Ultraviolet revolution: Could “far-UV” light provide widespread, safe disinfection of airborne viruses?

A short-wavelength ultraviolet light technology, beyond the decades-old mercury lamp, may be the most promising breed of UV yet studied for killing airborne coronavirus and other viruses and bacteria. Two studies—one now under peer review, and one reporting preliminary results from an ongoing experiment—bolster the case that this UV is safe for human eyes and skin.

If the initial promise of so-called “far-UV” pans out, prepare to hear the words “krypton-chlorine excimer lamp” more widely later this year and into 2021. That’s because KrCl lamps produce 222-nanometer UV efficiently enough that they might soon be broadly deployed for disinfection of hospitals, doctors offices, grocery stores, office buildings, shopping centers, airports, trains, airplanes, public transit, elevators or potentially anywhere that people gather in indoor spaces.

Or not. All safety and efficacy findings for far-UV are, for all their promise, still preliminary. And far-UV ultimately stands or falls on its own terms, apart from the well-established UV-C technologies—whose effectiveness against viruses and bacteria are well known, although direct exposure to longer-wavelength UV-C is also known to be harmful to human skin and eyes.

That difference only increases the possible leap forward that far-UV light fixtures could represent with respect to taming the coronavirus pandemic, says David Brenner, director of Columbia University’s Center for Radiological Research.

Brenner and collaborators have been studying far-UV’s sterilizing effects against microbes and viruses for more than a decade, he said in an interview this week.

“All the research we’ve been doing over the years was obviously not about COVID-19 or SARS-CoV-2,” he said. “We were focused on influenza, which in some sense is the same story: airborne transmission. So we had anticipated that by the next flu season, these things would start to be installed.”

According to a new study produced by Brenner’s group that is currently undergoing peer review, a lamp emitting far-UV light, bathing a room in 222-nm ultraviolet light at levels beneath the current industry threshold limit, would inactivate 99.9 percent of coronaviruses in the air in under 25 minutes. (The group’s work examined far-UV’s effect on other airborne coronaviruses, which they say will very likely have the same response as the SARS-CoV-2 novel coronavirus. Nevertheless, Brenner said, they’re now extending their work to include the effect of far-UV on the novel coronavirus, too.)

Far-UV light has a shorter wavelength than traditional UV-C, which means it carries more energy per photon. That effectively also translates to a shorter distance traveled through human and animal tissue — according to Brenner’s colleague Manuela Buonanno, associate research scientist at the Center for Radiological Research and specialist in far-UV’s effects on biological systems.

Buonanno said that the proteins in tissue cut the intensity of a 222-nm far-UV beam in half after traveling just 0.3 micrometers. Compare that to traditional UV-C light (with a 254-nm wavelength), whose tissue penetration depth is ten times longer, at 3 µm.

While far-UV won’t make it through even a fraction of the 5-to-10-micrometer-thick layer of dead skin on a person’s body, at least some UV-C light could make it through to live skin cells, where that UV-C might either kill them or render them cancerous.

Apparatus for exposing airborne virus to a far-UVC lamp
Photo: Manuela Buonnano/Columbia University

Apparatus for exposing airborne virus to a far-UVC lamp.

The same goes for the eyes; direct exposure to regular UV-C can cause eye damage. However, Buonanno said, “We have an ongoing study in which mice are exposed to [far-UV] light five days a week, eight hours a day.” The 96 (hairless) mice in the study are given regular exams to discover if their skin has reacted to the radiation or if their eyes have been adversely affected.

Brenner shared a recent preliminary report after 43 weeks of far-UV exposure for the mice. “We see no difference between any of the [animals in the] exposure and the control [groups],” the interim report says. The research team, he said, will continue their mouse study for at least another few months—until they’ve collected data based on 60 weeks of daily far-UV exposure to the mice.

Thus far, the mouse finding is consistent with most human safety studies of far-UV, Brenner said, such as a letter in a recent issue of the journal Photodermatology, Photoimmunology & Photomedicine. In this study, Scottish scientists controverted a previous finding from 2015 which claimed that far-UV light can cause harm to human skin.

The new study performed computer simulations which found that the harm the 2015 paper discovered was most likely caused by longer-wavelength (UV-C) light that was secondarily generated by the far-UV lamp, casting the old finding into doubt.

“What needs to be done with 222-nm light is to have a filter,” Brenner said. “The companies that I’m aware of all do that. Which basically blocks out any of the 240- and 250-nm-wavelength light.”

Buonanno said that threshold levels of longer-wavelength UV light have yet to be established. If a far-UV lamp can filter out 99.9 percent of longer and more damaging UV-C rays, will that be enough? Or could the 0.1 percent of the other UV light that gets through still cause harm?

“We do not know yet,” Buonanno said. “We are planning a study to measure safety after exposure to other wavelengths—say from 225 to 255 nm—to address this question in a more systematic way.”

Brenner said that while KrCl eximer lamps are currently the only game in town for generating far-UV light, he remains hopeful that far-UV LEDs might soon be developed. LEDs have become available for wavelengths as short as 300 nm and lately even 250 nm light. But, he said, UV LEDs that can efficiently generate 230- or even 220-nm light have not yet been invented.

The main reason, according to Brenner: “Up till now there hasn’t been a huge demand for 230-nm LEDs. There hasn’t been that much work on it. But I’m very much hoping that folks—maybe your readers—will take this on board.”

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