Use all three — they do different jobs. A cartridge filter (20 microns or finer) removes particles but does not sanitize. Ozone and UV kill microbes but leave no lasting residual, so most setups add a low chlorine or bromine residual. Filtration clears the water; a sanitizer keeps it safe between plunges. Cold water slows growth but never replaces sanitation.
The short answer — filtration and sanitation are two different jobs
Most cold-plunge product pages use “filter,” “purify,” and “sanitize” as if they were interchangeable. They are not. A cartridge filter physically strains particles out of the water — skin cells, dust, debris. It does nothing to microbes it can't physically trap. Ozone and UV kill or inactivate microbes, but neither one leaves anything behind once it stops working: ozone off-gasses and breaks down, UV only affects water passing directly under the lamp. The only thing actually protecting the water between plunges is a chemical residual — chlorine or bromine. Everything below this line exists to make that distinction specific instead of vague. It also stays narrowly on treating the water itself — what cold exposure does for the body is a separate question, covered in our cold-plunge recovery evidence guide, not here.
Filtration ≠ sanitation (the category error)
What a cartridge filter does
A cartridge filter removes suspended particulate — the stuff that makes water look cloudy or feel gritty. It does this mechanically, by size: water passes through pleated media and anything larger than the media's rating gets caught. A cartridge rated at 20 microns — the size PlungeChill's replacement filter cartridge kit uses — is a common, easy-to-source starting point, typically swapped every 4–6 weeks under normal use and sooner under heavy use. Filtration is necessary for clear water. It is not sufficient for safe water.
Micron reality
Cartridge ratings for home cold plunges typically run 10–50 microns Cartridge-Filtration Convention. 20 microns is the common sweet spot: fine enough to clear visible particulate without clogging as fast as a 10-micron cartridge, and catching more than a 50-micron one leaves behind. Most consumer cartridge ratings are nominal, not absolute — a nominal rating describes typical capture efficiency at that particle size, not a hard cutoff, so two cartridges both labeled “20 micron” can perform differently depending on media and construction.
What filtration cannot do
A cartridge filter does not kill bacteria or viruses, and it does not provide any lasting protection against them — it only removes what's large enough to be physically trapped. Framing filtration as a water-safety measure on its own is the category error this article exists to correct ANSI/PHTA. A tub can run crystal clear on filtration alone and still be growing exactly what you don't want in it.
The sanitizers, and what each actually does
The table below is the article's reason to exist: it lines up what each method removes or kills, whether it leaves a residual, how it behaves in cold water, and roughly what it costs to keep running.
| Method | Removes / kills | Residual? | Cold-water behavior | Cadence | Rough cost |
|---|---|---|---|---|---|
| Cartridge filter | Suspended particulate only | No — not a sanitizer | Unaffected by temperature | Swap 4–6 wk | Low, ongoing |
| Ozone | Bacteria, viruses; attacks biofilm as a strong oxidizer | No — only while the generator runs | Still effective cold | Runs continuously; no consumable swap of its own | Moderate hardware, low to run |
| UV | Microbes, on direct pass-through only | No — point-of-contact only | Dose depends on flow / water clarity / lamp condition, not temperature | Bulb roughly 6–12 mo continuous use | Moderate hardware, low to run |
| Chlorine | Microbes; provides a residual | Yes | Tighter pH window; cheaper, widely available | Dose & test regularly | Low, ongoing |
| Bromine | Microbes; provides a residual | Yes | Slower to activate cold, but holds a residual well in low-turnover water | Dose & test regularly | Slightly above chlorine |
Ozone
Ozone is a strong oxidizer and one of the few methods that stays effective in cold water Pool/Spa Sanitation Convention. It attacks biofilm — the slimy microbial layer that can build up on tub walls and plumbing — more aggressively than a chlorine or bromine residual alone Mah & O'Toole, 2001. But it is non-residual: it works only while the generator is actively injecting it into the water, then off-gasses and breaks down. No Tier-1 source publishes a half-life figure for aqueous ozone at cold plunge temperatures, and this article isn't going to invent one — treat “protected only while running” as the operating assumption rather than waiting on a number nobody has published.
Some ozone-safety pages still quote a separate 0.3 ppm, 15-minute short-term exposure limit. That figure was set in OSHA's 1989 air-contaminants rule and was later vacated by the 11th Circuit — it is not enforceable today. The number that still applies is the 8-hour time-weighted average above. Practically: run an ozone generator in a ventilated space regardless of which figure you've read, and treat this as a general workplace air-exposure limit, not a plunge-specific safety spec.
UV
UV kills or inactivates microbes only as water passes directly under the lamp — it is non-residual in the same way ozone is, just by a different mechanism. Dose depends on flow rate, water clarity (UVT), and lamp condition — not on the wattage printed on the box EPA UV Guidance Manual, 815-B-21-007. A dirty, low-clarity tub degrades UV performance even with a healthy bulb.
“The UV lamp should be replaced after two years, regardless of hours of use.”UV Dynamics, Lamp Life FAQ
Manufacturer guidance splits the difference on exactly when. UV Dynamics rates its lamps for a 9,000-hour useful life and tells owners to replace after two years regardless of hours logged; Jacuzzi/Sundance's CLEARRAY system calls for an annual bulb swap Jacuzzi/Sundance CLEARRAY. For a plunge running its UV system close to continuously, that lands in a rough 6–12 month practical window. Output fades measurably before a bulb fully fails, so “it still turns on” is not the same claim as “it's still doing the job.”
Chlorine / bromine
Chlorine and bromine are the only methods on this list that leave an actual residual — ongoing protection in the water itself, not just at a point of contact. Chlorine is cheaper and more widely available, but runs a tighter effective pH window, roughly 7.2–7.6, and its byproduct (chloramine) is harsher. Bromine is slower to activate in cold water but holds its residual well in the low-turnover, frequently-topped-up conditions of a home plunge, with a milder byproduct across a wider pH tolerance Pool-Industry Convention.
Bromine advocates argue that bromine's residual holds up better in cold, low-turnover plunge water and its milder byproduct is worth the extra cost. Chlorine advocates counter that chlorine does the same job for less money, so long as you hold its tighter pH window. What both agree on: whichever halogen you choose, it's the residual doing the work between plunges — ozone and UV are not a substitute for it. Practitioner sentiment leans bromine for cold plunges specifically, but the honest framing is cost versus convenience, not one being objectively “better.”
Why UV/ozone still need a residual
ANSI/APSP-11-2019 sets a spa baseline of 2–5 ppm free chlorine or 4–6 ppm bromine ANSI/APSP-11-2019, dropping to roughly 0.5 ppm free chlorine or 1.0 ppm bromine when a supplementary UV, ozone, or mineral system is doing part of the work — but a halogen residual is required either way, never zero. Separately, pool-industry convention for day-to-day dosing commonly cites a tighter band: chlorine around 1–3 ppm, bromine around 3–5 ppm Pool-Industry Convention. Both figures come from real sources; treat the code range as the outer compliance boundary and the convention figure as what most operators actually dose to day to day.
This is also why ozone and UV get marketed as if they replace chemistry entirely, and why that marketing is wrong: both are genuinely effective at what they do, and neither does the one job a residual does — guarding the water you're actually sitting in, in the hours between when the generator or lamp last ran and now.
UV/ozone add-ons plug into the same 120V, GFCI-corded circuit as the chiller itself — no separate electrical treatment needed. The wiring itself is covered in our chiller sizing guide, not here, since this article covers what the gear does to the water. One habit worth carrying over from that piece: check a brand's own product spec page for electrical or performance claims, not its blog — brand blogs contradicting their own hardware specs is a documented problem elsewhere in this hub.
pH and the basics that make any sanitizer work
Target range, test cadence, and why cold water slows chemistry
Spa pH runs 7.2–7.8 ANSI/APSP-11-2019. Outside that window, a chlorine or bromine residual doesn't work as advertised — too low and it corrodes and irritates, too high and it stops sanitizing effectively even at the “right” ppm reading. Test before every session, or at minimum every few days on a lightly-used tub.
Cold water slows sanitizer chemistry — roughly threefold at 50°F versus 80°F The Cold Standard, with reduced chlorine efficacy at low temperature corroborated separately NCCEH, 2024. That cuts both ways: your residual works more slowly, but so does whatever it's fighting — microbial growth also slows below 50°F NCCEH, 2024. Slower is not the same as stopped. The intervals below assume that slower baseline; they are not a reason to skip testing.
How often to change the water
This ladder is for above-ground, self-contained plunges — an in-ground or gunite installation runs under different, contractor-gated code requirements and is out of scope here.
The interval ladder
| Setup | Typical interval |
|---|---|
| No filtration or sanitation | 3–5 days |
| Basic filter only | 2–4 weeks |
| Filter + chemical sanitizer | 4–8 weeks |
| Ozone or UV + residual | 8–12 weeks |
| Full chiller + managed chemistry | 2–3 months, up to ~6 |
Cold-Plunge Maintenance Convention — figures corroborated in practice by Renu Therapy's and Morozko's own published maintenance guidance.
Override triggers
Cloudy water, odor, or visible slime override any schedule — drain and refill regardless of where you are in the interval. Colder water (below 50°F) slows the growth that causes those symptoms NCCEH, 2024, which is part of why the ladder above runs longer than a typical warm spa's — not a guarantee that colder means safe indefinitely. For a printable week-by-week routine that turns this ladder into a checklist, see our maintenance routine guide.
What it costs to keep clean
Ongoing cost splits into two buckets: filtration consumables (the cartridge itself) and sanitation consumables (chemicals, test strips, and eventually a UV bulb if you're running one). Both sit in the entry tier as recurring costs — the real budgeting question is cadence, not per-item price, since a filter changed every 4 weeks costs more over a year than the same filter changed every 8.
If your setup uses an external chiller with its own inlet filter, budget for that as a separate line item — Chilly GOAT's external chiller filter is the same idea applied to the chiller side rather than the tub itself, and it sits at the low end of the entry tier.
FAQ
Do I still need chlorine or bromine if I already run UV or ozone?
Yes. Both UV and ozone are non-residual — they treat water only at the point of contact or while actively running, then leave nothing behind. ANSI/APSP-11-2019 permits a reduced residual target (around 0.5 ppm free chlorine or 1.0 ppm bromine) when a supplementary UV/ozone/mineral system is present, but some residual is always required ANSI/APSP-11-2019.
What micron rating should a cold plunge filter cartridge be?
20 microns is the common sweet spot for home cold plunges — fine enough to clear visible particulate without clogging as fast as a 10-micron cartridge, or leaving as much behind as a 50-micron one. Ratings in this range are typically nominal rather than absolute, so capture varies somewhat by cartridge construction Cartridge-Filtration Convention.
How often should I change my cold plunge water?
It depends on what's protecting it: no filtration or sanitation, 3–5 days; a basic filter alone, 2–4 weeks; filter plus a chemical sanitizer, 4–8 weeks; ozone or UV backed by a residual, 8–12 weeks; a full chiller with managed chemistry, 2–3 months and sometimes longer Cold-Plunge Maintenance Convention. Cloudy water, odor, or visible slime override the schedule regardless of setup.
Is ozone or UV better for a cold plunge?
Neither wins outright — they're different tools for the same non-residual job. Ozone is the stronger oxidizer and reaches into biofilm, but needs ventilation and off-gasses once the generator stops. UV is simpler and set-and-forget, but only treats water passing directly under the lamp. Both still need a chlorine or bromine residual behind them.