Nonuniform UV-C dose across N95 facepieces can cause 2.9-log variation in SARS-CoV-2 inactivation

During public health crises like the COVID-19 pandemic, ultraviolet-C (UV-C) decontamination of N95 respirators for emergency reuse has been implemented to mitigate shortages. However, decontamination efficacy across N95s is poorly understood, due to the dependence on received UV-C dose, which varies across the complex three-dimensional N95 shape. Robust quantification of UV-C dose across N95 facepieces presents challenges, as few UV-C measurement tools have sufficient 1) small, flexible form factor, and 2) angular response. To address this gap, we combine optical modeling and quantitative photochromic indicator (PCI) dosimetry with viral inactivation assays to generate high-resolution maps of “on-N95” UV-C dose and concomitant SARS-CoV-2 viral inactivation across N95 facepieces within a commercial decontamination chamber. Using modeling to rapidly identify on-N95 locations of interest, in-situ measurements report a 17.4 ± 5.0-fold dose difference across N95 facepieces in the chamber, yielding 2.9 ± 0.2-log variation in SARS-CoV-2 inactivation. UV-C dose at several on-N95 locations was lower than the lowest-dose locations on the chamber floor, highlighting the importance of on-N95 dose validation. Overall, we couple optical simulation with in-situ PCI dosimetry to relate UV-C dose and viral inactivation at specific on-N95 locations, informing the design of safe and effective decontamination protocols.