UVC is really nasty stuff - you shouldn't be exposed to it

"UVC is really nasty stuff - you shouldn't be exposed to it,". "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.

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.

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.

No one knows how long it takes to deactivate Covid-19 with sunlight, or what strength is needed

All this means that using sunlight to disinfect surfaces is extremely problematic.

First of all, no one knows how long it takes to deactivate Covid-19 with sunlight, or what strength is needed. And even if they did, the amount of UV in sunlight varies depending on the time of day, the weather, the season, and where in the world you live - especially which latitude - so this wouldn't be a reliable way to kill the virus.

Finally, it goes without saying that disinfecting your skin with any kind of UV will lead to damage, and increase your risk of skin cancer.

And once the virus is inside your body, no amount of UV is going to have any impact on whether you're infected.

RAJENDRA K TRAKRU. [email protected]