Shielding of Viruses such as SARS-CoV-2 from Ultraviolet Radiation in Particles Generated by Sneezing or Coughing: Numerical Simulations of Survival Fractions

SARS-CoV-2 and other microbes within aerosol particles can be partially shielded from UV radiation. The particles refract and absorb light, and thereby reduce the UV intensity at various locations within the particle. Shielding has been demonstrated in calculations of UV intensities within spherical approximations of SARS-CoV-2 virions that are within spherical particles approximating dried-to-equilibrium respiratory fluids. The purpose of this paper is to calculate the survival fractions of virions (i.e., the fractions of virions that can infect cells) within spherical particles approximating dried respiratory fluids, and to investigate the implications of these calculations for using UV light for disinfection. The particles may be on a surface or in air. In this paper the survival fraction ( S ) of a set of virions illuminated with a UV fluence ( F , in J/m ² ) is approximated as S= exp( -kF) , where k is the UV inactivation rate constant (m ² /J). The average survival fractions ( S _p ) of all the simulated virions in a particle are calculated using the calculated decreases in fluence. The results show that virions in particles of dried respiratory fluids can have significantly larger S _p than do individual virions. For individual virions, and virions in 1-, 5-, and 9-µm particles illuminated (normal incidence) on a surface with 260-nm UV light, the S _p = 0.00005, 0.0155, 0.22 and 0.28, respectively, when kF= 10. The S _p decrease to <10 ⁻⁷ , <10 ⁻⁷ , 0.077 and 0.15, respectively, for kF =100. Calculated results also show that illuminating particles with UV beams from widely separated directions can strongly reduce the S _p . These results suggest that the size distributions and optical properties of the dried particles of virion-containing respiratory fluids are likely important in effectively designing and using UV germicidal irradiation systems for microbes in particles. The results suggest the use of reflective surfaces to increase the angles of illumination and decrease the S _p . The results suggest the need for measurements of the S _p of SARS-CoV-2 in particles having compositions and sizes relevant to the modes of disease transmission.