Traditionally, efforts around disinfection methods were motivated by reducing infections resulting from pathogenic organisms. With the COVID-19 pandemic, there has been higher demand for documented and controlled disinfection methods to treat air and surfaces responsible for virus transmission. UVC light is proven to effectively inactivate most pathogens including drug-resistant bacteria and most strains of viruses. Conventional UV disinfection based on mercury (Hg) lamps have been used for such applications but safety concerns and regulations around banning the use of mercury have driven the development of alternative UV sources for disinfection.
UVC LEDs are seen as the natural replacement for mercury lamps for several reasons: they are mercury-free, offer advantageous operating features (e.g. instantaneous on/off, ability to cycle without impacting lifetime, extraction of heat in the opposite direction of UVC light, high-performance reliability control), and low costs of maintenance. These benefits have enabled the integration of UVC LEDs into a variety of disinfection applications in water and high-touch surfaces, increasing the quality of products and features to the end-users and reducing costs to OEMs.
However, recent studies have investigated the use of krypton-chlorine (Kr-Cl) excimer lamps as an alternative method to disinfect. The shorter wavelengths of these lamps are thought to limit human health hazard due to the strong absorbance in biological material, meaning the light does not penetrate far enough into multi-cell organisms (such as humans) to create long-term damage. However, the shorter wavelengths also lead to a different disinfection process. Here we review both technologies and their usefulness in a variety of applications.
The UV spectral region ranges from 100 nm to 400 nm and is usually divided into three sub-regions based on absorption in the atmosphere and the biological action of radiation:
UVA and UVB are transmitted through the earth’s atmosphere and have limited germicidal effects. On the other hand, UVC is completely absorbed by the earth’s atmosphere and is highly disruptive for live organisms because it is strongly absorbed by proteins (principally 210 nm to 230 nm) and the nucleic acids of DNA and RNA (principally 250 nm to 280 nm). The latter wavelength range is commonly referred to as the “germicidal UVC range.”
The spectral sensitivity of a microbe is the relative ability of the microbe to absorb a photon as a function of wavelength over a range of wavelengths. Pathogens have a unique radiation absorption “fingerprint,” which means that they absorb photons differently at varying wavelengths. While different, each pathogen shows a peak absorption near 265 nm and diminishes rapidly above 280 nm in the UVB range. For the most pathogens, there is a steep drop in sensitivity below 250 nm.
Within the germicidal UVC range, 260 nm to 270 nm is seen as an ideal wavelength, with only a small drop in efficacy in damage to the nucleic acid across that wavelength range (peak DNA/RNA absorption is observed between 263 nm to 265 nm) while, outside that range, the efficacy of longer or shorter wavelengths starts to fall drastically.
For comparison, UVC in that range is two to three orders of magnitude more effective than UVA at inducing DNA damage.
The primary process of disinfection in the germicidal range is by the generation of cyclobutane pyrimidine dimers (CPD), the dominant form of UV-induced genomic damage. These dimers interrupt the replication of DNA/RNA and lead to bacterial cell death and viral inactivation.
Recently, scientists have studied the application of Krypton-Bromine and Krypton-Chlorine excimer lamps to generate primary photon emission peaks at 207 nm and 222 nm, respectively. UVC in the 207 nm to 222 nm range is commonly referred to as Far UVC. While photons emitted in this range are absorbed to some degree by the nucleic acids of DNA/RNA, the principal factor in reducing infectivity is thought to result from absorption and resultant damage to proteins. This has been demonstrated notably on adenovirus, methicillin-resistant staphylococcus aureus (MRSA), and influenza virus of type H1N1.
In water applications, the use of 222 nm does not seem likely since the UV transmissivity (UVT) in water becomes unacceptably large. UVT for filtered water is approximately constant down to 260 nm and starts to drop dramatically at shorter wavelengths due to common chemical contaminates such as nitrates. In addition, the pathogens of interest are biofilm-forming bacteria such as pseudomonas with peak absorption between 260 nm to 265 nm which exhibit lower photon absorption at shorter wavelengths.
Employing a 205 nm to 230 nm photon source to treat pathogens is far more likely to depend on the protein aspect of a pathogen, which can have substantially different absorption coefficients, rather than the proven nucleic acid DNA/RNA approach utilizing the absorption peak in the 260 nm to 270 nm wavelength range which has been shown to consistently and predictably inactivate pathogens.
UVC LEDs can be seen as attractive for a wide range of applications due to their low cost of ownership/maintenance and commercially viable price. For example, today, Crystal IS offers UVC LEDs at negotiated volumes in the price range of 10 to 15 cents per mW.
Commercially available UVC LEDs are based on semiconductors fabricated from Al1-xGaxN alloys and their emission wavelength is controlled by their alloy content, which means that UVC LEDs can also be made to emit at wavelengths below 225 nm including 222 nm. Therefore, the question of wavelength is not simply that of excimer lamp versus UVC LEDs. A higher Al mole fraction is required for UVC LEDS to emit at these shorter wavelengths and this results in lower efficiencies. For example, today, Crystal IS commercial UVC LEDs are about one order of magnitude (a factor of 10) more efficient at 265 nm than at 230 nm and two orders of magnitude less powerful; wavelengths below 225 nm are expected to suffer further degradations in efficiency and power.
Thus, for the vast majority of pathogens, the disinfection level achieved will be much higher in the germicidal range when using today’s UVC LED technology. The adenovirus is a special case where the efficacy of 222 nm radiation is as high as a factor of 10 (1 order of magnitude) compared to the germicidal range but, even here, the lower powers and shorter lifetimes of current LEDs below 230 nm will substantially increase the cost of an LED solution in this wavelength range compared to the germicidal range.
When comparing excimer lamps to UVC LEDs, there are other factors to consider carefully. The footprint of excimer lamps (typically tubes longer than 10 cm) compared to UVC LEDs (typically cuboid with square base of 0.3 cm) means that the flexibility in installation will be quite different. For early applications of excimer lamps with direct exposure on skin (only limited research has been carried so far, although results seem to indicate no permanent damage is seen). Excimer lamps will require expensive bandpass filters to remove longer wavelength (for example, the KrCl lamp for 222 nm emission has secondary emission peaks in the UVC around 258 nm and in the UVB). This will add to the cost of a product already in the thousands of dollars.
The question on preferences for a specific UVC wavelength (e.g. 222 nm vs. 265 nm) depends on the application. Excimer lamps seem to find relevance for treatment of large areas with continued human passage, however limited studies have been investigating the effects of prolonged exposure on humans.
Jose Morey, M.D., Chief Medical Innovation Officer for Liberty BioSecurity and advisor for MIT Solve and NASA iTech, said that while Far-UVC technology shows a lot of promise, it’s not quite ready for prime time just yet. “The angle and duration of the exposure are still yet to be determined,” he said. “The exposures to date have been controlled, and [there have been] mixed results depending on the type of surface, fabric, and curvatures.”
Compared to mercury lamps, the use of UVC LEDs is not only greener, it is commercially more attractive in several applications. While humans should not be directly exposed to UVC light, the small footprint of UVC LEDs and nearly point-like generation of light allows the design of targeted disinfection applications where the UVC radiation is well-controlled and unwanted exposure is eliminated to prevent health hazards. Additionally, while the wall-plug efficiency (WPE) of UVC LEDs is lower than that of mercury lamps when run continuously, the ability to turn the LEDs on/off on demand without a warm-up time translates into higher electrical efficiency over the lifetime of the application and allows for documented, predictable, and reliable disinfection.
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University of Colorado Boulder
University of Hong Kong