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Ozone disinfection uses ozone (O3), a highly reactive form of oxygen, to inactivate bacteria and viruses, mainly in water and wastewater treatment. It works through oxidation, leaves no harmful residual to remove in wastewater, and is generated on site because ozone decomposes quickly. According to the EPA, it is not an effective way to disinfect the air in occupied rooms.
This page is general reference information from public regulators, not medical or engineering advice. It is written for lab, clinic and infection-prevention staff who meet ozone in water systems, air-cleaner claims or device-sterilization discussions.
Definition and scope
Ozone is a gas made of three oxygen atoms. The U.S. EPA wastewater fact sheet describes it as formed when oxygen molecules are split by an energy source and the free atoms collide with other oxygen molecules; most plants pass an oxygen-bearing gas through a dielectric discharge gap at 6 to 20 kilovolts AC. Because ozone is unstable and decomposes back to oxygen in a short time, it is generated where it is used rather than stored or shipped.
Ozone disinfection is applied to water, wastewater and, with important limits, air. Ozone as a sterilizing agent for heat-sensitive medical devices is a separate topic, covered in low-temperature sterilization method selection and the broader sterilization technology overview.
How ozone inactivates microorganisms
The EPA fact sheet lists several mechanisms: direct oxidation and destruction of the cell wall, with leakage of cellular contents; reactions with radical by-products of ozone decomposition; damage to the purines and pyrimidines of nucleic acids; and breakage of carbon-nitrogen bonds that leads to depolymerization. The result depends on dose, mixing and contact time, which is why the fact sheet says systems should be pilot tested and calibrated before installation.
A water treatment system has four parts according to the fact sheet: feed gas preparation, ozone generation, ozone contacting, and destruction of leftover ozone in the off-gas. The off-gas step matters because the EPA describes ozone as extremely irritating and possibly toxic.
Where it is used, and what the regulators say in each setting
The same oxidant is described differently depending on the setting. The table sets out what three public sources state; the entries are not interchangeable.
| Setting | What the source says | Source |
|---|---|---|
| Wastewater effluent | Generally used at medium to large plants after at least secondary treatment. Contact time of roughly 10 to 30 minutes. No harmful residual that needs removal, and no regrowth of microorganisms except those protected by particulates. Described as the least used method in the U.S., with high capital and power cost. | EPA fact sheet 832-F-99-063 |
| Drinking water | A very strong biocide, more effective than chlorine, chloramines and chlorine dioxide against viruses and protozoa. A residual cannot be maintained in a distribution system because ozone has low solubility and is unstable above pH 9. Ozonation can form bromate and other brominated by-products when the source water contains bromide. | NHMRC Australian Drinking Water Guidelines, section 1.6 |
| Air in occupied rooms | No federal agency has approved ozone generators for occupied spaces. At levels within public health limits, ozone does not effectively remove viruses, bacteria or mold. | EPA, ozone generators sold as air cleaners |
Note the difference between the first two rows. The EPA wastewater sheet reports no regrowth after treatment, while the NHMRC notes that ozonation can produce compounds that promote bacterial regrowth in a distribution system. These statements describe different systems, an effluent discharge versus a long pipe network with no residual, so they are not merged here.
Why ozone is used sparingly
The EPA lists these limits on wastewater use: a low dose may not inactivate some viruses, spores and cysts; the process is more complex than chlorine or UV; ozone is corrosive and needs corrosion-resistant materials such as stainless steel; it is uneconomical where suspended solids, BOD, COD or TOC are high; and off-gas must be destroyed. Gaseous ozone is explosive at 240 g/m3, so systems usually run at 50 to 200 g/m3, and the feed gas must be very dry (dew point of -60 C or lower) to avoid corrosive condensate.
The NHMRC adds operating conditions for drinking water: turbidity at or below 1 NTU at the point of ozonation, pH below 9, online monitoring of pH, turbidity, ozone concentration and end-of-contact residual, and an alternative disinfectant where bromide is high. It also notes ozone is not used in Australia as the primary disinfectant for a sizeable supply.
Checklist before relying on an ozone claim
Built from the three sources above:
- Which setting is the claim about: effluent, drinking water, air or a device? The table shows the evidence differs by setting.
- Is there off-gas destruction and routine leak checking? The EPA says ambient ozone should stay below applicable safety limits.
- Is the feed gas dried, and are system materials corrosion resistant?
- For drinking water, has bromide in the source water been assessed, given the bromate risk?
- Does the water meet the turbidity and pH conditions for effective disinfection?
Common misunderstandings
Ozone generators are not proven room air cleaners
The EPA states that decontaminating air with ozone would take concentrations 5 to 10 times higher than public health standards allow, and that ozone may not affect contaminants embedded in porous materials. Inhaled ozone can damage the lungs; low exposure can cause chest pain, coughing, shortness of breath and throat irritation, and can worsen asthma. The limits the EPA cites are 0.05 ppm ozone output for indoor medical devices (FDA), 0.10 ppm as an 8-hour average for workers (OSHA), 0.10 ppm not to be exceeded (NIOSH) and 0.08 ppm as the outdoor 8-hour standard.
“It only leaves oxygen” is incomplete
Ozone does decompose to oxygen, but in water containing bromide it can form brominated by-products and bromate, per the NHMRC. For a different disinfection approach, see ultraviolet sterilization.
FAQ
What is ozone disinfection?
It is the use of ozone gas, generated on site, to oxidize and inactivate microorganisms, most often in water and wastewater treatment.
Is ozone more effective than chlorine?
For viruses and protozoa in drinking water, the NHMRC states ozone is more effective than chlorine, chloramines and chlorine dioxide. The NHMRC also notes an ozone residual cannot be maintained in a distribution system.
Does ozone sterilize medical equipment?
Ozone is used as a sterilizing agent in some low-temperature device sterilizers, a separate topic from water treatment; see sterilization validation and sterility assurance level.
Sources
- U.S. EPA, Wastewater Technology Fact Sheet: Ozone Disinfection, EPA 832-F-99-063, September 1999, https://www.epa.gov/sites/production/files/2015-06/documents/ozon.pdf (checked 2026-10-30).
- NHMRC, Australian Drinking Water Guidelines, 1.6 Disinfection with ozone, https://guidelines.nhmrc.gov.au/australian-drinking-water-guidelines/part-4/disinfection/1.6-disinfection-with-ozone (checked 2026-10-30).
- U.S. EPA, Ozone Generators that are Sold as Air Cleaners, https://www.epa.gov/indoor-air-quality-iaq/ozone-generators-are-sold-air-cleaners (checked 2026-10-30).
