Inactivation of virions in host particles in air using 222- and 254-nm UV: Dependence of shielding on particle size and UV wavelength

UV light at 222 nm inactivates SARS-CoV-2 virions at intensities appearing to be benign to skin and eyes. It can disinfect air without requiring protection of persons from UV as is needed at longer UV wavelengths. The lower risk of damage from 222-nm UV results from enhanced absorption of this UV in the outermost parts of eyes and skin. Microbes can be partially shielded from 250- to 285-nm UV when in host particles. Does the higher absorption at 222-nm increase the shielding of microbes within particles, and if so, to what extent? We use literature values to approximate the compositions and optical properties of SARS-CoV-2 virions and four types of fluids from the respiratory tract and mouth. We model virions within spherical host particles (homogeneous except for the virions), calculate the UV absorbed by the virions, and use these values to calculate survival fractions of virions in particles (Sp). We find that shielding is relatively small for particles generated in quiet breathing (typically 1.5-µm or smaller). For larger particles, shielding increases as the host-particle diameter increases. Shielding is greater at 222- than at 254-nm for all pairs of UV-fluence and particle-diameters calculated. The higher eye-safety thresholds at 222-nm allow use of larger fluences and shorter inactivation times. Because shielding increases as host-particle size increases, and because significant fractions of airborne disease-transmitting particles have diameter >1.5-µm, it appears that the sizes and optical properties of host particles must be considered in designing and verifying UV inactivation systems for such particles.