Why does my pool smell of chlorine?

The smell is not what you think

That strong swimming-pool smell is not caused by clean chlorine. It comes from chloramines - chlorine that has already reacted with sweat, urine, suncream and other waste. Chloramines are also what cause red, stinging eyes and itchy skin.

Properly treated water smells of almost nothing. So a strong smell usually means you need more free chlorine, not less.

Reading free and total chlorine

Your 6-in-1 strip has two chlorine pads. Free chlorine is the active chlorine still available to sanitise. Total chlorine is free chlorine plus the used-up combined chlorine.

Subtract free from total to get combined chlorine. Ideally the two readings are the same. If total is noticeably higher than free - around 0.5 ppm or more - chloramines are building up and it is time to shock.

Common causes

• Lots of bathers, especially without showering first.
• Free chlorine kept too low for too long.
• Indoor pools with poor ventilation, where chloramines hang in the air.
• Hot tubs, which get a lot of body oils and sweat in a small volume of warm water.

How to fix it, step by step

• Check and correct pH to 7.2-7.6 first - shock works much better at the right pH. Wait an hour after any pH adjustment.
• Shock the water in the evening with the pump running. The dose needs to be roughly 10 times the combined chlorine reading - the app's plan works it out for your volume.
• Dissolve shock in a bucket of pool water (chemical to water) and pour it in front of a return. Never mix it with other chemicals or add it through a skimmer with a chlorine floater in it.
• Leave the cover off and keep the pump running overnight. In a hot tub, run the jets with the lid open.
• Retest the next day. Swim once free chlorine is below 5 ppm (hot tubs: back in the 3-5 ppm range) and total chlorine matches free.

Preventing it

Keep free chlorine in range, ask everyone to shower before swimming, shock weekly in busy periods and after parties, and ventilate indoor pools well. For hot tubs, a weekly shock is part of the normal routine.

When to call a professional

If combined chlorine keeps returning within a day or two of shocking, or an indoor pool hall always smells strongly, ask a professional to look at your circulation, filtration and ventilation.

Frequently asked questions