Could ultraviolet light kill the coronavirus in our buildings and homes?

Heliotherapy in the 20s
© Ray Therapy in the 30s/ Fox Photos/Getty Images

Yes, but be careful.

In a recent post on the future of hotels after the coronavirus, I proposed with tongue in cheek that designers should "stick a big ultraviolet light in the middle of the room that runs when guests are out of the suite and it could be continuously disinfected." In fact, it may not be such a silly idea.

We are exposed to ultraviolet light from the sun all the time; it's why we wear sunscreen. But most of what gets through the atmosphere and down to us is the longer wave UV-A like you get from black lights, and UV-B, which causes skin damage and can lead to skin cancer. Doctors and scientists thought small doses were good for us, and between the World Wars, "heliotherapy", or treatment with UV, was a popular thing; but as Sir Henry Gauvain, Britain’s heliotherapist noted in 1922, you can have too much of a good thing. “Like a good champagne. It invigorates and stimulates; indulged in to excess, it intoxicates and poisons.”

WHO on ultraviolet lightWorld Health Organization/Public Domain

Doctors and scientists don't suggest that anymore, and the World Health Organization is quite explicit that using UV on people is dangerous.

But UV-A and UV-B are not very useful for disinfecting; it works, but it does not have enough energy. UV-C is another story, and has been used for a hundred years to treat water and air. Fortunately, it doesn't get through the atmosphere or we might not be around. Allison Bailes of Energy Vanguard explains:

Bacteria, fungi, and viruses can be killed by exposure to UVC. The UVC photons are energetic enough to damage the DNA and RNA of microorganisms, destroying their ability to replicate...And yes, this does work.

But it works on humans, too, which is why the lamps are buried in ductwork and not often put out in the open, and it is not that effective at killing the germs or viruses in the air; Bailes explains:

The main reason for that is that the UV lamp won't have the intensity needed to provide a high enough dose to kill the various germs. The air moves through most air handlers and duct systems at 500 to 900 feet per minute. The faster it moves, the more power you need in your UV lamps to zap the speedy little germs. So for UV lamps integrated into the HVAC system, the main benefit is to keep stuff from growing on the surfaces, especially the coil and drain pan. It's not going to kill much coronavirus or other baddies that get pulled into the ducts.

A bus is disinfected with ultraviolet rays© A bus is disinfected with ultraviolet rays in Shanghai/ HECTOR RETAMAL/AFP via Getty Images

But UVC lamps do work well on things that stay put; in China they are disinfecting entire buses, which seems like a very good idea. An industry group, The International Ultraviolet Association, says UVC will help prevent COVID-19 "based on existing evidence":

All bacteria and viruses tested to date (many hundreds over the years, including other coronaviruses) respond to UV disinfection. Some organisms are more susceptible to UVC disinfection than others, but all tested so far do respond at the appropriate doses...UV light, specifically between 200-280nm (UVC or the germicidal range), inactivates (aka, ‘kills’) at least two other coronaviruses that are near-relatives of the COVID-19 virus: 1) SARS-CoV-1 and 2) MERS-CoV.

VioLED uvC LED© Seoul Semiconductor VioLED

Most of the devices emitting UVC are clunky and dangerous mercury lamps, but LEDs have been developed, and according to a recent study, they are very effective at killing the SARS-CoV-2 virus (the official name for the coronavirus) that causes COVID-19. A Korean company claims that its UV LEDs (VioLEDs) provide "99.9% sterilization of coronavirus (COVID-19) in 30 seconds." They were in fact designed for automotive use, to sterilize the interiors of unoccupied vehicles. These LEDs are still under development and, according to researcher Christian Zollner, are not ready for prime time: "Many technological advances are needed for the UV LED to reach its potential in terms of efficiency, cost, reliability and lifetime."

But the company says otherwise in a press release; the CEO announced that "to reduce the spread of COVID-19, we decided to temporarily launch the product by drastically reducing the process of merchandising such as molds and customer delivery."

These VioLED car lights are designed to "safely detect for absence of occupants before activating lamps." It's not a stretch to imagine these fixtures being installed over toilets in public washrooms, bathrooms in homes, kitchens or really, just about everywhere (including hotel rooms), all hooked up to motion detectors to ensure that nobody is getting fried when they are running.

Far-UVC might be the answer

Even shorter wavelength and higher in energy than UV-C is what's called Far-UVC, which doesn't cause as much damage to people. Researchers at the Center for Radiological Research at Columbia University led by David Brenner are quoted:

Several years ago, Brenner and his colleagues hypothesized that far-UVC could kill microbes without damaging healthy tissue. “Far-UVC light has a very limited range and cannot penetrate through the outer dead-cell layer of human skin or the tear layer in the eye, so it’s not a human health hazard. But because viruses and bacteria are much smaller than human cells, far-UVC light can reach their DNA and kill them,” Brenner said.

According to the study, Far-UVC light: A new tool to control the spread of airborne-mediated microbial diseases, released in 2018 before this crisis, they found that it did indeed kill viruses:

We show for the first time that far-UVC efficiently inactivates airborne aerosolized viruses, with a very low dose of 2 mJ/cm2 of 222-nm light inactivating >95% of aerosolized H1N1 influenza virus. Continuous very low dose-rate far-UVC light in indoor public locations is a promising, safe and inexpensive tool to reduce the spread of airborne-mediated microbial diseases.

If this is truly the case, and if we get affordable Far-UVC LEDs, they might be installed everywhere, from subway cars to shopping malls to hotels. I personally hope that there is also some Far-UVC sunscreen.

Could ultraviolet light kill the coronavirus in our buildings and homes?
Yes, but be careful.

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