An Invisible War - UV-C vs COVID-19
The invisible pathogen COVID 19 has suddenly taken our entire planet by surprise at the beginning of 2020. The invasion was so sudden that even the World Health Organization (WHO) took time to declare this as a pandemic. At a time when the entire world is trying to arrive at plausible solutions to arrest the spread of this rampaging virus, our Lighting fraternity has joined the fight by concentrating more on studies of non-visual impacts of light.
An invisible part of the optical radiation spectrum, ultraviolet radiation has more energy than its visible counterpart due to its shorter wavelengths. The ultraviolet spectrum comprises UV-A (400 nm to 315 nm); UV-B (315 nm to 280 nm); UV-C (280 nm to 100 nm) – classification as defined by CIE. Whenever ultraviolet radiation is used for germicidal purposes, it is known as GUV (Germicidal Ultraviolet ) radiation. It has been found to be highly effective in deactivating or killing viruses, bacteria, fungi, and protozoa.
Due to complex photochemistry of UV- C component of sunrays with oxygen molecule at stratospheric level (about 16 km to 32 km above the earth surface) formation of ozone layer takes place, which in turn blocks irradiance of the actinic component of UV on human occupants. If nature had failed to stop the actinic radiance, it would have lead to physical hazards caused by photochemical and thermal reactions on human skin, cornea, and retina. So while trying to artificially generate UV-C, scientists have to monitor the wavelengths, angular subtense, exposure time, and finally physiological sensitivity to various wavelengths.
The radiant exposure which determines the dosage is dependent on the relative humidity and the kind of infectious agent it is required to tackle. Earlier studies have demonstrated the effectiveness of UV-C at 254 nm on surfaces contaminated with the Ebola virus and also during the influenza outbreak in Jordan in 1961.UV-C has proved its usefulness in the treatment of water disinfection and also air (CIE 2003) in Air Handling Units for quite some time now. Upper-air disinfection of circulating air by UV-C has been successful in limiting the spread of tuberculosis and has the recommendation of WHO.
Though serious research is underway to arrest the spread of SARS-CoV-2 with the application of UV-C for surface disinfection, its effectivity is yet to be published and proved. The major difficulty in containing the spread of SARS-CoV-2 arises from the observation recorded on the duration of its stay on fomites, which again vary on environmental factors and surface material. The successful disinfection of touch surfaces, therefore, depends on the determination of the location of the UV equipment and also proper reduction of shadow effects. Shadow effects play a deterrent role, for example, on frequently touched surfaces like door handles, latches, etc.
It has, however, been proved time and again that in actual practice UV-C radiation can deactivate pathogens by damaging their DNA. Shorter the wavelengths, more effective are the damage to pathogens. But due to complicated oxygen-ozone photochemistry, it has been observed that a significant increase in ozone production occurs at a wavelength of 200 nm or lower. That UV-C, at wavelength produced by the mercury resonance line of 253.7 nm, possesses the ability to kill bacteria was observed by researchers way back in 1877. Protein, particularly DNA was found to strongly absorb this wavelength. As UV radiation is found to trigger photochemical and thermal reactions on human skin it is always safe to keep the area being sterilized free from occupants. CIE set a threshold limit of allowable daily exposure dose to 60 J-sqm for 8-hour continuous exposure to UV-C radiation at 254 nm. Hazards associated with UV-C overexposures are transient corneal irritation(photokeratitis); conjunctival irritation (photo conjunctivitis) and skin irritation (erythema). These effects disappear within a 24-48 hour period, as per findings in CIE 187:2010 Photo carcinogenesis Risks from Germicidal Lamps. It is also mentioned in the report that the penetration in human skin is superficial only and, therefore, does not produce lasting biological damage. Since UV-C has the ability to deactivate any microbe, virus, fungus, or spore a proper dose has to be determined. The parameters which have to be considered for working out the required dose are UV-C wavelength, type of pathogen, the environment around pathogen, aerosol droplet size (with the virus inside).
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