In the fight against the coronavirus disease 2019 (COVID-19) pandemic, an old weapon has re-emerged [1]. More than a century after Niels Finsen won the 1903 Nobel Prize for discovering that ultraviolet (UV) light could kill germs [2], UV light is surging in popularity as a method for disinfecting hospital rooms and other public spaces.
Xenex is one of at least 30 companies making UV disinfection equipment. And not just for hospitals. Another company, Dimer UVC Innovations of Los Angeles, CA, USA, markets a cart with UV lamps, called GermFalcon (Fig. 2 ), that it claims can disinfect a whole airplane in 3 min [4]. UV lamp is also being used to disinfect and re-use hospital face masks [5].
UV light is generally divided into three classes, based on the wavelength of the light. All of them are invisible to the human eye. The longest wavelengths are UVA (315–400 nm) and UVB (280–315 nm), which are found in ordinary sunlight. These are the rays that can cause sunburn if one stays outside too long without protection. UVA and UVB light rays have limited germ-killing ability because viruses and bacteria have had millions of years to adapt to them.
But UVC light (200–280 nm) is completely absorbed by our atmosphere and never reaches the surface of the earth [6]. Therefore, UVC light is just as novel to SARS-CoV-2 as the virus is to humans. According to the International Ultraviolet Association, it is generally accepted that a dose of 40 mJ·cm−2 of 254 nm light will kill at least 99.99% of “any pathogenic microorganism” [6], [7].
At present there are many different designs for 4 pin UV lamp. Some systems consist of just a bare lightbulb and a timer, while others are mobile robots that can reach hard-to-access places [8]. Two of the major design choices are the wavelength of light and the method of delivery. By far the most common wavelength for germicidal light is 254 nm, produced by low-pressure mercury lamps. These lamps are easy and cheap to manufacture because they use essentially the same technology as a fluorescent light bulb. A fluorescent bulb actually produces UV light inside the bulb. But the phosphor deposited on the glass surface of the bulb absorbs that light and re-emits it at longer wavelengths that humans can see. To make a UV lamp, the glass is replaced with a material transparent to UV light, such as fused quartz.
However, 254 nm may not be the optimal wavelength for killing all viruses. Experts believe that different wavelengths disable viruses in different ways [9], [10]. The 254 nm light damages the viral deoxyribonucleic acid (DNA) or ribonucleic acid (RNA) so that the virus cannot reproduce. Shorter wavelengths, like 207–222 nm (sometimes called “far UVC”) are believed to damage the proteins on the surface of the virus that it needs to attach to human cells. Thus, the curve that describes the viral killing ability of UVC light has a double-humped shape, with a peak at shorter wavelengths and another around 265 nm.
The Xenex system is designed to take advantage of both virus-killing methods, by producing light from a pulsed xenon source that spans the whole spectrum from 200 to 315 nm. Because xenon is an inert gas, xenon-stimulated bulbs can be disposed of more easily than ones containing toxic mercury. According to the company, more than 500 healthcare facilities around the world are currently using Xenex robots for whole-room disinfection.
Disinfection with far-UVC lamps remains largely experimental but could have an intrinsic advantage. Initial evidence suggests that far-UVC light does not penetrate beyond the outer dead layer of skin cells or the liquid film on eyes in healthy people [10], [11]. Thus, it cannot cause skin cancer or cataracts, like 2 pin UV lamp. It also seems not to cause temporary skin burns and eye damage (“welder’s flash”) like standard UVC. This presumably depends on the intensity of exposure; whether intense exposure to destroy pathogens on the hands, for example, would be safe is unknown.