From Science Direct, Nov 2020
- Light-based technologies currently are cost-effective and widely available in market.•
- Photons can be used to inactivate SARS-CoV-2 in air, liquids and on surfaces.•
- Phototherapy can be used as an adjuvant to control virus infection and to modulate host immune system.•
- Light-based solutions can significantly contribute to mitigate the impacts of COVID-19 pandemic
Ultraviolet Germicidal Irradiation (UVGI)
UV-C lamps have long been used in hospital and industrial settings for decontamination purposes. In the context of a mitigation approach to infection spreading, UV-C can be particularly helpful in the inactivation of virus-containing aerosols and surfaces.
Air disinfection via upper-room germicidal UV-C light fixtures may be able to reduce viral transmission via the airborne route. Accordingly, an observational study during the 1957 influenza pandemic reported that patients housed in hospital wards with upper-room UV-C had an infection rate of 1.9%, compared to an infection rate of 18.9% among patients housed in wards without UV-C . However, it is important to note that the germicidal effect of UV-C seems to be strongly dependent on the relative humidity of the air, with UV-C effectiveness against influenza virus decreasing with increasing relative humidity .
The potential of viral spreading via contaminated surfaces depends on the ability of the virus to maintain infectivity in the environment, which in turn is influenced by several biological, physical, and chemical factors, including the type of virus, temperature, relative humidity, and type of surface . Importantly, single-stranded nucleic acid (ssRNA and ssDNA) viruses were more susceptible to UV inactivation than viruses with double-stranded nucleic acid (dsRNA and dsDNA). Also, the UV dose necessary to achieve the same level of virus inactivation at 85% relative humidity (RH) was higher than that at 55% RH .
In a recent study, Fischer et al. showed that UV-C light can inactivate more than 99.9% of SARS-CoV-2 viral particles deposited over the filtering material of N95 masks and stainless steel surface . As expected, inactivation kinetics over stainless steel was much faster (i.e., more than 99.9% for (0.33 J/cm2). However, after sufficient exposure (1.98 J/cm2) UV-C could promote germicidal efficacy levels that were similar to those promoted by ethanol, dry heat or vaporized hydrogen peroxide. Older studies have hypothesized that the necessary dose to inactivate 90% of viruses present in N95 filtering facepiece respirator (FFR) material would be about 30 times higher than over the surface of non-porous materials . This was an interesting estimation, but we should keep in mind that UV-C emission spectrum and irradiance of different UV-C equipment as well as material composition are widely variable . Therefore, such estimatives cannot be used as a robust procedure and experimental demonstrations must always be presented. Indeed, a recent in silico study demonstrated that for effective and fast decontamination one should consider the FFR shape besides the optical properties of the FFR model, which has to be determined at the UV-C specific wavelength . Even though UV does not seem to affect the filtrating capacity of FFRs, it is important to note that high UV-C doses can lead to reduced tensile strength of its materials [42,43].
The combination of multiple light wavelengths has been explored for cosmetic, environmental (water disinfection) and clinical (microbial catheter disinfection) applications. However, the precise photobiological mechanism of action and the experimental workflow to develop a marketable application is still missing [44,45].
It must be remarked that UV-C light at 254 nm is harmful to the eyes and skin and, therefore, it is recommended to use it in setups that avoid direct human exposure. Although, far-UV-C (207–222 nm) has been proposed as a disinfection technology that seems to be safer to human exposure . This has been claimed because far-UV-C range is strongly absorbed by amino acid residues and, therefore, is further blocked by the acellular stratum corneum of the skin and the cornea of the eye, leading to lower levels of UV-C light reaching the cellular layers of eyes and skin. However, as far as our knowledge goes, robust studies showing the actual safety of far-UV-C towards animal tissues in short and long terms have not been strongly established and degradation of proteins can also lead to serious eye and skin damages. Thus, we can only recommend UV-C application to inanimate objects. Additionally, far-UV-C technology is not broadly available in the market yet and the cost is far higher than common LP-Hg lamps. On the other hand, UV-C LED technology is limited to very compact applications. The shortest wavelengths available are around 255 nm, with the price per Watt being up to 1000 times higher than that of LP-Hg lamps, while displaying an energy efficiency (< 1%) far lower than that of LP-Hg lamps (25–40%) at 254 nm.