“You would literally be frying people,” says Dan Arnold, laughing in disbelief. 

Arnold works for UV Light Technology, a company that provides disinfecting equipment to hospitals, pharmaceutical companies and food manufacturers across the UK. Recently, as the global anxiety about Covid-19 has reached extraordinary new heights, he’s found himself fielding some unusual requests.

“We had an enquiry from a private individual about our equipment, saying ‘Well, why can't we just get one of your UV lights and put it up on the exit to the supermarket – people can stand under it for a few seconds before they go in’,” he says.

Among the abundant “health” advice currently swarming around the internet, the idea that you can disinfect your skin, clothing or other objects with UV light has proved extremely popular. In Thailand, a college has reportedly even built a UV tunnel that students can walk through to disinfect themselves.

So is this a good way to protect yourself from Covid-19? And is it true that since “the new coronavirus hates the sun”, sunshine will immediately kill it, as some reports on social media have claimed?

In short: no. Here’s why. 

Dangerous rays

Sunlight contains three types of UV. First there is UVA, which makes up the vast majority of the radiation reaching the Earth’s surface. It’s capable of penetrating deep into the skin and is thought to be responsible for up to 80% of skin ageing, from wrinkles to age spots.

Next there’s UVB, which can damage the DNA in our skin, leading to sunburn and eventually skin cancer (recently scientists have discovered that UVA can also do this). Both are reasonably well known, and can be blocked out by most good sun creams.

There is also a third type: UVC. This relatively obscure part of the spectrum consists of a shorter, more energetic wavelength of light. It is particularly good at destroying genetic material – whether in humans or viral particles. Luckily, most of us are unlikely to have ever encountered any. That’s because it’s filtered out by ozone in the atmosphere long before it reaches our fragile skin.

Or that was the case, at least, until scientists discovered that they could harness UVC to kill microorganisms. Since the finding in 1878, artificially produced UVC has become a staple method of sterilisation – one used in hospitals, airplanes, offices, and factories every day. Crucially, it’s also fundamental to the process of sanitising drinking water; some parasites are resistant to chemical disinfectants such as chlorine, so it provides a failsafe.

Though there hasn’t been any research looking at how UVC affects Covid-19 specifically, studies have shown that it can be used against other coronaviruses, such as Sars. The radiation warps the structure of their genetic material and prevents the viral particles from making more copies of themselves.

As a result, a concentrated form of UVC is now on the front line in the fight against Covid-19. In China, whole buses are being lit up by the ghostly blue light each night, while squat, UVC-emitting robots have been cleaning floors in hospitals. Banks have even been using the light to disinfect their money.

At the same time, UV equipment suppliers have reported record sales, with many urgently stepping up production to fill their orders. Arnold says UV Light Technology has run out of all of its equipment already.

But there’s a major caveat.

“UVC is really nasty stuff – you shouldn't be exposed to it,” says Arnold. “It can take hours to get sunburn from UVB, but with UVC it takes seconds. If your eyes are exposed… you know that gritty feeling you get if you look at the sun? It’s like that times 10, just after a few seconds.”

To use UVC safely, you need specialist equipment and training.

Sunshine solution?

Would UVA or UVB work instead? And if so, does this mean you can disinfect things by leaving them out in the sun?

The short answer: possibly – but you wouldn’t want to rely on it.

In the developing world, sunlight is already a popular means of sterilising water – it’s even recommended by the World Health Organisation (WHO). The technique involves pouring the water into a clear glass or plastic bottle, and leaving it out in the sun for six hours. It’s thought to work because the UVA in sunlight reacts with dissolved oxygen to produce unstable molecules such as hydrogen peroxide, the active ingredient in many household disinfectants, which can damage pathogens.

Without water, sunlight will still help to disinfect surfaces – but it may take longer than you’d think.

The problem is we don’t know how long, because it’s still too soon for many studies to have been done on the new coronavirus. Research on Sars – a close relative of Covid-19 – found that exposing the virus to UVA for 15 minutes had no impact on how infectious it was. However, the study didn’t look at longer exposures, or UVB, which is known to be more damaging to genetic material.

Instead, other viruses might provide some clues. Take the flu. When scientists analysed hospital admission records in Brazil, they found that the number of flu cases tended to go up during the burning season, when there is more smoke in the atmosphere from forest fires and the UV in sunlight is diluted.

Another study found that the longer flu particles were exposed to sunlight for – and the more concentrated it was – the less likely they were to remain infectious. Alas, the study looked at flu suspended in the air, rather than dried onto objects.