[Adam Murphy (OP)]

Darren sent us a message asking:

"The COVID crisis made me think. Here in NZ we are on tank water (gathered off our roof)  which is sterilised by passing through a tube exposed to a UV light. Could this system be used for the backlight in keypads, touch screen in places like McDonalds, information boards in airports/shopping centres etc? "

Thoughts?

[alancalverd]

We've seen this question before, and answered it.

UVC is a great surface and short-range sterilant. Fine in an enclosed water treatment plant, factories, laboratories, and even some wards, but not in a public place - it will fry exposed skin just as quickly as any other biological material.

[Bored chemist]

We've seen this question before, and answered it.

Yep
https://www.thenakedscientists.com/forum/index.php?topic=79388.msg601220#msg601220

[Bored chemist]

I have been thinking about this.
It's conceivable that, at some wavelength the path length in biological tissue would be so short that, a human would only have their stratum corneum (About 20 microns thick) damaged (and it's already dead) but a virus or bacterium (say 100 nm to 5 micron) would be fried all the way through.

[set fair]

I've heard that UVC will do just as Bored Chemist suggestss and that it won't even penetrate the lubricating layer of the eye. This seems at odds with it being able to penetrate water, which it must do to kill the germs.

ps 200 nm was the wavelength mentioned

[Bored chemist]

I've heard that UVC will do just as Bored Chemist suggestss and that it won't even penetrate the lubricating layer of the eye. This seems at odds with it being able to penetrate water, which it must do to kill the germs.

The commonest source of UVC is a mercury lamp emitting at 253.7nm.
That radiation certainly is harmful to people. A shorter wavelength might, paradoxically, not be as harmful.

The layer of tears over the eye isn't pure water. It's the impurities that might block UV.

[alancalverd]

From the man who laughs at danger. Or at least rubber gloves. I still read the manufacturer's handbook and wear a full visor and heavy overalls when dealing with any medical UV sources.

[Bored chemist]

Do you know of any medical UV sources that produce, for example, 220 nm UV?
Or are you talking about something irrelevant again?

[alancalverd]

Pulsed xenon discharge.

[Bored chemist]

Pulsed xenon discharge.

So, broadband UV/ vis/ IR including all the stuff that's known to be nasty. (rather than stuff that's relevant).


Funny thing is, I'd have thought you knew enough medical physics to recognise the idea.
High energy Xrays don't always cause more harm and narrower energy spectra can be beneficial too.

[alancalverd]

If I need 200 nm UVC to kill a bug, I'm perfectly happy to fry, boil or photolyse any other biological material in the vicinity.  Sterilisation is usually high altitude carpet bombing, not a precision drone strike.  If you can't tolerate a bit of collateral damage, by all means develop a specific antibody.

If your target virus is exquisitely sensitive to a wavelength that won't penetrate the tear film on your eyes, we're not going to kill it under a layer of sweat or fat, or if it's hiding behind a hair, inside a pore, or lurking in a droplet of aerosol snot.   

[Bored chemist]

Do you accept that, at a short wavelength, it's possible that light might be absorbed within the stratum corneum before it hit anything important in the skin?

[alancalverd]

Possibly. But as

the stratum  corneum is now understood to be live tissue that performs protective and adaptive physiological functions including mechanical shear, impact resistance, water flux and hydration regulation, microbial proliferation and invasion regulation, initiation of inflammation through cytokine activation and dendritic cell activity, and selective permeability to exclude toxins, irritants, and allergens

I'd prefer not to barbecue mine.

[Bored chemist]

I'd prefer not to, but I might be prepared to sacrifice the outermost micron to stave off a deadly virus.
UVC might be a way to make that balance.
So is washing your hands, or using alcohol gel...