Germicidal UVC tech

OK there are 4 notable subclasses of UV devices for this.

Let’s just START with the TLDR takeaway: Cheap, proven 254nm germicidal UVC fluorescent tubes have been around for decades and definitely kill germs, but are totally NOT for occupied rooms, they’re not eye-safe nor skin-safe, can damage many surfaces, and make the room stink. “UVC LEDs” are probably going to ship you bunk, but if they do work, they’ve got the same drawbacks as the 254nm tubes that will definitely work. But there is a new 222nm excimer UVC lamp that does seem to be effectively germicidal, and many CLAIM is skin-safe and even eye-safe but this is not well proven, and AFAIK real ones are not currently obtainable

OK so germicidal power is in the UVC range, 100-280nm. 265nm was deemed to be the “peak” of germicidal activity, as DNA/RNA of all sort of life has a very strong absorption at that specific point

  1. Safe but useless UV: UVA range is just “blacklights” (LED or purple fluorescent tubes), it’s nearly visible and causes posters to glow. It does nothing to skin or eyes and doesn’t fade things or crack plastics, but it doesn’t do anything to bacteria/viruses/fungi either. UV-cured glues and resists are specially designed to be cured by UVA exposure. UVB range can cause tanning, only “sort of” dangerous to skin and eyes, but also no significant germicidal power at all.

  2. Traditional, proven germicidal fluorescent tubes of low pressure mercury (there is another type) are UVC, specifically 254nm, very close to the golden 265nm DNA/RNA target. They are cheap and powerful and PROVEN as highly effective given adequate time and exposure, but surfaces must be exposed of course. Opaque things block the UV. It can, with adequate exposure intensity*time, destroy airborne pathogens.

254nm UVC cannot be used in an occupied room. It is harmful to skin and eyes, and creates a funny “burning hair” smell found to come from the UVC breaking down the oil in fingerprints and human skin shedding found in dust (basically burning it). It can damage surfaces and make plastic/rubber crack. The dangerous UVC portion is completely blocked by any common window glass, but they also emit a harmless, non-germicidal visible bluish wavelength that you can see through the glass.

Germicidal UVC tubes have been used inside air ducts so no light enters a room, after HEPA air filters that remove dust that would make the burning smell. But at that point, well-filtered air is generally not thought to have a significant risk that needs UVC sterilization. It could take a lot of light for a fast airflow.

  1. “Germicidal UVC LEDs” are supposed to be similar wavelength to germicidal UVC tubes- is exciting, but is not well established, proven tech, and diluted by scammers with junk products. If “real” UVC, it’s certainly going to have the same problems with not being eye-safe, damages surfaces, and create a smell- and may miss the 265nm peak so far that it is NOT germicidal.

It is difficult to build a practical UVC LED device, LEDs are typically sealed with silicone tops now, the older tech was plastic. Both block UVC as well as degrade rapidly, so the diode itself has to be sealed with sapphire or fused silica. So true UVC LED devices are very specialized construction. Unfortunately, the market is flooded with undocumented “germicidal LEDs” with either no claim of wavelength, outright false claims about wavelength (some found to be basically common, useless UVA “blacklights”), or possibly accurate claims of wavelength BUT with dubious, unproven powers of being germicidal at that wavelength, and are too far from the 265nm peak.
It’s pretty likely some Chinese mfgs did make germicidal UVC LED die (the actual teeny tiny silicon diode) but could not seal them appropriately with an exotic process and just slapped standard clear silicone on top, opaque to UVC so it kills the UVC rendering it useless, but they still claim it’s UVC.
Hackaday DID check one big seller:

AAND it’s bunk. Common UVA blacklight LEDs, zero germicidal power (but safe to look at!)

It would be hard to tell what wavelength and emitted power you actually get. It will have the same drawbacks as germicidal 254nm UVC tubes if it works though, so it begs the question why not just use a cheap, proven, powerful 254nm tube if you have an application that can target it. Its advantage would only be that it might be a more convenient form and won’t break if dropped.

  1. There is very new hype surrounding “far UVC”, 222nm, which claims it can destroy bacterial and viral particles but claims should not be harmful to people (in theory). It seems pretty certain it can damage some common surfaces (cracking plastic/rubber, fading colors)

This is NOT well studied, especially the “is it actually safe to be around?” part, especially being eye-safe. goes out of its way to be clear it’s for unoccupied rooms only.

These are 222nm excimer tubes, not to be confused with the traditional 254nm UVC germicidal tubes.

Far-UV 222nm excimer light is actually pretty interesting, but don’t look obtainable yet. Sterilray and Ushio are two companies marketing genuine (I believe) 222nm excimers, and they don’t seem to be obtainable off-the-shelf anywhere. Chinese vendors are spamming “222nm lamps!” that in the description-detains-fine-print often admits “254nm”, so it’s the same traditional germicidal UVC lamp you can’t be in a room with, and NOT a real 222nm. I’m certain whether or not the details admit it, they’re only going to ship you a traditional 254nm tube, not a 222nm excimer. They probably wouldn’t have that tech but if they did they wouldn’t be peddling it as a no-name vendor on Alibaba.

Note that if you did order and receive a fake “222nm” tube that is in fact 254nm (almost certainly the case), it will not be easy to tell the difference, and you may be exposing people to dangerous UVC.