Negligible increase in indoor endotoxin activity by 222 nm far-UVC illumination on bioaerosols

Far-UVC irradiation (222 nm) is an emerging approach for disinfection due to its effectiveness and potentially harmless nature to humans by direct irradiation compared with other UV wavelengths. However, the indirect risk caused by 222 nm irradiance, such as changes in the inhalation hazard of irradiated bioaerosols, is poorly studied. In particular, Gram-negative bacteria (GNB) lysis releases endotoxins, which can cause respiratory diseases via inhalation. Herein, we measured the endotoxin activity of illuminated GNB bioaerosols in a chamber using the limulus amebocyte lysate (LAL) assay. A 4-fold higher endotoxin activity ratio (EAR) of the UV-irradiated cells to fresh cells was found in the GNB bioaerosols illuminated by 222 nm than 254 nm, which can be explained by the different inactivation mechanisms. Compared with 254 nm, 222 nm illumination excited the cell membrane components (e.g., proteins) more effectively, leading to the formation of more reactive oxygen species (ROS) and membrane damage, followed by exposing lipid A to release the free-endotoxins into the aqueous environment of the aerosols. The increase in EAR, membrane damage, and ROS level at 222 nm were significantly higher than at 254 nm (p < 0.05). The EAR increased linearly with the UV dose up to 50 mJ cm⁻². The EAR increase rate constant (k_EAR) was insensitive to the KCl concentration in the aerosol, but it decreased by more than 80% when the droplets became solid particles. At a 222 nm UV dose of 50 mJ cm⁻², the increase in indoor endotoxin activity due to illuminated GNB in typical indoor environments was estimated to be 3 orders of magnitude lower than the pre-existing endotoxin activity. Thus, our work suggests a negligible increase in endotoxic risk due to 222 nm indoor disinfection. © 2023 The Author(s).