In late 2020 the new coronavirus, SARS-CoV-2, began affecting patients worldwide. As of 31 December 2023, the WHO COVID-19 dashboard totaled about 770 million reported cases of the disease.
Due to the massive worldwide effect of COVID-19, research is growing in the area of rapid, safe limitation of airborne and surface viral transmissions. One usage is a type of ultraviolet (UV) light device for disinfecting surfaces and objects, such as the cabinets used by hospital staff upon arrival at work to sanitize their mobile phones and small personal items.
Reproduced from Atmosphere, Volume 13, Number 1411, Sep 2022 (Creative Commons 4.0)
UV radiation is defined as the portion of the electromagnetic spectrum from 100 to 400 nm, and it is further divided into three major photobiological bands (A, B, and C). A subsection of the UV‑C band from 200 to 230 nm (“far” UV‑C) shows potential for inactivation of human pathogens. Conventional UV‑C germicidal applications typically range from 250 to 280 nm; however, the commonly used wavelength of 254 nm poses a health hazard to skin and eyes.
Far UV‑C light from 207 to 222 nm kills airborne coronavirus effectively and doesn’t appear to harm exposed mammalian skin. This wavelength range demonstrates limited penetration of biological tissues, such as the stratum corneum (the outermost layer of human skin), compared to penetration of 254 nm light.
In July 2023 a team of researchers demonstrated a filter-free, harmless, single-wavelength far UV‑C germicidal light source at 207 nm that inactivated airborne pathogens, including coronavirus, adenovirus, and vaccinia virus. The UV‑C module operates by drawing in air and irradiating airborne aerosols, then the treated air undergoes a second round of irradiation, resulting in a high level of disinfection. Their technology is proposed for high-traffic public spaces such as hospitals and airports, which may have high levels of microorganisms.
The scientists used human virus strains amplified in Vero cells or HeLa cells in the study preparations. Known concentrations of virus were exposed to UV‑C using an excimer lamp source at wavelengths of 207, 222, and 254 nm for varied time durations. The outcome demonstrated an increased disinfection rate over time for all three viruses, with the human coronavirus exhibiting 82% inactivation within one second, 96% inactivation at 30 seconds, and 99% at 10 minutes.
The researchers also conducted safety evaluations of these wavelengths in enclosed environments using human keratinocyte and human corneal epithelial cells, with a safety tolerance of 20% cell loss. The two higher wavelengths resulted in greater cell mortality as well as morphology and cultural deterioration, but the 207 nm irradiation did not significantly impact morphology, and apoptosis and cell death were absent by Western blotting analysis. The 254 nm wavelength induced cytotoxicity, and severity increased with energy.
In general, the compared wavelengths showed 207 nm as the best germicidal wavelength for the vaccinia virus. Previous studies suggest that viruses with larger genomes are more susceptible to irradiation, as they provide more targets for photochemical damage to occur rapidly. However, the adenovirus and coronavirus samples were effectively inactivated at 99% or higher by the 207 nm lamp with no evidence of photoirradiation damage to the human cells.
David Brenner, director of Columbia University’s Center for Radiological Research and coauthor of a 2022 paper on far UV‑C and viral diseases, notes the strong evidence in a discussion with IEEE Spectrum: “If the technology had been well developed five years earlier and far UV‑C lights had been widely installed, potentially the COVID crisis could’ve been unlike what it turned out to be.”
IEEE Spectrum identifies some challenges with far UV‑C, including the expense of krypton excimer lamps, which are the main type of far UV‑C source studied to date. The lamps cost thousands of dollars, have relatively short lifetimes, and a few other limiting issues such as use of caustic gases and overall bulk. However, if these challenges can be overcome, Brenner likes the devices’ potential ease of use: “Masks, vaccination, and social distancing all involve people having to make decisions. Far UV‑C is a passive device; it’s just up there in the ceiling…Far UV‑C light could maybe prevent the next pandemic or the next yearly influenza epidemic.”