Why does my pool smell like chlorine? Chloramines and how to clear them
The sharp "chlorine" smell most people associate with pools isn't free chlorine. It's mostly chloramines: compounds that form when chlorine reacts with sweat, urine and other nitrogen-containing waste from swimmers. A strong smell, stinging eyes and itchy skin usually mean the pool needs more chlorine, not less.
Free, total and combined chlorine
- Free chlorine (FC) is the active sanitizer, available to kill germs.
- Combined chlorine (CC) is chlorine that has already reacted with waste, mainly chloramines. It sanitizes poorly and causes the smell and irritation.
- Total chlorine (TC) is the two added together.
Most pool-care guidance aims to keep combined chlorine at 0.5 ppm or less. To measure it you need a test that reads both free and total chlorine, such as a DPD or FAS-DPD drop kit. Many simple test strips only show one number. See how to test pool water.
Calculate the dose to clear it
The "10 times" rule
The traditional way to destroy chloramines is breakpoint chlorination: adding enough free chlorine to oxidize them completely. A long-standing rule of thumb from the pool industry is to raise free chlorine by about 10 times the combined chlorine reading. With 1 ppm of combined chlorine, that's about 10 ppm of extra chlorine. Adding too little can make things temporarily worse, because partly reacted chloramines can linger.
In outdoor pools with stabilizer (CYA), many owners skip the exact ratio and simply hold the shock level for their CYA until combined chlorine drops to 0.5 ppm or less. Both approaches aim for the same end: enough free chlorine, long enough, to finish the job.
Worked example
A 15,000-gallon pool reads 2 ppm free and 3 ppm total chlorine, so combined chlorine is 1 ppm. Ten times that is 10 ppm. With 12.5% liquid chlorine, 10 ppm in 15,000 gallons takes about 10 × 56,781 L ÷ 1,000 ÷ 0.125 ≈ 4,540 mL, or about 1.2 gallons. Free chlorine rises to about 12 ppm, then falls as it destroys the chloramines.
Step by step
- Test free and total chlorine, pH and CYA. Bring pH to about 7.2–7.4 first, since chlorine works better at lower pH.
- Add the calculated chlorine in the evening with the pump running, poured slowly in front of a return jet.
- Keep the pump running and leave the pool uncovered if you can. Sunlight helps break down chloramines.
- Retest after several hours. If combined chlorine is still above 0.5 ppm, raise free chlorine again and hold it.
- Swim again once free chlorine is back down to your normal level and within your product label's limits.
Non-chlorine shock (potassium monopersulfate) also oxidizes waste and is sometimes used to lower combined chlorine. Be aware that it can make DPD total chlorine tests read high for a while after it's added. Follow its label for test timing.
Preventing chloramines
- Keep free chlorine at the right level for your CYA every day, so waste is oxidized as it arrives. The chlorine and stabilizer guide gives the numbers.
- Shower before swimming. The CDC encourages swimmers to rinse off, and not to pee in the pool, because sweat, dirt and urine use up chlorine and form chloramines.
- Indoor pools need good ventilation: chloramines in the air above the water cause much of the irritation and smell.
Frequently asked questions
Is a strong chlorine smell a sign of too much chlorine?
Usually not. Well-chlorinated pools with clean water often have little smell. A strong smell is a common sign of chloramines, meaning there's more waste than the free chlorine has dealt with.
Why are my eyes red after swimming?
Chloramines are a common cause. pH far out of range, especially low pH, can also sting. Test combined chlorine and pH.
Do saltwater pools get chloramines?
Yes, the chemistry is the same. A salt cell adds chlorine steadily through the day, which helps, but heavy swimmer use can still outpace it.