Far-UVC efficiently inactivates an airborne pathogen in a room-sized chamber
researchsquare.com/article/rs-908156/v1
researchsquare.comPosted Sep 16, 2021, 1:23 PM UTC · locked
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researchsquare.comAbstract
Many infectious diseases, including COVID-19, are transmitted by airborne pathogens. There is a need for effective environmental control measures which, ideally, are not reliant on human behaviour. One potential solution is Far-UVC which can efficiently inactivate pathogens, such as coronaviruses and influenza, in air. When appropriately filtered, and because of its limited penetration, there is evidence that Far-UVC does not induce acute reactions in the skin or eyes, nor delayed effects such as skin cancer. While there is laboratory evidence for far-UVC efficacy, there is limited evidence in full-sized rooms. In the first study of its type, we show that Far-UVC deployed in a room-sized chamber effectively inactivates aerosolised Staphylococcus aureus. At a room ventilation rate of 3 air changes per hour (ACH), with 5 filtered sources the steady-state pathogen load was reduced by 92.1% providing an additional 35 equivalent air changes (eACH). This reduction was achieved using Far-UVC intensities consistent with current regulatory limits. Far-UVC is likely to be more effective against common airborne viruses, including SARS-CoV-2, and should thus be an effective and “hands-off” technology to reduce airborne disease transmission. The findings provide room-scale data to support the design and development of safe and effective Far-UVC systems.
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Is far UV-C eye and skin safe? I’m guessing it’s not and it’s important, because be it Cobalt sources or Zyklon B there will be stupid people blasting it against unsuspected bystanders if adequate warnings, regulations, and foolproof designs are not present
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UVC is interesting because it is a spectrum and the efficacy of pathogenic inactivation spans a broader range than the range of danger for human exposure.
Traditional sources are at 254nm from gas lamps and are highly effective at both pathogenic inactivation, and inducing photokeratosis (makes your eyeballs hurt), which is sort of a self-limiting activity, as well as risk to skin.
LED UVC sources designed to operate at 222nm are much safer for human exposure, but are more costly and quite inefficient at the current manufacturing maturation level.
From a functional standpoint, these sorts of systems are much better implemented in contained air handling systems to minimize risk while minimizing cost (using traditional lamps); in the future the deployment of LEDs may be possible, but the sorts of energy densities we're talking about here are not currently feasible.
Total dosing levels need to be about 1-2 J/cm2 using 254nm light based on my past readings, which is consistent with the High scenario here, and it's 30min exposure for total viral inactivation. I have read some papers suggesting inactivation may be possible in much smaller dosing levels, more like <10 mJ/cm2 using 222nm light, but those seem to be quite specialized circumstances.
I do think there is a lot of interesting possibilities in using LEDs, but for now those are mostly in enclosed spaces (phone cleaning cases are popular and the math checks out), but for large spaces that is probably some years off.
Still, this is a good study to add to the literature.
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Sticking UV in air enclosed air handling systems immediately means the effectiveness is limited by the air flow rate. At best you get the equivalent of a filter with no pressure drop or 100% outdoor air but you lose the other functions of filters and fresh air. Enclosed UV is helpful to prevent growth of microbes in HVAC systems but not hugely useful for air cleaning. Likewise for UV in a closed box to disinfect items - it does a job that is quite easy to do by other means.
The huge advantage of UV is that it doesn’t need to be enclosed. That’s what enables air cleaning at the equivalent of 30+ air changes per hour in a room while most mechanical ventilation systems max out at more like 2-4 and going over 5-10 is noisy and very expensive. Upper room UV is highly effective already but very rarely used, I’d guess largely because most people never cared about airborne infections. If people start caring, 222nm might be even better without needing the same safety precautions. UV in a box/duct will always be a niche thing though.
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You raise excellent points about the airflow being the limiting factor in enclosed systems! I should have specified that it's a relatively cheap and easy addition to lower rated air handlers (say MERV 10 or below) that would require significant upgrades to the full system to bring them up to levels which would make them effective at removing airborne viruses. I agree the future of this application is in open spaces, and presumably with 222nm due to the reduced risk.
Indeed there are other ways to do most surface cleaning, but the one area the enclosed box situation really shines is in resource constrained situations where N95 mask reuse is necessary - a few mins in a simple UV box effectively inactivates viral load through all surfaces of the masks, without degrading the overall utility significantly until multiple reuses and disinfection cycles have taken place. There are many papers showing total inactivation at ~ 1J/cm2 dose levels, which neatly coincides with this paper as well, here is one.
Once the UV LED material is matured and made more efficient/powerful, one could envision them being placed inside existing bulb structures so that any LED light installed indoors has some inactivation effect, and by having them throughout a space there is a constant cleaning effect, but this of course would require a change of glass to being UV transmissive.
Fascinating area of study!
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As with everything, safety is dose-dependent, but even within the current very conservative limits it seems to be quite effective. It's likely to be safe at much higher levels.
The peak 8-hour exposure dose in this “High” scenario is outside the current exposure limits but within, and indeed motivated by, the proposed increase in the American Conference of Governmental Industrial Hygienists (ACGIH) Threshold Limit Value (TLV) for the skin (415 mJcm^−2 at 222 nm over 8 hours)
I think a reasonable standard could be set based on equivalence to a few minutes of sunlight. Most of the evidence seems to point to effective far-UVC irradiation being much less damaging than this.
This modelling study suggests that DNA damage in the skin from 10 minutes of sunlight (UV index 4) is equivalent to 30,000 hours filtered far-UVC at the basal layer (where most skin cancers form) or 300 hours at the top of the epidermis. This lab study found significant damage to the corneas of rats from 254nm UVC but none from 222nm UVC.
It seems more work may be needed, particularly animal studies to find the true upper safe limits of 222nm UVC, but overall it looks like a highly effective system might have a tiny additional impact on the total UV risk we're already exposed to. Also much lower risk than other hazards we accept in buildings like electricity and gas appliances.
Existing upper-room germicidal UV systems (using 254nm lamps pointed away from people) can be very safe and effective too, though there's more risk from misuse. Should be much more common in well-controlled environments though IMO. We've had good evidence these can greatly reduce transmission of highly contagious airborne viruses since the 1930s.
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How long does this process take? Some of the previous UVC research said it takes as long as 15 min to de-activate the vast majority of human coronaviruses. If the virus primarily transmit by direct inhalation between two people (droplet model), that would be too long. If the primary mode of transmission is more like smoke build-up (airborne model), any amount of reduction in virus concentration would help. It's pretty frustrating that a year and a half later we still don't know which method is the more common way for Covid to spread.
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It's more important to consider the total energy density, if the energy density is low it takes a long time, if the energy density is high the time is shorter.
In this experiment, the highest power scenario shows >99.9% viral inactivation after 30 mins, but to be fair even the high power scenario is relatively low power density.
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I supose a interesting aproach would be a closed box with very high density UVC lamps inside (with the assumption of more energy-less time to deactivate), and recirculate room air througth it. Being a closed box, should be "secure", it could even be used with a more energetic source than UVC too...
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We need more research and data like this to reduce transmission of all respiratory viruses. Vaccines alone won’t be enough; air quality and prophylactic treatments will be important tools that we have not yet focused enough on.
There’s a lot of architecture and societal changes that happened after the 1918 flu that make a lot of sense to me now (for example “cold dorms” that legally required students to sleep with the windows open year around). Buildings from 1920 often have tall windows for ventilation that buildings from 1890 lack. I wonder what changes we’ll see this time.
Yes. I'm interested in achieving a standard for household gadgets, too, because I'm not fully convinced that a plug-in fan becomes antiviral just by having a UVC bulb inside.