine combined-chlorine tests and some regular water replacement stop the smell coming back.
We will test free and combined chlorine, check the balance, and give you a written plan to fix the cause — not just the smell.