How to fight Covid with light

The empty room was sealed. The scientists started the clock. For nearly two hours, they pumped the roughly four-by-three-meter chamber full of aerosols carrying a nasty pathogen. It was a pared-down replica of the kind of situation that many of us have tried diligently to avoid for the last couple of years.

Unable to use the Covid-19 virus in their experiment due to biosafety restrictions, the researchers had opted, instead, for Staphylococcus aureus, a bacterium that can cause a range of sometimes deadly infections. Then, with the press of a button, the scientists activated lamps they had fixed to the ceiling, sending a death ray beaming down upon the cloud of germs — a light, invisible to the human eye, with a wavelength of 222 nanometers.

Light with such a wavelength — called far-UVC, from the ultraviolet portion of the electromagnetic spectrum — is deadly for microbes, researchers are learning, but appears safe for human beings. And that offers tantalizing possibilities as the Covid-19 pandemic rolls on.

The researchers took air samples from the room every five minutes. Then they grew cultures to count how many living bacteria were present in samples retrieved before and after switch-on. These showed that the light had worked — with astonishing effectiveness. “We were pretty gobsmacked at the reduction in the pathogen,” says Ewan Eadie, a medical physicist at NHS Tayside in Dundee, Scotland.

Eadie and colleagues tested various levels of exposure to far-UVC, all within the guidelines set by the American Conference of Governmental Industrial Hygienists. The highest exposure saw pathogen levels plunge by about 98 percent in mere minutes.

The team published their results in the journal Scientific Reports in March 2022. Since then, they have tested the technology on two other pathogens — Pseudomonas aeruginosa and a bacteria-infecting virus called Phi 6 — with similar success. The researchers are confident that the lamps would also destroy the virus that causes Covid-19: Experiments in a different setting by other teams have shown that far-UVC does inactivate SARS-CoV-2, perhaps because UV light damages the genome of the virus.

Some health experts argue that disinfecting indoor spaces with light could be a game-changer as the world opens up and the memory of lockdowns — for most of us — slowly fades. SARS-CoV-2, the virus that causes Covid-19, can become airborne via the floating particles emitted when people breathe, speak, shout or sing. Virus-laden plumes can hang in the air indoors and spread disease. For more than two and a half years, scientists and government officials around the world have promoted a string of preventive measures — handwashing, vaccination, social distancing, mask-wearing, ventilation. None of them is perfect; they are often described as layers in a “Swiss cheese model” of risk reduction. Some experts now suggest we should consider adding far-UVC into the mix in certain indoor spaces.

“We are kind of like the pharmacists of light,” says Eadie, as he describes his and his colleagues’ roles in determining how far-UVC could help to decontaminate indoor spaces, as well as how other forms of UV light can sometimes be used to treat certain diseases, such as the skin condition psoriasis. The key, he says, is to set just the right wavelength and exposure.

What is far-UVC?

Light radiates in waves, and the length of those waves determines what color we will see in the light — from red (around 650nm) to violet (around 400nm). But when the waves get shorter still, the light becomes invisible and is known as ultraviolet (UV). There are different kinds of UV light, depending on its precise wavelength between 10nm and 400nm. Some forms (UVA and UVB) are present in sunlight, and exposure to them can cause skin aging and sunburn. Shorter-wavelength UVC (100-280nm) also contains portions that are dangerous, but it’s absorbed by the ozone layer, so we’re rarely exposed to it. Far-UVC — generally considered to be between 200 and 230nm — is considered a less dangerous form of UVC.

Past work using UVC at 254nm has been shown to slow the spread of airborne pathogens. William F. Wells, a researcher at Harvard University, published multiple studies on this effect back in the 1930s and 1940s and even used the lamps to clean the air in the upper airspaces of school classrooms, dramatically reducing the spread of measles.

But though Wells and other early pioneers of this technology could shine their 254nm lamps aloft to clean the air floating in the upper portion of rooms, or use the light inside vents or ducts, they couldn’t actually shine them downward, directly onto the occupants of rooms, without ill effect. That’s because although 254nm inactivates pathogens, it also causes skin and eye damage at certain doses.

Today, UV light around 260nm is commonly used for disinfection but generally only in similarly restrictive contexts — such as in upper-room germicidal units, sometimes fixed to the walls of a hospital room, or in lamps used to clean surfaces. In 2020, Transport for London, which operates the London Underground, installed UV devices to clean the handrails on escalators in its stations. UV lamps are also used to kill pathogens in wastewater and to disinfect laboratory equipment such as safety goggles. The common theme is that all of these applications avoid shining the light directly onto human skin or eyes.

Recent experiments using mice and human participants suggest that 222nm, far-UVC, is much safer, which is partly why Eadie and colleagues were keen to use it in their study. A clinical trial currently underway in Canada will also use far-UVC lamps set at 207 to 222 nm to find out whether they reduce the transmission of infections including influenza and Covid-19 in long-term care facilities for elderly people. Researchers are to install the lights in communal areas such as corridors and dining rooms, but in some locations they will install placebo lights that look exactly the same but don’t emit far-UVC. The objective is to find out whether residents living in settings with the far-UVC lamps experience a reduction in Covid-19, flu and various respiratory illnesses, or not.

It’s important to continue studying far-UVC in order to ensure that it really is safe to shine directly onto people of various ages over long periods, says Amanda Weaver, an environmental epidemiologist and PhD candidate at the University of California, Berkeley. She praises the experiment by Eadie and colleagues but suggests that some settings may wish to take advantage of the other benefits that come from cleaning the air with ventilation systems that use HEPA filters; these also remove allergens and pollutants such as dust along with viruses and bacteria.

“Filters are more cost-effective and you can sort of get at these other, long-term, constant, chronic exposures,” she says. Still, the possibility that the lamps could zap airborne pathogens is “amazing,” she adds, and in places where infection control is particularly important, such as hospitals, far-UVC could come into its own.

Matt Butler, consultant physician at Cambridge University Hospitals in the UK, agrees. “From an infection-control perspective, it seems that it’s a no-brainer, really,” he says. Butler and colleagues are currently running a study evaluating the efficacy of HEPA filters on a ward at Addenbrooke’s Hospital in Cambridge. Existing ventilation systems often can’t reach required rates of air changeover, he says. And there is discussion among healthcare professionals over how many air changes per hour are required to make a space safe anyway (the transmission risk depends on highly variable factors such as the density of human crowding in a room and the number of infectious people present). Butler says he would be keen to use far-UVC lamps in future work to find out if this further reduced infections.

Far-UVC is “something that should work together with ventilation, not instead of ventilation but to support ventilation,” says Lidia Morawska, a physicist at Queensland University of Technology. In August 2021, she and colleagues published a paper suggesting that current ventilation standards alone are not sufficient to properly control Covid-19 transmission.

Morawska raises the possibility that far-UVC, when shone directly between people indoors, might even reduce short-range airborne transmission — in other words, inactivate the virus so quickly that an airborne particle loaded with virus could become safe in the short time that it takes to travel, say, a meter or so from an infected person who is speaking or coughing to an uninfected individual nearby.

“It’s possibly one of the only technologies, other than a mask, that could stop that,” says Eadie. Supplementing the preventive actions and changes in behavior that keep people safer —since these have become politicized and divisive during the pandemic — could be very powerful, says Weaver.

Eadie, however, argues that we need more studies demonstrating the effectiveness of the lamps before we start using far-UVC devices for infection control. He and his colleagues hope to investigate that in the next phase of their research — by modeling the possible impact of far-UVC on disease transmission.

“I think we need some more real-world studies,” he says, “before we were to say, ‘Yes, let’s go ahead and let’s start using these lamps.’”