Why your pool smells of chlorine: chloramines, not too much chlorine

The counterintuitive answer

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.

The familiar strong pool odour often comes from chloramines, which form when chlorine reacts with nitrogen-containing contaminants. They can build up in the water and indoor air. Use measured free and total chlorine readings to assess the water; odour alone does not establish the chlorine dose or disinfectant level.

The CDC explains that chloramines can leave the water and accumulate in indoor air when ventilation is inadequate.

Free, combined, total: the three chlorine numbers

To follow the fix, you need the vocabulary your test kit uses.

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.

Where the smell actually comes from

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.

Breakpoint chlorination, in plain terms

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.

1. Test. Measure free and total chlorine; the difference is your combined chlorine.
2. Dose to ten times combined. If combined chlorine reads 0.8 mg/L, the target is about 8 mg/L of free chlorine — calculated for your pool volume and dosed in one deliberate addition, with the pool closed.
3. Circulate and hold until combined chlorine collapses towards zero.
4. Return to range. Let free chlorine decay back to the normal operating band, confirm by test, and reopen.

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.

Indoor pools: treat the air as well as the water

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.

Prevention: less in, more destroyed

• Showers before swimming. Rinsing off sweat and cosmetics removes chloramine ingredients before they reach the water. It is the single cheapest improvement a facility can make — the CDC's healthy swimming guidance leads with it.
• Toilet breaks and no peeing in the pool. Urine plus chlorine is the chloramine recipe. This is a hygiene rule with a chemical reason.
• Hold free chlorine in range continuously, so oxidation keeps pace with the load instead of falling behind between doses. Automatic dosing helps here — see dosing systems.
• Test combined chlorine routinely, not just when it smells. A rising combined reading is the early warning; the smell is the late one.
• Replace some water regularly. Dilution removes the dissolved by-products that chlorine chemistry alone cannot.

Common questions