Active oxygen genuinely does disinfect — just differently from chlorine: faster, more gently and for a far shorter time. In a hot tub at 38 °C it breaks down within one to two days, so it calls for more frequent dosing and is usually combined with an activator or an algaecide. This article sums up how chlorine-free methods work, where their real limits lie and who they pay off for.
What active oxygen is and how it disinfects
Two different chemical families are sold under the trade name “active oxygen”. Granules are most often potassium persulfate or potassium monopersulfate (MPS), while tablets and liquids are usually based on hydrogen peroxide, or on sodium percarbonate, which releases peroxide in water.
The shared principle is oxidation. Both substances release highly reactive oxygen species in water that attack the cell walls of bacteria and break down organic contamination — sweat, cosmetics, sunscreen residues. Chlorine does essentially the same thing, only with a different oxidising agent and with considerably greater persistence.
And persistence is precisely the key difference. Once dosed, chlorine stays in the water as a residual disinfectant — a measurable reserve that neutralises bacteria brought in by the next bather. Active oxygen, by contrast, is consumed in the reaction and then breaks down into water and oxygen. The water is clean, but with no protective reserve.
Why active oxygen is combined with an activator
Peroxide on its own, at pool concentrations, is not aggressive enough against algae and some of the more resistant microorganisms. Manufacturers therefore supply an activator alongside it — usually a solution of silver or copper ions, or a quaternary ammonium salt (an algaecide) that also acts as a wetting agent and prevents algae growth between doses.
This is essential information for anyone buying a “chlorine-free” kit in the hope of water without chemicals: such a system contains at least two active substances, and one of them is typically a biocide with a long-lasting effect. Chlorine-free, yes — chemical-free, absolutely not.
Active oxygen vs. chlorine vs. bromine: an honest comparison
The following table compares the three most common methods under the conditions of a domestic hot tub (1,000–1,500 l, 36–38 °C) and a seasonal pool. Prices are indicative retail levels in 2026.
| Criterion | Chlorine | Bromine | Active oxygen |
|---|---|---|---|
| Effectiveness against bacteria | High, proven by decades of practice | High, comparable to chlorine | Good at a sufficient dose, weaker against algae without an activator |
| Stability at 38 °C | Days (faster loss without a stabiliser) | Highest — bromine is more stable than chlorine when hot | Low — the effect typically fades within 24–48 hours |
| Dosing frequency in a hot tub | 2–3× a week | Continuously via a floater/dispenser | After every soak, or at least every other day |
| Residual disinfection | Yes, measurable | Yes, long-lasting | Practically none |
| Smell | Typical, intensifies as chloramines form | Milder, but distinctive and clings to swimwear | Lowest — the water is practically odourless |
| Gentleness on skin and eyes | The most reported irritation | Medium | The gentlest of the three |
| Testing | Easy (strips, drop and photometric tests) | Easy | Harder — requires special peroxide/MPS strips |
| Indicative annual cost (hot tub) | €60–100 | €100–160 | €160–280 |
| Main weakness | Smell and irritation when pH is wrong | Cost and the need for a dispenser | Short lifespan, higher cost, risk under heavy loading |
Chlorine and bromine are not interchangeable, and you should never mix them in a single dispenser. The differences between them are covered in detail in a separate article, Bromine in a hot tub, and the complete procedure for setting up your water in the guide Hot tub chemistry step by step.
Why high temperature is a problem for active oxygen
Both hydrogen peroxide and persulfate break down faster the warmer the water is. While the effect lasts roughly three days in a pool at 26 °C, in a hot tub at 38 °C it is often a matter of 24 hours — and even less in intensive use. Breakdown is further accelerated by UV radiation and the presence of metals in the water.
The practical consequence: a hot tub running on active oxygen needs a dose after practically every soak and a clearly defined routine. Going away for a week and leaving the water to “run itself” is considerably riskier with this method than with a bromine dispenser, which releases the active substance continuously.
The second consequence is economic. Lower stability means higher consumption, so the annual cost tends to be two to three times that of chlorine. It is not a make-or-break sum, but it is only fair to know it up front — tips on where to save elsewhere are gathered in the article How to save on pool chemicals.
Who active oxygen makes sense for
Active oxygen is not an inferior method — it is a method with a different profile. It works well wherever two conditions are met: a low to medium bather load and an owner who doses regularly.
- Small volumes and low occupancy. A hot tub for two people used several times a week is the ideal scenario. A five-seat tub with weekend guests, on the other hand, is already borderline for active oxygen alone.
- Users with sensitive skin. The most common reason for switching. Alternatives for allergy sufferers and people with atopic skin are examined in detail in the article Sensitive skin and alternatives to chlorine.
- Seasonal and smaller pools. In an above-ground pool used for a few months a year, the lower water temperature is an advantage and active oxygen lasts longer here.
- As a complement to a UV lamp or ozone. The strongest combination of all. Physical disinfection kills microorganisms as the water flows through the equipment, while active oxygen handles oxidation in the rest of the tank. Details in the article UV lamp and ozone.
Conversely, it makes no sense in a rental hot tub, in tanks with a heavy and irregular load, and anywhere the owner cannot guarantee dosing every other day.
An essential warning: “chemical-free” does not exist
The marketing of chlorine-free products often leans on words like “natural”, “chemical-free” or “just oxygen and water”. Physically, that does not add up. Any water in which people bathe at 30–38 °C needs one of two things: a residual disinfectant, or very intensive filtration and physical disinfection with a short turnover time.
Without it, biofilm multiplies in the pipework within a few days in warm water, and with it bacteria of the genus Legionella, which are transmitted by inhaling aerosol from the jets. The risk is not theoretical and concerns hot tubs above all, where the most aerosol is created — so read Hot tub hygiene and legionella before you drop chlorine altogether.
A sensible compromise used by many owners: everyday operation on active oxygen plus a chlorine shock once every two to three weeks or after every larger gathering. The water stays free of chlorine smell most of the time, but the biofilm takes a regular hit.
Other chlorine-free alternatives and their limits
Copper and silver ionisation
Electrolytic ionisers release copper and silver ions into the water with a demonstrable bactericidal and algicidal effect. It sounds elegant, but there are three catches: the ions act slowly (they cannot handle acute contamination), copper at higher concentrations and with unsuitable pH stains the water, the grouting and light hair a greenish blue, and many countries have limits on silver content in bathing water. Ionisation is therefore suitable as a supplement, not as the sole disinfectant.
Biguanide (PHMB)
Polyhexamethylene biguanide is a standalone system that works without chlorine and without oxygen and is fairly stable in water. Its drawbacks, however, are harsh: it is completely incompatible with chlorine, bromine and active oxygen (mixing produces cloudiness and precipitates), it tends to cost two to three times more, and in some pools it causes a slimy coating on the filter cartridge that shortens its life. Switching to biguanide and back again means a complete water change and a thorough clean of the filter.
A natural pool as the only genuinely chemical-free solution
If you want swimming without any added biocide at all, there is only one honest route: a biologically filtered pool, where the water is maintained by a planted regeneration zone and the microbial community in the substrate. The price is a higher initial cost, the need for extra space and a different character of water (lower temperature, natural appearance). You will find a complete analysis in the article Natural pool and swimming pond.
Practical dosing and testing of active oxygen
Indicative dosing always follows the instructions for the specific product, because concentrations differ between brands. The general framework for a 1,000 l hot tub looks like this:
- Balance the water first. Alkalinity 80–120 mg/l, pH 7.2–7.6. Active oxygen is less sensitive to pH than chlorine, but excessively low pH speeds up its breakdown and corrodes the equipment.
- Start-up dose after filling: as a rule twice the maintenance dose, so typically 50–100 ml of liquid product or 20–30 g of granules per 1,000 l.
- Maintenance dose after every soak or every other day, with the filtration running and the cover off for 15–20 minutes.
- Activator/algaecide usually once a week at the dose stated in the instructions — without it the method collapses into greenish cloudiness after a few weeks.
- Filter cleaning once every two to three weeks. Active oxygen binds more organic matter, which ends up in the cartridge.
Testing is the weak point of the method: ordinary free chlorine strips will not pick up peroxide content. You need specific active oxygen strips (H₂O₂ or MPS), which are sold separately and have a coarser scale. The ideal value is usually 5–8 mg/l. Always keep an eye on pH and alkalinity too — an overview of measuring methods can be found in the article How to test pool and hot tub water.
How to switch from chlorine to active oxygen
The switch is not simply about stopping one product and starting another. Residual chlorine and peroxide react with each other, destroy each other, and the result tends to be water protected by neither method.
- Change the water. In a hot tub this is the simplest and most reliable step — the article How often to change hot tub water will help you judge the intervals.
- Flush the pipework with a jet cleaner before you drain. Old biofilm would immediately burden the new system.
- Clean or replace the filter cartridge. One soaked in chlorine and organic matter is the most common cause of a failed switch.
- Do not mix methods. The only exception is a planned chlorine shock with a gap, not simultaneous dosing.
- Test more often for the first two weeks and watch how quickly the value drops. Set your realistic dosing interval accordingly.
Frequently asked questions
Is active oxygen really gentler on the skin than chlorine?
Yes, in most cases. It does not create chloramines, which are behind the typical smell and a large part of eye and skin irritation. The condition is correctly set pH — water that is too acidic or too alkaline irritates regardless of the disinfectant chosen.
Can I combine active oxygen with chlorine?
Not at the same time. The two substances oxidise and degrade each other in the water, so you pay for two chemicals and have neither working. They can only be combined separately in time — everyday operation on oxygen and an occasional chlorine shock with a gap.
Is a UV lamp or ozone enough so that I do not have to dose anything else?
No, it is not. Both UV and ozone act only on the water currently flowing through the equipment and leave no residual protection in the tank. They work excellently as a supplement that can easily halve disinfectant consumption, but not as the sole measure.
Why did my water turn green after switching to active oxygen?
Most often the activator or algaecide is missing, or the dosing interval is too long. Active oxygen on its own will not suppress algae in the long term, especially in an outdoor pool in the sun.
How do I know there is enough active oxygen in the water?
Only by measuring with specific peroxide or MPS strips — the appearance and smell of the water tell you nothing. The target value is usually 5–8 mg/l; the exact range is stated by the manufacturer of the particular product.
Conclusion: an alternative, yes; a miracle, no
Active oxygen is a legitimate and, in many households, a more pleasant way to keep water clean — at the price of greater discipline in dosing, higher costs and less tolerance of sudden loading. If chlorine smell bothers you and your hot tub sees calm use, it is worth trying. If you are looking for a system that forgives a week of inattention, stay with bromine or chlorine.
Products for all the methods described — active oxygen, activators, biguanide and classic chlorine and bromine chemistry including testers — can be found in the Water treatment category. Further comparisons and guides are in the Guide section.
