If far-UV is so great, why isn't it everywhere?

Far-UVC probably would have a large effect if a particular city or country installed it.

But if only a few buildings install it, then it has no effect because people just catch the bugs elsewhere.

Imagine the effect of just treating sewage from one house, and leaving all the untreated sewage from a million houses untreated in the river. There would be essentially no effect.

Why there isn't more ground-up interest: it's expensive and people can't easily tell if it's worth the cost. Also anything where UV is touching you has to overcome people's safety concerns.

Good question on the large effects are easily measured thing--has to do with the distinction between: 1) in what environments you are cleaning the air, 2) how much you are cleaning the air there, 3) how much pathogen people inhale, and 4) how much pathogen is required to actually make people sick. It's not just a far-UV problem, it'd be a problem for any air cleaner, it's just that far-UV is especially expensive to install and especially faces negative "UV" associations.

Far-UV has a large effect on airborne pathogen concentration, and that large effect is in fact easy to measure in a chamber! But once you add it to a room where people are moving around and talking to each other, how much pathogen are they actually inhaling? Is the air in the room well-mixed? Is the far-UV reaching the infectious air before people inhale it? Even if they inhale air that has living pathogens, are they getting sick? If they get sick, did they get sick from that room or from a different environment that they were in previously? Study endpoints matter a lot. 

Being able to understand intervention efficacy especially becomes a problem if a disease is largely spread via superspreaders/had high variance in infection sources. COVID, at least early in the pandemic, had very high variance, whereas eg seasonal flu doesn't usually. Therefore, if your study intervention is installed in public spaces, it's possible for it not to show much effect on seasonal flu but a large effect on a disease with early-COVID-like dynamics, which means you have to wait for that COVID-like disease to come along to see the effect--but that would be worth it; high-variance diseases are very concerning! 

Another way of saying all this is that it's not the case that the effect will be super hard to measure given enough people and time, it's that the effect is hard to measure given that you need a lot of people in your study to account for the distinctions listed above, and/or you need a highly controlled environment, and that's just expensive.

Had something of a similar reaction but the note about far-UV not having the same problems as other UV serilization (i.e., also harmful to humans) I gather the point is about locality. UV in ducks will kill viri in the air system. But the spread of an airborn illness goes host-to-target before it passed through the air system.

As such seems that while the in-duct UV solution would help limit spread, it's not going to do much to clean the air in the room while people are in it exhailing, coughing or sneezing, talking.... 

I suspect it does little to protect the people directly next/in front of a contagious person but probably good for those practicing that old 6 foot rule (or whatever the arbitray distancing rule was).

Just my guess though.

The word duct doesn't appear here because far-UV installation is most useful (compared with other wavebands) for whole-room application--agree that UV in ductwork has the potential to be very useful, but you'd use a longer wavelength if people weren't going to be directly exposed, because you can crank up the power on longer UVC wavelengths without producing tons of ozone. (Far-UV is specifically 200-230 nm, but UVC goes up to 280 nm.) I focus on far-UV because the excitement about far-UV specifically has to do with whether there's potential for it to be installed in such a way that it can stop in-room pathogen transmission before air entirely circulates or recirculates, which could be a big deal for pandemic prevention. 

Why in-duct UVC isn't more widely employed: I don't know a ton about its current adoption level but retrofitting ductwork is a hassle, even just in the simple sense of needing to contract technicians to do it. Anyway, its efficacy isn't guaranteed; you need to know the power of the light you're getting, which many consumers are not equipped to assess. The efficacy is a combination of the light's power and the speed of airflow through the duct, UV lights are often sold at an insufficient power to effectively disinfect air, and consumers can't necessarily evaluate whether it was a good investment (common problem across air cleaners in general).

My guess is that we're still on the leading edge of ordinary consumers thinking of air quality as something that they can and would want to control for anything beyond comfort, particularly for pathogen (or allergen, or mold) removal. All air quality tech faces that problem (and my blog post was trying to address the problems far-UV faces beyond that). Anecdote I found interesting: Some colleagues attended a global indoor air quality conference earlier this year and reported that something like 80% of the sessions were about comfort in the indoor environment, while about 5% were about pathogen control.

I'd be interested to know what the numbers on UV in ductwork look like over the past 5 years. When I had to get a new A/C system installed in 2020, they asked whether I wanted a UVC light installed in the air handler. I had, before then, been using a 70w UVC corn light I bought on Amazon to sterilize the exterior of groceries (back when we thought fomites might be a major transmission vector), and in improvised ductwork with fans and cardboard boxes taped together.
Getting a proper bulb--an optimal wavelength source--seemed like a big upgrade. Hard to come up with quantitative efficacy numbers, but we did have a friend over for the day, who turned out to have been in the early stages of covid, without getting infected. Our first infection was years later, at a music event.

My description was a pretty quick gloss, but yep, the government is large and I know partners have been inquiring with various offices. Getting money is always going to be a problem. Honestly part of it is, let's say it takes three years to get funding for [something you care about], it's not actually that long in government timelines but it feels like forever when you work at a small organization or company and your work revolves around that particular thing. 

(Let me know if I misunderstood; I'm reading your second sentence as "why aren't the companies...") On company size: The industry is split between emitter companies and consumer product companies; the emitter companies sell the far-UV emitter (basically the lightbulb) to a different company that builds the housing for consumers. The emitter companies are usually a branch of a larger electronics/lighting company; the consumer product companies are usually very small. 

Some companies have run their own studies, but most of their installations are much too small to be studies in themselves. One problem I've heard about in the case of at least one larger installation is that the customer who sought the installation wanted the data to remain confidential. Otherwise, large studies are indeed mostly too costly for these companies to self-fund entirely, but they may offer partial funding or provide their lamps at-cost or as donations to studies. 

Quiet air filters is an already solved problem technically. You just need enough filter area that the pressure drop is low, so that you can use quiet low-pressure PC fans to move the air. CleanAirKits is already good, but if the market were big enough cared enough, rather than CleanAirKits charging >$200 for a box with holes in it and fans, you would get a purifier from IKEA for $120 which is sturdy and 3db quieter due to better sound design.