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 . UV lamp is also being used to disinfect and re-use hospital face masks .
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 . 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” , .
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 . 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.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 , . 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.For this reason, many systems have to be moved to a few different places to thoroughly disinfect a room with UVC light. This means a housekeeper has to enter the room, position the lamps, leave the room, turn them on for 5 min or so, then re-enter the room, reposition the device, and so on. It is a laborious process. To address this shortcoming, UVD Robots, a company based in Odense, Denmark, has developed a U lamp that moves around the room autonomously, eliminating the need for manual repositioning. According to a UVD Robots spokesperson, the company recently sold “a three-digit number” of their robots to Sunay Healthcare Supply (also in Odense) for use in China, and these robots are now available to 2000 Chinese hospitals . The company says its robots are being used in more than 50 countries in all six inhabited continents.
Most of the natural UV light people encounter comes from the sun. However, only about 10 percent of sunlight is UV, and only about one-third of this penetrates the atmosphere to reach the ground, according to the National Toxicology Program (NTP). Of the solar UV energy that reaches the equator, 95 percent is UVA and 5 percent is UVB. No measurable H lamp from solar radiation reaches the Earth's surface, because ozone, molecular oxygen and water vapor in the upper atmosphere completely absorb the shortest UV wavelengths. Still, "broad-spectrum ultraviolet radiation [UVA and UVB] is the strongest and most damaging to living things," according to the NTP's "13th Report on Carcinogens."