Far-UVC: The impact of optical filters on real-world deployment

In 2015, a study showed that Krypton-Chloride (KrCl) excimer lamps could induce erythema and basal layer DNA damage in human skin. Later studies found that filtering out longer wavelength emissions from these lamps resulted in no acute skin effects. However, there is a limited understanding of how much to reduce unwanted emissions and which wavelengths are important. Accurate spectral irradiance data is therefore crucial for safety, as variance in optical filtering significantly affects the weighted irradiance of a lamp. To simplify the risk assessment process for Far-UVC lamps, we highlight the usefulness of the lamp exposure limit (H_LEL) and present this in the context of spectral emission data for 14 commercially available Far-UVC lamps. Our results demonstrate that relying solely on a radiometric measurement and a single-wavelength exposure limit at 222 nm could lead to over-exposure. The H_LEL is a practical metric which can be utilized to determine the exposure time before reaching the exposure limit. It can also be used in the determination of the minimum ceiling height for compliance with standards like UL 8802. Manufacturers are urged to provide H_LEL for their products; installers should adhere to H_LEL; and standards and regulatory bodies should insist on this information in new guidance.