“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. The World Health Organization (WHO) has issued a stern warning against people using UV light to sterilise their hands or any other part of their skin.

During the White House briefing on Thursday, the US president suggested that UV light could be brought inside the body to kill the coronavirus. It’s not clear which type he meant, but given what we know about the damage UVA, UVB and UVC can do to genetic material and living tissues, this would be a bad idea – let alone impractical, since Covid-19 mostly infects the lungs.

Recently, scientists have discovered a promising new type of UVC which is less dangerous to handle, and still lethal to viruses and bacteria. Far-UVC has a shorter wavelength than regular UVC, and so far, experiments with human skin cells in the lab have shown that it doesn’t damage their DNA (more research is needed to be sure).

On the other hand, bacteria and viruses don’t come off as well, because they are small enough for the light to reach. One study found that it could prevent mouse wounds from becoming infected with the superbug MRSA, while another found that it could kill flu viruses suspended in the air.

However, the vast majority of the UVC lamps on the market don’t use far-UVC yet – and again, it hasn’t been tested in actual humans, just on our cells in petri dishes and other animals. So this type of radiation probably won’t help you during the current pandemic either.

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.