Inactivation of Norovirus by ozone gas in conditions relevant to healthcare

Background

Norovirus is one of the most common causes of acute gastroenteritis outbreaks in settings where people live or stay in close quarters, including hospitals, aged care homes, hotels and cruise ships. The virus is notoriously hardy in the environment and can persist on surfaces for extended periods, making it difficult to control with conventional cleaning. Liquid disinfectants such as bleach and peroxide-based products are labour-intensive to apply, do not always reach every contaminated surface, and their real-world effectiveness against norovirus has been questioned. Researchers investigated whether ozone gas, which spreads through air rather than requiring surface-by-surface wiping, could offer a practical alternative for decontaminating rooms after a norovirus outbreak.

Study design

The study, led by Hudson, Sharma and Petric, tested ozone gas against dried samples of norovirus and its laboratory surrogate, feline calicivirus (FCV), since norovirus itself cannot be grown in standard cell culture. Rather than testing only in a laboratory, the team placed replicate virus samples at multiple locations - such as the bathroom, bed and table - within three realistic environments: an office (34 cubic metres), a hotel room (47.6 cubic metres) and a cruise liner cabin (36.4 cubic metres). Samples were applied to both hard surfaces (plastic, steel, glass) and soft surfaces (fabric, cotton, carpet). A portable ozone generator using corona discharge maintained concentrations of 20-25 parts per million, monitored with a calibrated ozone analyser, alongside a device that released bursts of water vapour to raise humidity. The standard treatment protocol was 20 minutes of ozone exposure, a 5-minute humidity pulse, 10 minutes of incubation and 15 minutes of surface scrubbing to recover the virus - a total of less than an hour. FCV infectivity was measured using plaque assays and cytopathic effect endpoint dilution tests, while both FCV and norovirus were also measured by quantitative reverse transcriptase real-time PCR (QRT-PCR), which detects viral genetic material.

Findings

Ozone gas substantially reduced infectious FCV across all three settings. In the office, infectivity fell to between 1.2% and 1.7% of untreated controls (a reduction of roughly 60- to 90-fold). In the hotel room, bathroom and table samples showed no detectable infectious virus at all, while the bed sample fell to below 0.02% of control. In the cruise cabin, infectious virus was reduced by more than 10,000-fold, again to the point of non-detection. Across different surface materials - plastic, fabric, cotton and carpet - FCV infectivity reductions ranged from roughly 30,000- to 600,000-fold, with all materials showing broadly similar sensitivity to ozone. Norovirus, measured only as surviving RNA rather than infectivity, showed smaller reductions: surviving fractions of around 5-7% in the office, around 7-8% in the hotel room, and 0.3-3% in the cruise cabin. The authors noted that QRT-PCR detects genetic material that is more resistant to damage than infectious virus particles, so the true reduction in infectious norovirus is likely understated by these RNA-based figures.

What the authors concluded

The authors concluded that ozone gas can inactivate norovirus (via its FCV surrogate) in dried samples across realistic healthcare-relevant settings, in some cases reducing infectious virus below the level of detection. They highlighted that the short treatment protocol - under an hour in total - makes the approach practically applicable to hospitals, hotels and cruise ships, provided the room can be temporarily sealed and vacated during treatment.

Limitations of this study

Because norovirus cannot be cultured in the laboratory, the study relied on feline calicivirus as a surrogate for measuring true infectivity; FCV may not behave identically to norovirus in every respect. The QRT-PCR method used to assess norovirus directly measures only genetic material, not infectivity, and the authors themselves note this likely underestimates ozone's real effectiveness against norovirus. Testing was conducted at temperatures below 23 degrees Celsius, so the effect of higher temperatures or more variable humidity was not extensively explored. Ozone gas is also toxic at the concentrations used, meaning treated rooms must be sealed and unoccupied for the full protocol, which may be operationally challenging in some settings. Finally, this was an experimental study using controlled sample placements rather than a clinical trial in an active outbreak setting, so real-world performance may differ.