Scientists at Fujita Health University told a news conference they had proven that ozone gas in concentrations of 0.05 to 0.1 parts per million (ppm), levels considered harmless to humans, could kill the virus.
The experiment used an ozone generator in a sealed chamber with a sample of coronavirus. The potency of the virus declined by more than 90% when subjected to low level ozone for 10 hours.
“Transmission of the novel coronavirus may be reduced by continuous, low-concentration ozone treatment, even in environments where people are present, using this kind of system,” said lead researcher Takayuki Murata.
“We found it to be particularly effective in high-humidity conditions.”
Ozone, a type of oxygen molecule, is known to inactivate many pathogens, and previously experiments have shown that high concentrations, between 1-6 ppm, were effective against the coronavirus but potentially toxic to humans.
A recent study at the Georgia Institute of Technology showed that ozone may be effective in disinfecting gowns, goggles and other medical protective equipment.
Fujita Medical University Hospital, in Aichi prefecture central Japan, has installed ozone generators to reduce infection in waiting areas and patient rooms.
The university has also performed a clinical trial of Fujifilm Holdings Corp's 4901.T Avigan drug on COVID-19 patients.
Previous studies show that norovirus has a high stability and resistance toward environmental stress and is much more resistant than coronavirus toward alcohols, chlorine, and ultraviolet disinfection (Li et al. 2020). On the other hand, Dubuis et al. have treated airborne norovirus with ozone, in a controlled chamber, under high RH conditions, low ozone concentration (0.46 mg/m3 or 0.23 ppm), and 40 min of contact time (Dubuis et al. 2020). The reduction of murine norovirus infectivity was as high as 99.8%. This fact shows that a large reduction of infectivity can be achieved treating the atmosphere with aerosolized norovirus even when the concentration of ozone is as low as 0.46 mg/m3 when the RH is as high as 85%. Therefore, considering the similitude between SARS-CoV-2 and murine norovirus, one might expect, from Fig. 1, that TC doses of 20 mg/m3 min could be efficient for SARS-CoV-2 under high RH conditions (this has to be confirmed by experimental studies).
Lee et al. have shown, using a human coronavirus (HCoV-229E) as a surrogate for SARS-CoV-2, that the virus present on contaminated masks lost its infectivity to a human cell line (MRC-5) when exposed to 240 mg/m3 ozone gas during 1 min. Similar results were obtained for influenza A virus (H1N1). In this case, this short exposure time did not fully degrade the viral RNA, and thus, the loss of infectivity was attributed to the damage of the viral envelope or envelope proteins, resulting in failure of the virus to attach itself to host cells (Lee et al. 2020).
In summary, from a conservative point of view, TC of 100–200 mg/m3 min are expected to assure a high inactivation of the virus, and they could be easily applied to decrease the viral load of PPE and other materials in small chambers during pandemic peaks, using small available ozone generators (2–3 g/h), at levels of 10–20 mg/m3 (5–10 ppm) for 10 min.
Effectiveness of ozone on reducing airborne bacteria The total airborne bacteria in the conference room was measured before and after each ozonation. Measurement was carried out using an Andersen N-6 single-stage sampler with Tryptone Soya Agar (Oxoid) in petri dish. 283L of air was taken for each sampling. The petri dish was incubated at 35o C for 48 hrs before counting. The disinfection efficiency of ozonation at different concentration was tabulated in Table 1.
Table 1. Reduction of Airborne Bacteria after Ozonation Ozone conc. 0.5 ppm 2.5 ppm 5 ppm Before Ozonation 592 CFU/m3 612 CFU/m3 552 CFU/m3 After Ozonation 169 CFU/m3 42 CFU/m3 57 CFU/m3 Reduction % 71.5% 93.1% 89.6% The results show that ozone is effective in reducing airborne bacteria. At higher ozone level, the sanitizing effect increased. Over 90% of airborne bacteria could be reduced at 2.5 ppm concentration. Further increase of ozone concentration to 5 ppm does not beneficial in bacteria reduction percentage. Unlike laboratory experiments conducted by Kowalski et al (1) that could remove 99.99% airborne bacteria after ozonation, the best reduction percentage in our case was around 93% only. High removal percentage could not be achieved because the conference room was not 100% sealed. Doors should be opened briefly during each air sampling (for placing a new agar dish on the sampler) and air exchange from outside was unavoidable. For safety reason, excessive high concentration ozone should be avoided and the lowest ozone concentration that could kill most of the microorganisms should be selected as optimum. Depends on the contamination level, 0.5 – 2.5 ppm ozone level is adequate for air disinfection.
Ozone (O3) is an unstable gas comprising three atoms of oxygen. It is unstable because the gas will readily degrade back to its stable state, diatomic oxygen (O2) with the formation of free oxygen atoms or free radicals. The free oxygen atoms or radicals are highly reactive and they will oxidize almost anything (including viruses, bacteria, organic and inorganic compounds) in contacts, making ozone an enormously powerful disinfectant and oxidizer. In fact, ozone is a much stronger oxidizer than other common disinfectants such as chlorine and hypochlorite. The usage of chlorine or hypochlorite in many countries has been decreased significantly due to the possibility formation of carcinogenic by-products such as trihalomethanes (THM) during Page 2 the disinfection process. In contrast, ozone disinfection does not produce any harmful residues, and all the residual ozone will be converted back to oxygen within a short time. Ozone is therefore considered as an environmentally friendly disinfectant.