Viruses such as SARS-CoV-2 can be partially shielded from UV radiation when in particles generated by sneezing or coughing: Numerical simulations

UV radiation can inactivate viruses such as SARS-CoV-2. However, designing effective UV germicidal irradiation (UVGI) systems can be difficult because the effects of dried respiratory droplets and other fomites on UV light intensities are poorly understood. Numerical modeling of UV intensities inside virus-containing particles on surfaces can increase understanding of these possible reductions in UV intensity. We model UV intensities within spherical approximations of virions randomly positioned within spherical particles. The model virions and dried particles have sizes and optical properties to approximate SARS-CoV-2 and dried particles formed from respiratory droplets, respectively. Wavelengths used are 260 nm (germicidal UVC) and 302 nm (solar UVB). In 5- and 9-μm diameter particles on a surface, illuminated by 260-nm UV light from a direction perpendicular to the surface, 10% and 18% (respectively) of simulated virions are exposed to intensities less than 1/100^thof intensities in individually exposed virions (i.e., they are partially shielded). Even for 302-nm light, where the absorption is small, 11% of virions in 9-µm particles have exposures 1/100^ththose of individually exposed virions. Calculated results show that shielding of virions in a particle can be strongly reduced by illuminating a particle either from multiple widely separated incident directions, or by illuminating a particle rotating in air (because of turbulence, Brownian diffusion, etc.) for a time sufficient to rotate through all orientations with respect to the UV illumination. Because highly UV-reflective paints and surfaces can increase the angular ranges of illumination, they appear likely to be useful for reducing shielding of virions.