By The Zagosa Team. Published .
Reviewed or updated .
It sounds backwards, but a strong chlorine smell nearly always means there is too little free chlorine, not too much. The pong comes from chloramines, which form when chlorine reacts with sweat, sun cream and other organics. They sting the eyes and skin, and they are the chemistry telling you it needs a hand.
Free, total, and the gap that counts
Free chlorine is the active sanitiser, the part doing the killing. Combined chlorine (chloramines) is what remains after chlorine has reacted with organics. Total chlorine is simply the two added together.
If free chlorine reads 3 ppm and total reads 5 ppm, you are carrying 2 ppm of chloramines. That is your smell. Anything over 0.5 ppm of combined chlorine is worth shocking.
Clearing it in one evening
Dose a non-chlorine shock at the label's per-litre rate, lift the cover, and run the jets for 20 minutes. The oxidiser breaks the chloramines back down into nitrogen gas, which vents off instead of recombining.
Leave the cover off for at least an hour afterwards, weather permitting. Shut it straight away and you trap the gases, so the smell will still be hanging around in the morning.
Test free and total chlorine the next day. They should now sit within 0.5 ppm of each other.
Stopping the smell returning
Shower before you get in. A 30-second rinse takes off most of the body oils and lotions before they ever reach the chlorine.
Shock weekly even when everything looks fine. Chloramines quietly accumulate between soaks.
Keep free chlorine in the 3 to 5 ppm band and pH between 7.2 and 7.6. Outside that range the sanitiser fights itself and chloramines form faster.
Common questions
Can I shock then jump straight in?
After a non-chlorine shock, yes, once 15 to 20 minutes have passed and free chlorine has fallen back into the bathing band. After a chlorine shock, wait until the level is back below 5 ppm.
Is the smell itself harmful?
Not at the levels a hot tub normally reaches, but it irritates eyes and airways and clearly flags that the water is off. Treat the smell as feedback, not a feature.