> The holy grail for this technology is a Far-UVC Diode.

Right. Currently, 222nm lamps are gas-discharge tubes, and they cost far too much. There are fake 222nm LED lamps available online. There are also units that produce what Naomi Wu calls "homeopathic levels" of UV. Beware of scams.

If this is to go anywhere, a low-cost tester is needed. You can get a hand held UV spectrograph for about $3000, but that's more of a lab instrument. Something like a detector on a stick that lights up green for enough 222nm UV to be useful, and red for too much UV in dangerous parts of the spectrum.

One idea is to have a unit up near the ceiling that emits UV in a horizontal plane, so that air that makes it up to the ceiling gets sterilized. Convection, maybe with ceiling fan help, can move air through that region. But humans aren't up near the ceiling much, so they don't look into a UV source.

This still seems kinda scary. The scatter off the walls seems like it would be a problem for eyes or even skin over time. Would it generate ozone? What are the by-products of ionizing dust and what happens when you breath them?

The good thing about far uvc is it isn't penetrating. It doesn't it make it through your your outermost cornea or epidermis layers.

Scatter is also negligible as long as its set up competently: reflectivity is very different for UV and visible light.

Ozone production certainly limits the maximum power that can be used, but as I understand it you don't need to crank anything close to max power to attenuate the airborne viral load.

You can get 265-275nm LEDs- they are effective but you have to check them since there are constant scams.

https://twitter.com/RealSexyCyborg/status/154177301370350387...

You also have to dump a lot of heat so there's fan noise and Upper-Room GUV is already quite cheap so they aren't really game changers in that application- but they do work.

https://twitter.com/RealSexyCyborg/status/153744305018765722... https://twitter.com/RealSexyCyborg/status/153944537069847756...

222nm excimer lights are pretty costly per joule compared to 254-275nm given the 3,000hr or so lifespan of typical Chinese bulbs (although USHIO claims 10,000 is on the way) but from working with the factories they aren't quite as expensive as people think- the distributor markup is huge. You can also squeeze about 30% more out of them by switching to PTFE reflectors:

https://twitter.com/RealSexyCyborg/status/156236957519840051...

Best would be no reflector at all and pump as many joules into room air as we can with some air circulation, but right now we can only do that with unfiltered excimer lights- which aren't horrible but not something you'd want to sit with for several hours a day:

https://twitter.com/RealSexyCyborg/status/157656585826391244... https://twitter.com/RealSexyCyborg/status/159832965174278963...

Cylindrical 222nm cut filters would solve this but so far we can only get flat panes- and they are quite costly so building a square or triangular enclosure of filters costs more than you gain.

For what it's worth I make pretty extensive use of (filtered) 222nm- both portable and at home and I haven't caught COVID yet. Just an anecdote, but I've got no reason to stop using it- at least personally.

https://twitter.com/RealSexyCyborg/status/159942679977541222... https://twitter.com/RealSexyCyborg/status/159708372174773452... https://twitter.com/RealSexyCyborg/status/158850109792860160... https://twitter.com/RealSexyCyborg/status/161091221064529920...

The math on wearable Far-UVC didn't work out (exposure time is too short for the power, high power exceeds the TLV at that distance)- but it was a fun side project for a couple of days:

https://twitter.com/RealSexyCyborg/status/157297688745831219... https://twitter.com/RealSexyCyborg/status/158296013877611315...

As far as 222nm safety, I'm comfortable with it based on current data, used within recommended TLVs and have tested it on myself at length. But the one area I have concerns is folks with Lupus and other UV-sensitive immune disorders. It should be ok given that UVB seems to be the problem region, but we really have no information and I'd like some before any real widespread roll out of this technology.

https://twitter.com/RealSexyCyborg/status/155949983593741926...

> but from working with the factories they aren't quite as expensive as people think- the distributor markup is huge.

That is good to know. In the US these things are sold by high-markup "lifestyle" suppliers. Gas-discharge tubes are not inherently expensive, but the market is not yet large.

> A visible light filter that allows 222nm through. ... No blue UV death glow...

Yes. For long-term deployment that's needed.

Hat tip for the David Brenner link. The bottom of that page reads:

    Over the past six years, we have been working on a very exciting new approach to killing drug-resistant bacteria - superbugs- , as well as airborne viruses such as influenza, using a unique type of ultra-violet light. We have always known that ultraviolet light can efficiently kill all microbes, but conventional germicidal ultraviolet light is hazardous to our health, causing skin cancer and cataracts. We have identified a particular wavelength of UV light that has the best of both worlds - it's safe and it kills microbes - including superbugs.

While he doesn't say it directly, I guess he is talking about 222nm UV.

Wonder what it does to human health if the microbes on your skins surface are constantly exposed to Far-UV?

Update: as a result of another comment, I stumbled into this: https://mdpi-res.com/d_attachment/ijms/ijms-23-09112/article...

My quick take on their experimental results is in the other comment, but in short it appears there will indeed be a trough of disillusionment somewhere down the line.

Unless I'm misreading it, this paper looked at the effect on cells in isolation? But the reason we think 222nm is safe isn't that it has no effect on living cells, but that the living cells in our body are protected by a thin layer of dead cells that 222nm can't get through.

Here's the kind of paper I think is much more relevant, where they shined light into eyes directly: https://pubs.acs.org/doi/10.1021/acsphotonics.2c00041

This is a recent interesting paper: https://doi.org/10.1111/php.13767

Showing effectiveness on surfaces, while remaining safe for humans.

I’m following closely the development, it’s still extremely costly to make a product but hopefully will see more adoption in the near future, starting at enterprise level. I have a device in my home, planning to permanently install it in the entrance where we leave shoes & co.

Feel free to dm me if you’re hacking in the space, happy to chat more.