Ultraviolet light — specifically UVC — is great for killing pathogens. It is used as a heavy-duty cleaning solution in hospitals. It's also used to clean drinking water, purify air and disinfect surfaces. But that's usually far from direct human contact.

The challenge is making it safe indoors, in spaces that are used by lots of people. That includes schools, gyms, airports and large offices. Researchers have been working on the idea for years.

It's not the same as the idea proposed early on in the COVID-19 pandemic by former US President Donald Trump.

Trump asked whether it was possible to inject people with disinfectants and expose our bodies directly to UV light. The science says that is not a great idea. UV can be dangerous. It can have serious health implications if we're exposed to too much, directly.

A new study, however, suggests researchers may have found a way to use a type of UVC to kill COVID-like viruses and bacterial in a way that is safe for people.

Three types of UV light

We're exposed to ultraviolet light from the sun every day. UVC is one of three types of ultraviolet light.

The three types of ultraviolet light are UVA, UVB and UVC. All three types can damage the skin and eyes.

UVA makes up most of the ultraviolet radiation in the Earth's atmosphere. It reaches the Earth's surface and can, for instance, cause our skin to develop anything from wrinkles to cancer. It can cause cataracts in the eyes.

UVB rays burn the top layer of the skin. They make you tan or sunburnt.

UVC rays are the most dangerous of the three. But the atmosphere absorbs natural UVC completely, so we're safe from it outdoors.

Small-scale research on UVC

David Brenner, director of Columbia University's Center for Radiological Research in New York City, has been studying the potential of UVC technology to kill viruses in settings safe for humans.

Brenner has discovered that although UVC light is harmful to people, there is a form of UVC that can kill viruses without harming our skin or eyes — and it's called far-UVC. And Brenner's team has developed and tested a far-UVC lamp.

UVC light does its damage when it is able to go beyond the surface of the skin and eyes, reaching cells deeper in the body.

But far-UVC light is absorbed by the outer layers of the skin and eyes. As a result, far-UVC is prevented from reaching those deeper, living cells, and is less likely to cause any damage.

Researchers have tested the theory on mice, and it appears to work.

There have been two small studies testing it on people as well. And those appear to show that far-UVC is safe for human exposure in the short term. But long-term studies on humans, and those with groups larger than 20 people, have yet to be published.

Far-UVC lamps: First real-world tests

Brenner's team has demonstrated in a study published in March that their far-UVC lamps kill bacteria in full-size rooms.

Using five such lamps, the researchers found that it took less than five minutes to reduce the level of indoor airborne microbes by more than 98%.

The researchers sprayed a continuous mist of bacteria into the room, then let it stabilize for an hour before turning on the far-UVC lamps. They took air measurements over the course of the next 50 minutes, testing different light exposures.

Even at a low exposure, the lamps killed 92% of the bacteria in 15 minutes, they say.

The study is the first to test the technology in a full-sized room. Past experiments using far-UVC light have been conducted in laboratory settings.

International safety standards

UV radiation is measured in wavelengths, and the unit for that is nanometers (nm).

In the US, where Brenner and his team are based, the maximum daily exposure to UV is set at 222 nm. They used that as a standard for their experiments. And at those levels, they could put their far-UVC lamps on the US market.

The standards vary internationally. In Germany, for instance, the maximum allowed UV exposure is lower.

But Brenner says that staying within the German regulatory limit would still significantly decrease microbial levels in occupied rooms.

"If far-UVC lights had been installed in multiple public locations in Germany and operated within the current German regulatory limits, there's a good chance that a significant number of the people who have died from COVID-19 would still be alive," said Brenner.

Other labs and companies, like Philips, have developed technology that distributes the UVC light indirectly — things like upper air disinfection luminaires and chambers.

Edited by: Zulfikar Abbany