I wrote a tweetstorm on why 222nm LEDs are not around the corner, and given that there has been some discussion related to this on Lesswrong, I thought it was worth reposting here.
People interested in reducing biorisk seem to be super excited about 222nm light to kill pathogens. I’m also really excited—but it’s (unfortunately) probably a decade or more away from widespread usage. Let me explain.
Before I begin, caveat lector: I’m not an expert in this area, and this is just the outcome of my initial review and outreach to experts. And I’d be thrilled for someone to convince me I’m too pessimistic. But I see two and a half problems.
First, to deploy safe 222nm lights, we need safety trials. These will take time. This isn’t just about regulatory approval—we can’t put these in place without understanding a number of unclear safety issues, especially for about higher output / stronger 222nm lights.
We can and should accelerate the research, but trials and regulatory approval are both slow. We don’t know about impacts of daily exposure over the long term, or on small children, etc. This will take time—and while we wait, we run into a second problem; the Far-UVC lamps.
Current lamps are KrCl “excimer” lamps, which are only a few percent efficient—and so to put out much Far-UVC light, they get very hot. https://link.springer.com/article/10.1134/1.1448635 This pretty severely limits their use, and means we need many of them for even moderately large spaces.
They also emit a somewhat broad spectrum—part of which needs to be filtered out to be safe—https://pubmed.ncbi.nlm.nih.gov/33465817/ - further reducing efficiency. Low efficiency, very hot lamps all over the place doesn’t sound so feasible.
So people seem skeptical that we can cover large areas with these lamps. The obvious next step, then, is to get a better light source. Instead of excimer lamps, we could use LEDs! Except, of course, that we don’t currently have LEDs that output 222nm light.
(That’s not quite true—there are some research labs that have made prototypes, but they are even less efficient than Excimer lamps, so they aren’t commercially available or anywhere near commercially viable yet, as I’ll explain.)
But first, some physics!
The wavelength of light emitted by an LED is a material property of the semiconductor used. Each semiconductor has a band-gap which corresponds to the wavelength of light LEDs emit.
It seems likely that anything in the range of between, say, 205-225nm would be fine for skin-safe Far-UVC LEDs. So we need a band-gap of somewhere around 5.5 to 6 electron-volts. And we have options. Here’s a list of some semiconductors and band-gaps; https://en.wikipedia.org/wiki/List_of_semiconductor_materials.
Blue LEDs use Gallium nitride, with a band-gap of 3.4 eV. Figuring out how to grow and then use Gallium nitride for LEDs won the discoverers a Nobel Prize—so finding how to make new LEDs will probably also be hard. https://www.nature.com/articles/nphoton.2014.291
Aluminum nitride alone has a band gap of 6.015 eV, with light emitted at 210nm. So Aluminum nitride would be perfect… but LEDs from AlN are mediocre. https://physicsworld.com/a/leds-move-into-the-ultraviolet/
Current tech that does pretty well for Far-UVC LEDs uses AlGaN; Aluminium gallium nitride. And when alloyed, AlGaN gives an adjustable band-gap, depending on how much aluminum there is.
Unfortunately, aluminum gallium nitride alloys only seem to work well down to about 250nm, a bunch higher than 222nm. This needs to get much better. Some experts said a 5-10x improvement is likely, but it will take years.
That’s also not really enough for the best case, universal usage of really cheap disinfecting LEDs all around the world. It also might not get much better, and we’ll be stuck with very low efficiency Far-UVC LEDs, at which point it’s probably better to keep using Excimer lamps.
But fundamental research into other semiconductor materials could allow much better Far-UVC LEDs. One candidate is hexagonal Boron nitride crystals. Another is diamond—which I don’t think will be practical to work with or build LEDs from, but “Diamond LEDs” sound awesome.
If we do find a new promising material, getting a good manufacturing process to make it and create the PN junctions will be critical. And unlike AlGaN, advances in other areas won’t provide benefits for a new material.
Plus we won’t have the existing knowledge of how to make it work. Remember the Nobel-prize for Blue LEDs? It’s hard to figure this stuff out. But people haven’t had a strong reason to do so—disinfecting air changes that.
There’s a bunch of cool physics and simulation tech that lets research explore which possible semiconductors could be viable. That seems very worth doing, in case AlGaN doesn’t work, or something better can be found.
Unfortunately, there’s another (half) problem, which is really the first problem again. Remember, whichever LED semiconductor material we find that works, if we find any, probably won’t emit light at the same wavelengths as KrCl excimer lamps.
How do different light sources affect safety? We don’t know exactly. A better LED is likely to be higher output than 222nm lights, and will be at a slightly different wavelength. We might even need entirely new safety studies done at whichever new wavelength we find.
And even if we get those safety studies, getting from there to commercial viability will take time—and it’s unclear how expensive or difficult it will be to make these new LEDs.
This is not to say I’m pessimistic about the idea! I think there’s a >50% chance we find LED materials that work at significantly better than 10% efficiency, are cheap, and are safe for humans. (Conditional on 222nm being found safe.) But it’s a decade or more away.
That’s OK. We can plan for a decade or more in the future. As attention to the areas grows, people are doing exactly that. So as usual, I’m excited that the future will be awesome, and can be made much safer than the present, at least from biorisks.
But we definitely don’t want to fool ourselves into assuming there is a silver bullet around the corner. And even once it’s around, it won’t eliminate the need for multi-layered protection against future pandemics—and we should be investing in those other parts now as well.