Cold plunge chemicals: what to use and how much

Type “should I put chlorine in my cold plunge” into a search engine and you’ll find two camps: pool-industry voices saying dose it like a spa, and cold-plunge influencers saying cold water keeps itself clean. Both are wrong. Here’s the honest version.

The core fact: cold slows bacteria, it doesn’t stop them

Refrigeration is why your milk lasts a week instead of a day — but it still goes off. The same applies to your plunge. At 5–10°C, bacteria multiply much more slowly than in a hot tub, but every dip adds skin flora, sweat, skin oils and whatever’s on your feet. Biofilm — the slippery layer that eventually coats unmanaged tubs — forms happily in cold water; it just takes longer.

So the real question isn’t “chemicals or not”. It’s: how long do you want to keep the same water, and how many people are getting in it? Chemicals buy time between water changes. If you’re happy changing water often, you can buy that time with a hose instead.

Option 1: low-dose chlorine — cheapest and best understood

For a 300–1,000L residential plunge, the usual guidance is 1–3 ppm free chlorine — deliberately below the 3–5 ppm typical for hot tubs, because cold water loses chlorine far more slowly, so it’s easy to overshoot. Use stabiliser-appropriate granules (sodium dichlor is the common choice for small tubs), dose small, wait, and test before adding more.

Costs are trivial: a 1kg tub of chlorine granules is roughly £10–£20 in the UK and lasts months at plunge doses; test strips are £5–£10 per 50. Keep pH at 7.2–7.6 — outside that band chlorine works poorly and skin complains. Total chemical spend for a maintained plunge is realistically £5–£15 a month.

Downsides: faint chlorine smell, occasional dry skin, and you do have to actually test — an unmeasured “capful now and then” is how tubs end up either septic or stinging.

Option 2: bromine — more stable, more expensive

Bromine at roughly 3–5 ppm is the spa-world alternative: it stays effective across a wider pH range and is stable in low-turnover cold water, and some people find it gentler on skin. Bromine tablets cost more than chlorine granules in the UK (typically £20–£30/kg) and bromine lingers on skin longer. It’s a reasonable choice; for most plunges it solves a problem low-dose chlorine didn’t have.

Option 3: hydrogen peroxide — gentler feel, weaker evidence

Peroxide is popular in the plunge world: no chlorine smell, and it degrades to water and oxygen. Typical practice is roughly 30–100 ppm, checked with peroxide-specific test strips, often alongside an ozone system. Be aware this is enthusiast practice rather than an established spa-treatment standard, the concentrated product needs careful handling, and — this is the important one — never combine peroxide with chlorine or bromine; they react with each other and you lose sanitisation from both.

Option 4: ozone or UV hardware

Many plunge chillers now include an ozone generator or UV lamp inline. Both kill microbes effectively in the water passing through the unit — but leave no residual sanitiser sitting in the tub between passes. Treat them as a multiplier: they let you run the low end of a chlorine dose, or stretch chemical-free water changes further, rather than replacing either. If you’re choosing a chiller anyway, it’s a worthwhile feature; retrofitting hardware just to avoid £10 of granules rarely pays. Running-cost context is in cold plunge running costs.

Option 5: no chemicals at all

Entirely workable if you accept the routine: shower (properly, with soap) before every dip, run filtration if you have it, keep a lid on, and do a full drain, wipe-down and refill on a schedule — roughly every 1–2 weeks for one clean-showering user with a filter, and every few days without one or in summer. Water itself is cheap: refilling 500L costs on the order of £1–£2 on a UK meter, and unmetered homes pay nothing marginal. The failure mode is human, not chemical — the week you skip is the week the biofilm arrives. Any cloudiness, slipperiness or smell overrides the schedule.

What we’d actually do

• One user, ice-barrel style, no filter: no chemicals; change water every few days; pre-shower religiously.
• One or two users with a filtered chiller: low-dose chlorine at 1–3 ppm (or lean on built-in ozone/UV with fortnightly changes); test twice a week.
• Family or shared plunge: chlorine or bromine, tested and logged, plus regular changes. Shared warm-blooded bodies in unsanitised water is how folliculitis and ear infections happen.

Whichever route you pick, sanitiser is only one leg of the stool — filtration, water changes and tub cleaning carry the rest. The full routine lives in cold plunge maintenance and hygiene.

Sources: HSE — HSG282: controlling legionella and other infection risks in spa-pool systems