How chlorine keeps pool and hot tub water safe: hold free chlorine at 3-5 mg/l with pH 7.2-7.6, shock weekly and test 2-3 times a week. UK dosing guide.
Chlorine keeps pool and hot tub water safe by maintaining a constant disinfectant residual that kills bacteria and viruses on contact. In a UK hot tub, hold free chlorine at 3–5 mg/l; in a swimming pool, 1–3 mg/l is typical. Keep pH at 7.2–7.6 and total alkalinity at 80–120 ppm so the chlorine stays active, test 2–3 times a week, and shock weekly to remove the bather waste that causes the harsh "chlorine" smell.
When chlorine dissolves it forms hypochlorous acid, which penetrates the cell walls of bacteria and other microorganisms, destroying the proteins and enzymes they need to survive. That constant killing power between doses is what makes chlorine the most widely used sanitiser for pools and spas. It also helps prevent algae growth and oxidises dirt and organic debris, keeping the water clear as well as safe.
The forms UK owners actually use are dry: stabilised chlorine granules dissolve almost instantly and suit hot tubs, chlorine mini tablets feed slowly via a dispenser, and calcium hypochlorite granules are the classic unstabilised choice for pools and shock dosing.
Test 2–3 times a week and before every use with 6-in-1 test strips, and follow the step-by-step dosing in our guide to adding chlorine to a hot tub.
The harsh smell around a poorly kept pool comes from chloramines – compounds created when free chlorine combines with sweat, body oils and urine. Chloramines irritate eyes and airways and have almost no disinfecting power. The fix is a periodic oxidising dose: either a chlorine shock or a non-chlorine shock, which burns off the combined chlorine and dissolved bather waste. Shock weekly and after heavy use – our guide to chlorine vs non-chlorine shock explains which to reach for.
Used as directed, chlorine is safe. Store it cool, dry and away from other chemicals, never mix different chlorine types or chlorine with bromine in dry form, always add chemicals to water (never water to chemicals), and dose with the pump running so the product disperses. Keep everything locked away from children and pets.
Bromine is the main alternative for hot tubs – more stable in hot water and milder-smelling, held at 4–6 mg/l. See chlorine vs bromine for UK hot tubs for a full comparison. Mineral cartridges, ozone and UV systems can reduce how much sanitiser you need, but none of them replaces a measurable chlorine or bromine residual in the water.
Hold free chlorine at 3–5 mg/l in a hot tub. Swimming pools run lower, typically 1–3 mg/l, because they operate at cooler temperatures with a lower bather load per litre.
A strong smell usually means chloramines – used-up chlorine bound to sweat and oils – not too much free chlorine. Shock the water to oxidise the chloramines and the smell disappears.
Yes. At pH 7.2–7.6 chlorine is highly active; above 7.8 more than half its killing power is lost. Correct total alkalinity to 80–120 ppm first, then adjust pH.
Test 2–3 times a week as a minimum, plus before each use and after heavy bathing sessions. Test strips take seconds and prevent almost every common water problem.