Ultraviolet light can kill microscopic creatures like bacteria and viruses by destroying the molecular bonds in their genetic material. But UV light also damages human DNA, causing eye and skin damage and increasing our risk of cancer. It turns out, though, that there’s a loophole: a specific wavelength of UV light that’s safe for people but capable of killing coronaviruses, both on surfaces and in the air.
Since the start of the pandemic, you’ve probably spent a lot of time on sites like Amazon, which means you’ve probably seen countless advertisements for devices that promise to disinfect stuff in your home using ultraviolet light—specifically, a range of short UV wavelengths called UVC (that’s light with a wavelength between 200 and 280 nanometers). Earth’s atmosphere thankfully blocks UVC from reaching us, though we still have to worry about longer-wavelength UV light, which damages our skin and eyes. Disinfecting using UVC light works: It’s been employed by hospitals and medical facilities for decades to clean personal protective gear, tools, equipment, rooms, and even water. Depending on how it’s deployed, UVC can be especially good at getting into tiny nooks and crannies that are otherwise very hard to sanitize.
There are some issues with using UVC light to disinfect, however. You need a certain intensity to quickly kill microbes. With the low-powered devices being sold to consumers, you’d need to leave a device like a smartphone locked inside a glowing box for a very long time for it to be effectively sanitized—and that’s assuming it’s properly exposed to UVC light on all sides. The other issue is a little more concerning: germicidal UVC light with a wavelength of 254-nanometers is considered to be carcinogenic, causing DNA mutations in our skin and eyes. Hospitals take extreme measures to use it safely, where as the average consumer undoubtedly wouldn’t.
As a result, experts like Jim Malley, professor of civil and environmental engineering at the University of New Hampshire and the founding president of the International Ultraviolet Association, instead recommend social distancing, proper hand washing, masks, and even disinfectant wipes to minimize one’s exposure to covid-19—at least for the time being. Two recent studies, one conducted at the Vagelos College of Physicians and Surgeons at Columbia University Irving Medical Center and one at Hiroshima University in Japan, have found that a very specific wavelength of UVC light—222 nanometers—is unable to penetrate the eye’s tear layer or the dead-cell layer of skin, preventing it from reaching and damaging living cells in the human body.
The Columbia University study, published in June 2020, found that even low exposure to 222-nanometer UVC light was able to kill two common coronaviruses (which cause seasonal colds) that had been aerosolized. Exposure to this far-UVC light for about eight minutes killed 90% of airborne coronaviruses; it took about 25 minutes of exposure for 99.9% of the viruses to be killed. Last week, another study from Hiroshima University confirmed that far-UVC light was effective at killing SARS-CoV-2 (the virus that causes covid-19), although the tests were done in a more controlled setting. The researchers exposed a viral culture on a polystyrene plate to a far-UVC lamp at a distance of 24 centimeters, which killed 99.7% of the virus in just 30 seconds.
Despite the success tests, the Japanese research team believes that more studies need to be conducted on far-UVC light using real-world surfaces and environments before it’s adopted as an effective tool for disinfection. Nevertheless, it’s an exciting discovery, as far-UVC light could theoretically be safely deployed in public places even when people are present. It would also make UVC sterilization devices targeted at consumers safer and cheaper to produce, if failsafes (e.g. locking mechanisms) are no longer needed.