Indoor Air Quality Implications of Germicidal 222 nm Light

A known strategy for mitigating the indoor transmission of airborne pathogens, including the SARS-CoV-2 virus, is irradiation by germicidal UV light (GUV). A particularly promising approach is 222 nm light from KrCl excimer lamps (GUV222); this inactivates airborne pathogens, but is thought to be far less harmful to human skin and eyes than longer-wavelength GUV (e.g., 254 nm). However, the potential for GUV222 to affect the composition of indoor air has received little experimental study. Here, we conduct a series of controlled laboratory experiments, carried out in a 150 L Teflon chamber, to examine formation of oxidants and other secondary species by GUV222. We show that GUV222 generates ozone (O3) and hydroxyl radicals (OH), both of which can react with volatile organic compounds to form oxidized volatile organic compounds and secondary organic aerosol particles. Results are consistent with predictions from a simple box model based on known photochemistry. We use this experimentally-validated model to simulate the effect of GUV222 irradiation under more realistic indoor air scenarios, spanning a range of light and ventilation conditions. We demonstrate that under some conditions, GUV222 irradiation can lead to levels of O3, OH, and secondary organic products that are substantially elevated relative to normal indoor conditions, especially when ventilation is low and GUV222 intensity is high. Thus, GUV222 should be used at the lowest intensities possible and in concert with ventilation, decreasing levels of airborne pathogens while mitigating the formation of air pollutants in indoor environments.