Far-UVC light (222 nm) efficiently and safely inactivates airborne human coronaviruses

Abstract

A direct approach to limit airborne viral transmissions is to inactivate them within a short time of their production. Germicidal ultraviolet light, typically at 254 nm, is effective in this context but, used directly, can be a health hazard to skin and eyes. By contrast, far-UVC light (207-222 nm) efficiently kills pathogens potentially without harm to exposed human tissues. We previously demonstrated that 222-nm far-UVC light efficiently kills airborne influenza virus and we extend those studies to explore far-UVC efficacy against airborne human coronaviruses alpha HCoV-229E and beta HCoV-OC43. Low doses of 1.7 and 1.2 mJ/cm² inactivated 99.9% of aerosolized coronavirus 229E and OC43, respectively. As all human coronaviruses have similar genomic sizes, far-UVC light would be expected to show similar inactivation efficiency against other human coronaviruses including SARS-CoV-2. Based on the beta-HCoV-OC43 results, continuous far-UVC exposure in occupied public locations at the current regulatory exposure limit (~3 mJ/cm²/hour) would result in ~90% viral inactivation in ~8 minutes, 95% in ~11 minutes, 99% in ~16 minutes and 99.9% inactivation in ~25 minutes. Thus while staying within current regulatory dose limits, low-dose-rate far-UVC exposure can potentially safely provide a major reduction in the ambient level of airborne coronaviruses in occupied public locations.

Figure 1

Coronavirus survival as function of the dose of far-UVC light. Fractional survival, PFU_UV / PFU_controls, is plotted as a function of the 222-nm far-UVC dose. The results are reported as the estimate plaque forming units (PFU)/ml using the conversion PFU/ml = 0.7 TCID₅₀ by applying the Poisson distribution. Values are reported as mean ± SEM from multiple experiments (n = 3 alpha HCoV-229E and n = 4 for beta HCoV-OC43); the lines represent the best-fit regressions to equation (1) (see text and Table 1).

Figure 2

Infection of human lung cells from irradiated aerosolized alpha HCoV-229E as function of dose of far-UVC light. Representative fluorescent images of MRC-5 normal human lung fibroblasts infected with human alphacoronavirus 229E exposed in aerosolized form. The viral solution was collected from the BioSampler after running through the aerosol chamber while being exposed to (a) 0, (b) 0.5, (c) 1 or (d) 2 mJ/cm² of 222-nm light. Green fluorescence qualitatively indicates infected cells (Green = Alexa Fluor-488 used as secondary antibody against anti-human coronavirus spike glycoprotein antibody; Blue = nuclear stain DAPI). Images were acquired with a 10× objective; the scale bar applies to all the panels in the figure.

Figure 3

Infection of human lung cells from irradiated aerosolized beta HCoV-OC43 as function of dose of far-UVC light. Representative fluorescent images of WI-38 normal human lung fibroblasts infected with human betacoronavirus OC43 exposed in aerosolized form. The viral solution was collected from the BioSampler after running through the aerosol chamber while being exposed to (a) 0, (b) 0.5, (c) 1 or (d) 2 mJ/cm² of 222-nm light. Green fluorescence qualitatively indicates infected cells (Green = Alexa Fluor-488 used as secondary antibody against anti-human coronavirus spike glycoprotein antibody; Blue = nuclear stain DAPI). Images were acquired with a 10× objective; the scale bar applies to all the panels in the figure.