Disinfection with Far-UV (222 nm Ultraviolet light) (2020) [pdf]
boeing.com
boeing.com
The holy grail for this technology is a Far-UVC Diode. The guy who won the Nobel Prize for the blue LED (IIRC) is working with other physicists at UCSB to try and create said diode: https://www.news.ucsb.edu/2020/019949/uv-lights-way
If you want to learn more about Far-UVC itself (safety, etc), David Brenner is the real MVP: http://www.columbia.edu/~djb3/
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.
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.
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...
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.
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.
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.
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
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.
https://www.amazon.com/Blue-Tube-UV-Light/dp/B00D48XDO0/
Thought it might be applicable to computers, but realized that UV breaks-down PLASTICS as well, which is why you mostly find in ducting.
Very cool stuff if you're into clean/sterile living.
Note: Most of the dust in your home is comprised of human skin. Unless you've had an industrial vacuuming, other people's skin is moving around your house or gumming up the walls of your ducts. UV lights (and chemicals) are the only way to break that shit down.
> When inhaled, ozone can damage the lungs. Relatively low amounts can cause chest pain, coughing, shortness of breath and throat irritation. [0]
[0] https://www.epa.gov/indoor-air-quality-iaq/ozone-generators-...
[0] https://en.wikipedia.org/wiki/Ultraviolet_germicidal_irradia...
Where did you find that 185nm number? Everything I see on google says ~250nm, not to mention the original post is about 222nm UV.
Smaller than 240nm creates it, to various degrees, with the ideal frequency being 160nm. 185nm is 'produces a noticeable amount'.
[https://onlinelibrary.wiley.com/doi/full/10.1111/php.13391]
I'd imagine it breaks down molecular oxygen (some of which goes on to form ozone), and it breaks down ozone (some of which goes on to re-form ozone).
At any rate, that seems like altogether too radical to be desirable near living creatures whose well-being one cares for.
In others, no foreseeable issues even if run 24x7.
So, uh, maybe don’t buy cheapo units off Amazon unless you have some way to see/test what they’re doing.
The article is about KrCl 222-nm lights, which people want because they're safe [1] to shine on people, unlike Hg 254-nm lights.
[1] Probably -- I'd like to seem some specific additional experiments before widespread deployment.
254nm is also not safe.
254nm destroys ozone, interestingly.
240nm and smaller (160nm being the highest producing freq.) produce ozone.
[https://onlinelibrary.wiley.com/doi/full/10.1111/php.13391]
It cleaves DNA the same as UV-B, and there is no reason to think it isn't cancer causing. There are a number of areas that can get exposed that have very thin epidermis, or none at all (eyes), though eyes would get retinal keratosis not cancer.
That said, hopefully no one is spending enough time close to a high enough power UV-C source for this to REALLY be a problem.
Welding Cite - [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5640727/]
Notably, while having a higher skin cancer risk despite being inside a lot (shop welders, not shipyard welders), the vast majority of welders in that sampled population will have been wearing heavy protective clothing continuously. There was a noticeable increase in risk of skin cancer on the neck, which is one of the few areas that is not always adequately covered.
Anecdotally, I knew folks who didn't wear proper full coverage PPE when welding and welded a lot (auto body repair in one case, farm equipment repair in another), and both died in their early 40's from multiple malignant melanomas. One of them, it was 10+ all at once, and he died in less than a year. No one was surprised, unfortunately. They were ALWAYS sunburned from it, and they didn't spend a huge amount of time outside otherwise. That is a pretty broad spectrum source though.
UV-C As Potentially Mutagenic/Causing Damage not caught by normal replication suppression mechanisms - [https://pubmed.ncbi.nlm.nih.gov/9951833/]
UV-C will also have no problem converting all sorts of organic chemicals into interesting, and often more toxic versions (albeit killing any organisms relying on their original structure in the process), same as UV-B or UV-A.
Presumably what’s going on is that longer wavelength UVC from welding is extremely dangerous.
> UV-C As Potentially Mutagenic/Causing Damage not caught by normal replication suppression mechanisms - [https://pubmed.ncbi.nlm.nih.gov/9951833/]
p53-/- human keratinocyte cell line means that those were in vitro experiments which don't account for the wavelength-dependent penetration depth, which is the point of discussion here.
*Source: https://www.ehime-iinet.or.jp/ehime_e/corp/toyo/ronbun/image...
We do know a lot about how dangerous different frequencies are, and when you combine the "risk to humans" curve with the "emissions by wavelength" curve you get very low risk.
I do still think we should run additional experiments here before rolling this out broadly for hours-a-day usage, but from what we know so far it looks very good.
Here's an example of some of the research here, where they shined 222 nm vs 254 nm into rat eyes and looked for corneal damage. While 254 had some harmful effects 222 was fine: https://www.tandfonline.com/doi/abs/10.1080/10715762.2019.16...
Have an uncle who is / was (recently retired) a welder. He worked everywhere, did all sorts of welding, and even got certified to do underwater stuff -- which he did for 2 years and GTFO out of as soon as he made enough $$$$
He got skin cancer on his next and on his right elbow. Cuz that's where the gloves and other PPE clothing had gaps, esp. when he leaned certain ways or use his right hand to move the torch.
The whole point of using 222nm is that it doesn't cause effects on either the eyes or the skin. Consequently, you can use it at higher concentrations without worrying about the leakage.
The issue, as I understand it, is simply that we don't have a decent LED monochomatic source. All of the currently available sources have broad spectrums that have to be filtered out.
On top of that, reasonable disinfectant timeframes are a few minutes to a few hours, tops. Safety timeframes for human exposures need to be measured in decades if this technology catches on. Claims that lethal dosages of 222nm will penetrate typical glops of pathogens in the environment in seconds, but that cancer causing dosages will not penetrate human skin over years are extraordinary.
Extraordinary claims require extraordinary evidence. Are there long term safety trials for this technology? The article claims “reduced damage” in the abstract, implying routine 222 nm exposure is unsafe.
also viruses (eg. HIV, Coronavirii,...) are often killed by oxygen in air, dry out or die in other ways when not floating in bodily fluids.
So is the combination of 222 nm + exposure times required for disinfection still enough to cause noticeable bleaching of paints, aging of plastics, etc. etc. over time?
It penetrates single cell walls but nothing deeper.
People are mostly interested in 222nm for doing things like sterilizing air circulation and handheld wands. You can put a very high flux without worrying about accidentally exposing someone to a hazardous level of UV light.
People aren't trying to sterilize skin directly (as far as I know), but, even there, it might help healthcare workers. Washing your hands or coating it with alcohol is nasty over time. Being able to hold your hands in a device for 15 seconds and sterilize it rather than having to dump alcohol on them could be an improvement.
KrCl excimer lamps produce a relatively narrow spectrum as is: https://pubs.acs.org/doi/10.1021/acsphotonics.2c00041
If it turns out we need it to be even narrower, we could put filters on them.
One concern I've heard about UV (and ionization) stuff is that you're adding 'active chemistry' into your ventilation system, which could possibly cause strange reactions you may not want.
Besides 'bugs' and dead skin, there are are VOCs and other chemicals that we use in our homes: how will those reaction? If these units are new and working properly, things may be fine, but how many homeowners will do (or have someone do) regular inspections/maintenance? Having this stuff in non-residential places may be fine because Facilities has a role in keeping HVAC working: regular people don't do that.
Having good filters (MERV ≥13) will get rid of most of stuff you don't want in a simpler fashion.
Paradoxically, a 'more efficient' filter will generally achieves lower real-world performance (CADR) because the airflow drops so much.
The MERV rating are often hidden, instead you may need to look for numbers from a company-specific rating system like 3M's 'MPR 1900' or Home Depot's (yes, really) 'FPR 10.'[0]
(I almost don't want to include this last paragraph of info, because by sounding like an ad it will automatically 'taint' everything else, but fuck it....)
I find myself generally buying the 3M version (which is ~30% more expensive locally) which I observe has a much higher pleat count. That means more surface area (lifetime) and better pressure drop (airflow). By my math I come out ahead in cost per area of medium, which for me is a better metric than cost per filter.
[0] https://airfiltersdelivered.com/blogs/helpful-tips/merv-mpr-...
Anyone got recommendations for a reputable vendor online? Something other than BezosMart?
It isn't just a backup; you want it there before the expensive fine-grained filter, to catch the big stuff and extend the life of the more expensive filter.
I wouldn't use a prefilter with any real pressure drop thought. Why not? Well...
In theory a two-stage filter is ideal, because you can cycle the filters through: swap the (mostly clean) post-filter over to the pre-filter during filter changes, optimizing both filtration level and using the full capacity of each consumable filter element. This is the procedure when changing the ISS water filters, incidentally.
There's a downside, of course...
Essentially it's the same as series and parallel resistors, so for two filter stages in series (to achieve the same rated pressure drop) you need double the rated size for each of the stages, therefore 4x the total filter area and size. In practice, nobody really wants to install that in their basement.
Some of the Chinese positive pressure systems have seemingly the ultimate low-consumables design: a washable stainless prefilter, washable electrostatic filter, two stages (supports cycling) of HEPA filter, and last a refillable granular activated carbon stage. Spent activated carbon could be used as a soil amendment, or returned to a local facility for regeneration into new activated carbon.
Very low consumables, but very costly up-front.
UV units intended for installation in return or supply ducts are snake oil. A waste of electricity and money (the bulbs have a pretty finite lifespan.) The velocity of air in most ducts is such that anything flying by wouldn't be sterilized, and you can just install a large air filter (like the Aprilair 413 others have mentioned) and it'll bring a lot more benefits to the table, namely much better reduction of dust.
The greatest problem with indoor air quality is offgassing of VOCs and other pollutants from construction materials, furniture, electronics, etc.
If you want the best indoor air quality: install a large filter like the Aprilair or some equivalent, an air exchange device, and if you have a gas stove/oven, switch that to electric.
If you just do the simple thing and point the lights in a self-facing ring in the duct, no, they will not work. The trick is to beam the light down the incoming pathway to drastically-increase the exposure time. Clean reflective surfaces in the ducting to even out the photon flux density will greatly help with this.
Of course all this ended up being useless, but it was a fun project during the pandemic.
*I mean, it requires a certain kind of stubborn determination to do a hardware startup, and I hope he's crushing it.
Rawdogging unfiltered air with true COVID or other bad virons will expose you to a potentially harmful virus--millions of people have been killed and many more disabled from it so far. You have no way of knowing if you will join their fate and are just rolling the dice.
Go to southeast Asia where drinking water isn't so great. Drink it as a tourist and you'll spend a few days running to the toilet. But a native will drink it just fine as they're exposed to it all the time and their immune system has learned to deal with it.
Low level exposure is good for that, you will find germs everywhere, not enough to make you sick but enough to give your immune system a heads-up for when it does come across a high level source of them like a contagious carrier.
If you really believed this you would tell your surgeon not to bother wearing gloves or wash hands because you want to ensure your immune system is functioning ideally,
When the surgeon goes home on the metro he doesn't wear gloves either. Or a mask in many cases.
Of course living in a sterile environment works but the problem is that you will then have to do it all the time everywhere. Otherwise the first time you're out of that environment your immune system will have to catch up on everything. And there's a whole spectrum in between. Perfect "safety" is not possible while having an actual life.
The question is, does the covid pandemic indicate we should move towards the more sterile side of that spectrum? I personally don't think so, on the contrary. If you see the isolationist tactic employed by China it has been far less effective at getting the situation over with than our approach in the west. We were enjoying our lives for a year while the Chinese were still brutally locked up.
In the case of Southeast Asia, just getting used to the water is far more easily achieved than fixing it everywhere due to the high cost of a good water system. Some people get ill but life always carries risk. And for foreigners there's always bottled water. In our case we make a different tradeoff.
But training the immune system against common threats by exposing it to low level sources is really a thing. If it couldn't do that we would have been extinct millennia ago :) Because threats appear much faster than evolution can cope with them.
'Southeast Asia' doesn't make a trade-off because they analyzed the data and came to the conclusion that their population is better off with half-filtered water, they do it because they are poor and have bad infrastructure. Would you like their water supply so you can get your immune system trained? While at it maybe start eating unrefrigerated food. My friend from Africa did that growing up and has an iron stomach -- it sure does work (too bad about all the kids who died though because they got unlucky).
The problem isn't 'it doesn't work', the problem is 'you can't just get rid of the awful bad ones and leave the kinda bad ones. Either you do it or you don't.
This is why your type of argument is terrible.
I'm ok with the risk and I don't want to go everywhere masked (which I find really horrible for historical trauma reasons) or bother with ventilation and sterilisation stuff. I will never do that kind of stuff willingly.
If anything covid told me it's better not to be too fussy because it's over quickly (unlike China which really tried to kill it at all costs).
But when people say the immune system can't be trained by exposure I just have to disagree.
But anyway your mind is set and so is mine so there is no point in continuing.
So you are arguing because you feel like it is correct, based on nothing more than your own intuition.
> And the cost of those HVAC sterilisations will of course be paid by all of us (customers of venues, transport etc).
Are you willing to give up your nice clean water and get some 'southeastern asian' water to save a few cents? I mean, who cares how many people die from it, cause life is risky eh?
Viruses are also linked to all sorts of nasty outcomes like Alzheimer’s, cancer, etc. I doubt we’ll have any means to knock transmission back a ton anytime soon but it’d be nice to do what we can.
With the Spanish flu in 1918 it took months for it to get around, mostly through rail and steam ships. With COVID-19 we saw some variants explode around the entire globe in a matter of _weeks_.
I spent a year on a rather isolated base on an island, with about 50 other people. Hygiene was certainly not as good as it is here in the civilised world as we often didn't have running water, ate wild meat that had simply been stored in the cold outside for days, food products with an expiration date a few years in the past, etc, and we just didn't get sick. We got sick when the supply ship came, though. It wasn't Antarctica either, the conditions are mostly similar to Northern Europe, not so different from Brittany.
I think the situation is quite similar to how humans lived for a long time, in small groups with only occasional contact with strangers, and in these conditions I'm sure infectious disease was much less common than it is now.
Of course disease did exist, but I think it would mostly have been diseases brought by insects or other vectors, not as much things like the flu or the common cold.
It's actually the exact opposite? Improved hygiene and vaccination means rates of illness have dropped.
Tried to get the NYC mayor's office interested, but they just pawned me off to a black hole of submitting forms that no one reads.
If anyone still wants to do it, shoot me an email, username is greg with the domain "qbix.com" or add me on Facebook
(Compared to the more normal situation of someone retrofitting an office, bar, or other normal space with 222-nm lighting.)
2) Ebola, Yellow Fever, HIV & Hepatitis aren't airborne, so probably wouldn't be the kind of viruses that the parent is talking about
I was snarky because the idea of "exercising the immune system" is misinformation and used by "immunity debt" proponents.
I mean, if you’re going to go the naturalistic angle, then what’s natural about squashing people into buildings with poor air flow?
We've known that for a long time[0]. I have friends that are plumbers and HVAC people, that have to do strange stuff, by law.
He worked on skyscrapers all over NYC.
AFAIK, there is no good evidence about any of those hypothesis, and weak ones for them all.
https://www.canada.ca/en/public-health/services/diseases/201...
> A whole room UV simulation demonstrated a far UV-C lamp (207-222 nm) could further reduce SARS-CoV-2 by 50-85% compared to ventilation alone and with both far UV-C and high ventilation the SARS-CoV-2 viral count was reduced by 90% in 6 minutes and 99% in 11.5 minutes Footnote 5.
I wonder if this is something that I could add to my pre-existing home ventilation system; no concerns at all about skin exposure (which doesn't seem to be an actual issue based on this Boeing paper), and it shouldn't be so hard.
* I found this (https://faruv.com/) but I don't know how reliable they are.
That said, if this really was that effective, I don't understand how viruses and bacteria are able to survive outside with our fusion-powered UV ray generator in the sky.
-edit- * I feel dumb, here it is at the end of the summary. Looks like Boeing is working w/ faruv.
> Boeing recently entered into patent and technology licenses with Healthe® Inc. and FarUV Technologies. Under these licenses, both companies will produce and distribute a commercial Far-UV 222 nm mobile wand, helping airlines and potentially others reduce the impact of the coronavirus pandemic.
Edit: The article's claim is that because far UV is absorbed in the upper atmosphere there's not really any evolutionary pressure for organisms to develop protection. Larger organisms just happen to be protected by virtue of our size.
222nm doesn't take even 30 seconds to wipe things out. Every dish was totally clean.
265nm needed about 2 minutes to sterilize.
275nm almost got the job done at 3 minutes and had the 5 minute dish totally clean.
290nm was frankly pathetic. All of the dishes survived with barely-decreasing colony count across increasing times. I estimate it'd take an hour of exposure to clean the dish.
There is a reason that even during the height of the pandemic, that outdoor events were not a large source of transmission.
People going into bars or houses after marching/protesting/partying were a problem, but standing around in the sunshine, even in a reasonably dense crowd, wasn't ever really a problem. (AFAIK, I remember news articles going "well that didn't turn out as bad as we all thought it would")
turn that wand into wall panels and let people walk through it - just like any other clean-room-style man-trap?
It seems that there is consensus on both effects, but I can't find anything conclusive that allows the scale of the effects to be measured sufficiently to do a benefit/cost analysis for each type of light spectrum and usecase.
Ozone is already a known and easily tested quantity, and generally only happens at 240nm and lower wavelengths. So 222 as listed might produce some, but doesn't seem optimal (180nm seems better).
Found a paper - seems likely that is the case [https://onlinelibrary.wiley.com/doi/full/10.1111/php.13391]
Note scale on the first graph.
edit: ah hah, but many of these lamps have defects that can result in higher freq. light emission and high ozone production! See later parts of the paper.
However, our skin heavily depends on friendly bacteria to do useful things. One concern that I have yet to see research testing is the impact of extended Far-UVC exposure on these microbial populations.
However, while I was trying to find the specifics on the DNA damage, I found this: https://mdpi-res.com/d_attachment/ijms/ijms-23-09112/article...
My only grief with this article is the conclusion they draw from the yH2AX staining. I can see several possible reasons for observing the higher proportion of cells with it.
Other than that, the paper raises a boatload of serious points and definitely observes negatively impacted cellular function. I still think FarUVC has incredible potential but just like cellphone towers the exact specifics to interact with it safely need serious further study.
However, cells exposed to 222-nm far UVC presented the highest degree of DNA damage as evidenced by yH2AX staining. Globally, these cells presented transcriptional changes in cell cycle and senescence pathways. Thus, the introduction of 222-nm far UVC lamps for disinfection purposes should be carefully considered and designed with the inherent dangers involved.
222nm may have the same capabilities, but not the same efficacy and efficiency as 254nm (or 185nm, the germicidal action if which is it creates ozone, which does the disinefecting). Still, if it works but just takes longer without the sunburn and blindness, whether by proximity to 254nm such that some 254nm is still present, and/or by proximity to 185nm such that ozone is still created, well, ok.
... which just happens to be exactly the second spectral line of a low-pressure mercury-vapor lamp. Actually 253.7. The other line is 184.5. Are you sure that the study you read isn't just reporting this wavelength as ideal because we have a convenient source of it? Efficient far-UV sources tend to be few and far between. It would be an awfully big coincidence if the two major spectral lines of an Hg plasma just happen to be the two ideal wavelengths for disinfection.
FWIW, zinc vapor has a persistent line at 214 nm, but I've never heard of this being applied in a commercial bulb. Other relevant possibilities are all toxic.
Studies, and yes. Mercury vapor's spectral emission is just a happy coincidence and why mercury vapor lamps are used for germicidal applications. The germicidal LED's they make are 255nm. But 254nm, not 255nm, not 253.7nm, is the most efficient wavelength for germicidal applications, in that it kills the most bugs the fastest. Please don't make me dig up the studies, please find them yourself, there are plenty, and it'll mean more to you.
https://avidbots.com/resources/what-you-need-to-know-about-u...
TLDR: The level of light insolation you need for disinfection makes them less useful for large area coverage and/or areas exposed to the public due to eye safety concerns.
Same as overuse of antibiotics - it only breeds more resistant strains and harms the immune systems because of lack of less resistant strains.
The UVC doesn't disrupt their metabolism or mutate their genetics subtly, it shreds their chemical bonds.
It's much more like worrying that germs will evolve resistance to hydrogen peroxide or ethanol or iodine disinfectants. It's not impossible of course, but it'll require the kinds of mutations that make them entirely other organisms.
But it does -- this only depends on an prompt dose, at which you have exactly one chance. How can you guarantee a lethal dose (for any definition of lethal) for every single microorganism? Even hard gamma was adapted for, compared to that UV is nothing.
Antibiotic resistance is a thing not because antibiotics are a molecular shredder that chews up the bugs. It's because they're some kind of metabolic poison.
I might be able to increase my resistance to Iocaine powder by slowly dosing up. But I can't increase my resistance to Andre the Giant tearing me limb from limb.
I believe they were asking specifically that. How do you ensure that?
In other words, how do you ensure all get the correct (lethal) dose?
Some may be partially hit. It would be very hard to tell, no?
If you put these in ductwork then you need to look at the flow rate through the duct and the required light flux and then ensure that the supplied flux is well in excess of that.
Remember we're not talking about your normal LD50 toxicity. It doesn't kill by metabolic disruption. It kills by turning the thing from organism into not organism.
If an organism survives it does so not by adapting its metabolism to the poison. It does so by enough of itself not being destroyed that it is able to rebuild.