The severity of the 2020 COVID–19 pandemic warrants the rapid development and deployment of effective countermeasures to reduce person-to-person transmission. We have developed a promising approach using single-wavelength far-UVC light at 222 nm generated by filtered excimer lamps, which inactivate viruses and bacteria, without inducing biological damage in exposed human cells and tissue 11–16. The approach is based on the biophysically-based principle that far-UVC light, because of its very limited penetration in biological materials, can traverse and kill viruses and bacteria which are typically micrometer dimensions or smaller, but it cannot penetrate even the outer dead-cell layers of human skin, nor the outer tear layer on the surface of the human eye 12.
In this work we have used an aerosol irradiation chamber to test the efficacy of 222-nm far-UVC light to inactivate two aerosolized human coronaviruses, beta HCoV-OC43 and alpha HCoV–229E. As shown in Fig. 1, inactivation of the two human coronavirus by 222-nm light follows a typical exponential disinfection model, with an inactivation constant for HCoV–229E of k = 4.1 cm2/mJ (95% C. I. 2.5–4.8), and k = 5.9 cm2/mJ (95% C. I. 3.8–7.1) for HCoV-OC43. These values imply that 222 nm UV light doses of only 1.7 mJ/cm2 or 1.2 mJ/cm2 respectively produce 99.9% (3-log reduction) of aerosolized alpha HCoV–229E or beta HCoV-OC43. A summary of k values and the corresponding D90, D99, and D99.9 values we have obtained for coronaviruses is shown in Table 2, together with our earlier results for aerosolized H1N1 influenza virus 23.
The results suggest that both of the studied coronavirus strains have similar high sensitivity to far-UVC inactivation. Robust linear regression produced overlapping 95% confidence intervals for the inactivation rate constant, k, of 2.5 to 4.8 cm2/mJ and 3.8 to 7.1 cm2/mJ respectively for the 229E and OC43 strains. As all human coronaviruses have similar genomic sizes which is a primary determinant of UV sensitivity 27, it is reasonable to expect that far-UVC light will show similar inactivation efficiency against all human coronaviruses, including SARS-CoV–2. The data obtained here are consistent with this hypothesis.
It is useful to compare the performance of far-UVC light with conventional germicidal (peak 254 nm) UVC exposure. We are aware of only one such study 33, which used an aerosolized murine beta coronavirus. The study reported a D88 of 0.599 mJ/cm2, which others 4 have used to estimate the D90 for the virus with 254 nm light as 0.6 mJ/cm2. This value is similar to those estimated in the current work (see Table 2), suggesting similar inactivation efficiency of 222 nm far-UVC and conventional germicidal 254 nm UVC for aerosolized coronavirus, and providing further support for the suggestion that all coronaviruses have similar sensitivities to UV light.
The sensitivity of the coronaviruses to far-UVC light, together with extensive safety data even at much higher far-UVC exposures 12–18, suggests that it will be feasible and safe to have the lamps providing continuous far-UVC exposure in public places to significantly reduce the probability of person-to-person transmission of coronavirus as well as other seasonal viruses such as influenza. For example, the current dose limit guideline for 222 nm light from the International Commission on Non- Ionizing Radiation Protection (ICNIRP) is 23 mJ/cm2 per 8-hour exposure 34. Interpreting this as an intensity of ~3 mJ/cm2/hour, and based on our results here for the beta HCoV-OC43 coronavirus, continuous far-UVC exposure at this intensity would result in 90% viral inactivation in approximately 8 minutes, 95% viral inactivation in approximately 11 minutes, 99% inactivation in approximately 16 minutes and 99.9% inactivation in approximately 25 minutes. Increasing the intensity by, say, a factor of 2 would halve these disinfection times, while still maintaining safety 12–18.
In conclusion, we have shown that very low doses of far-UVC light efficiently kill airborne human coronaviruses carried by aerosols. An exposure dose as low as 1.2 to 1.7 mJ/cm2 of 222-nm light inactivates 99.9% of the airborne human coronavirus tested from both subgroups beta and alpha, respectively. Together with previous safety studies 12–18 and our earlier studies with aerosolized influenza A (H1N1) 23, these results indicate that far-UVC light is a potentially powerful and practical approach for reduction of airborne viral transmission, without the human health hazards associated with conventional germicidal UVC lamps.
These results suggest the utility of deploying low dose rate far-UVC lights in highly occupied indoor public locations such as hospitals, transportation vehicles, airports and schools, potentially representing a safe and inexpensive tool to reduce the spread of airborne-mediated microbial diseases.