Intro to Far-UV
Wouldn’t it be nice if there was a lightbulb that you could install, it wouldn’t have any bad effects on people and still be able to kill airborne pathogens, like viruses?
We’ve been using UV for over a century to kill pathogens, but the standard UV that is used with low pressure mercury lamps (254 nm) can hurt people if they are exposed. You can’t just shine it in an occupied room. A newer way of producing UV has opened up new possibilities to create safe indoor spaces.
What is Far-UV?
We are most familiar with UV light that comes from the sun. UVA is light with wavelengths from 380–315 nm and is responsible for skin aging. UVB is light between 315–280 nm and can cause sun burns and skin cancer. Both of these can penetrate deep into your skin.
UVC consists of light between 100–280 nm. It is more energetic than UVA or UVB, so it can cause damage to DNA, but it does not penetrate that well, so does not pose a significant skin cancer risk to people. All the UVC light from the sun gets blocked by the atmosphere and does not reach the surface. Within the UVC band, there’s a special range between 200–230 nm which has been called far-UV (it’s further away from the light spectrum than UVA, UVB and much of UVC). Light from this range has special properties which opens up new possibilities to fighting off airborne diseases.
What’s Special About Far-UV?
UVC light (including far-UV) can be used against viruses. When used this way, it is usually called ultraviolet germicidal irradiation (UVGI) or germicidal ultraviolet (GUV). Far-UV is special because of how it interacts with human skin and eyes. Despite having enough energy to damage DNA, Far-UV does a really poor job at penetrating anything. As soon as it hits a surface, it stops almost right away.
For your skin, you have an outer layer of dead skin called the stratum corneum which is 10–30 µm thick. It will absorb a lot of the far-UV light so it won’t penetrate to the live tissue beneath.
Your eyes are also protected by a tear layer that is around 3 μm thick. This also helps block a lot of the far-UV light.
Because you have outer layers protecting your eyes and skin and far-UV is so poor at penetrating those layers, your body can handle a much higher dose of light between 200–230 nm than it can for other wavelengths of UVC light. For the most common UVC light at 254 nm, the threshold limit value (TLV) for the dose on your body and eyes is 6 mJ/cm². For far-UV, the TLV for your eyes is 160 mJ/cm² and your skin TLV is 479 mJ/cm². So the TLV from far-UV is 27 x higher than UV 254 nm on your eyes and 80 x higher on your skin.
A mathematical model compared the effects of far-UV light and sunlight on people’s skin. On the top skin layer, 10 minutes of sunlight is equivalent 300 hours of exposure of Ushio Care222 lamps and 30 hours of exposure to Sterilray lamps (used in Woods et al.). During an 8 hour day, using unfiltered Sterilray light, it’s equivalent to 2.5 minutes in the sun for the outer skin layer and 6 seconds in the sun for the basal layer which has the higher risk of cancer. For the filtered Ushio Care222 lamp, one 8 hour day’s exposure is equivalent to 15 seconds in the sun for the outer layer and less than 1 second of exposure for the basal layer.
At this dose, you can design a far-UV system to illuminate the whole room while occupied, provide a very high rate of virus inactivation and keep the exposure levels below the TLVs. This cannot be done with any other wavelengths of light. This is why far-UV is the most powerful tool we have against airborne diseases.
How effective is it?
A team of scientists performed an experiment where they filled a room about the size of a typical room in your house, with aerosolized Staphylococcus aureus. They then turned on 5 15 W far-UV lamps and the concentration dropped by 98%. They calculated that this is 128–322 equivalent air changes per hour. Using 1 15W lamp gave 33–66 equivalent air changes per hour.
These values were also confirmed with mathematical modelling showing you can reach greater than 100 equivalent air changes per hour with far-UV.
For a single 15 W fixture, this equates to around 1000–2000 m³/h, 600–1200 cubic feet per minute or 300–600 liters per second or equivalent clean air. For comparison, a box fan Corsi-Rosenthal box is 300–500 CFM, about half as effective.
There are two caveats when applying this to the real world. First, Staphylococcus aureus is much more difficult to inactivate than SARS-CoV-2, so it is possible that the equivalent air changes per hour for SARS-CoV-2 will be much higher than what is reported. Conversely, this experiment did not use respiratory aerosols which can be larger than the aerosols used here. It is unclear how much of an effect this will have. It can still be assumed based on this experiment and previous research that far-UV will be effective.
To compare far-UV effectiveness with other air cleaning methods, minimum ventilation requirements are usually around 2–3 air changes per hour, portable HEPA filters can usually provide 2–5 equivalent air changes per hour and upper room UV can provide 20–30 equivalent air changes per hour. All of these methods provide far less than 100 equivalent air changes per hour or more which is only achievable with far-UV.
How is it Created?
Currently, the main way far-UV light is created is through Krypton-Chloride (KrCl) excimer lamps. These lamps primarily release light at 222 nm (within the far-UV band), but there is some light outside of that band that is also produced. Wavelengths that are longer than 230 nm can more easily cause skin or eye damage and wavelengths below 200 nm can produce ozone and other byproducts at higher rates, so companies often include a filter to only allow wavelengths between 200–230 nm to be exposed to the room.
Where can you buy it?
The biggest supplier of far-UV lamps is Ushio, but it is sold by their partners found here. Another supplier is Eden Park. DBA Light Solutions sells products sold by UV Can Sanitize in Canada, First UVC and Quantadose in the US. While the previously mentioned companies sell filtered far-UV lamps, Sterilray sells unfiltered lamps.
I am not endorsing any of these products or manufacturers.
Personal Far-UV
There are products that have come out like the Lily (2 W), X-One (3 W) and Mobile Shield (10 W) which are portable and designed to be placed close to a person and disinfect the air before it is inhaled.
The experiment showing the effects of far-UV on a chamber indicates that a single device at low power reduced bacteria concentrations by only 13%, indicating it is not that effective. This would indicate that the Lily and X-One are not effective layers of protection when used in a typical room in your home. A single lamp at medium power was found to be more effective and reduced the bacteria concentrations by around 70%. This publication indicates that the fluence rate from the Mobile Shield is approximately 3x higher than the Lily (Hexagon) or X-One. It is more likely to be effective but it is still unclear how effective it will be.
However, these devices are not primarily designed to disinfect the whole room. The goal is to only disinfect the air close to the occupant with a short exposure time. I’m unsure about the irradiance values for the Mobile Shield, but I have done tests on the Lily (also sold as Hexagon) and X-One. At 40 cm away, which is a typical distance between a table and your nose, the irradiance values can be between 10–50 μW/cm². Assuming a 90% inactivation dose of 200 μJ/cm², a SARS-Cov-2 virion would need to be exposed for 4–20 seconds. I think you would inhale the virus within 4 seconds of it coming in contact with the far-UV light, so I don’t think there is enough contact time to inactivate the virus effectively.
If there are computational fluid dynamics models showing a high rate of inactivation using personal far-UV devices or if an experiment is performed showing these devices are effective, then they can considered as one of the methods to provide non-infectious air and protection from airborne diseases. I would not recommend using it as a personal device until there is evidence of its effectiveness.
Safe Use
Threshold Limit Values
As stated previously, the TLV for eyes is 160 mJ/cm² and for skin is 479 mJ/cm². If you are installing a lamp, you should know what the irradiance is at different distances. I have measured the irradiance for the Lily and the X-One. The values for the Carnation/Round Tile can be found here. Here are the irradiances for the Delphi lamps. I tested the industrial model and was not impressed because it required a noisy fan and adjustments to a timer instead of a simple on/off function like a light bulb.
Once you know what the irradiance is, the dose = irradiance x time in seconds. So if you are going to be 40 cm away from the 5W Delphi module, the irradiance is 48 μW/cm². If the maximum dose on your eyes is 160 mJ/cm², you need to make sure that you do not exceed exposure for more than 160000/48 = 3333 seconds, or about 1 hour. If the lamp will not be facing you or you have eyewear protecting your eyes, you will reach the TLV in 479000/48 = around 10000 seconds, or 3 hours.
I’ve tested different types of glasses and eye protection and all were very effective at stopping far-UV light. If you are wearing glasses and as long as the far-UV light is not bypassing around them, you can use the skin TLV of 479 mJ/cm² when far-UV light is shining at your face.
Whenever setting up these lamps, I recommend finding the irradiance from the manufacturer at the distance you intend to be, multiplying it by the expected exposure time during the day and ensuring you remain below the TLV.
Skin & Building Microbiome
Your skin has a microbiome consisting of bacteria, fungi, viruses and mites. It is good for you and helps prevent the growth of harmful pathogens. Constant exposure of your skin to UV light can affect the skin microbiome. However, showering, bathing, exposure to sunlight, disinfectants and anti-bacterial hand soap can all also affect the skin microbiome. Furthermore, people are mostly clothed, so much of the skin will not be exposed to far-UV light. It is currently not known the effect of exposure to far-UV would be, but there is a basis to have little concern, especially if the exposure is not constant every day.
Buildings also have a microbiome which helps prevent the growth of harmful pathogens and this can also be detrimentally affected by the use of far-UV.
However, when it comes to concern about the microbiome, this could also apply to anti-bacterial hand soap and the use of electrostatic sprays, fogging and deep cleaning and these methods are often used for disease mitigation. The extent of this concern is unclear.
Ozone
At the beginning of the COVID-19 pandemic when people were discussing far-UV as a possible solution, there were discussions about ozone generation. It seemed the consensus was that it isn’t a concern. For example, this publication came to the conclusion that “the contribution of the main 222-nm wavelength to the ozone generation is usually negligible.”
While ASHRAE did not have a clear policy about byproducts from far UV, their documents on UV lamps, resources on filtration and disinfection, and position on filtration and air cleaning all indicate that ozone is a concern for UV wavelengths below 200 nm, but they never state a concern above 200 nm. For example, their position on filtration and air cleaning states “A variety of UV light sources can be used in PCO, including black lights (UV-A: long-wave; 400 to 315 nm), germicidal lamps (UV-C: short-wave; 280 to 200 nm), and lamps that generate ozone (vacuum UV [UV-V]: under 200 nm).” It only states the UV-V can generate ozone. Far-UV, which is in the range between 200–230 nm was not explicitly listed as an ozone concern by ASHRAE.
However, recently there have been multiple publications which can be found here and here which directly measured ozone generation from far-UV lamps. The conclusions were that the ozone generation was significant and should be considered when designing a whole room far-UV system.
Safety Issues Conclusion
Because these issues are still not well known, caution is still warranted with far-UV. Upper room UV, while not being as effective, can still provide 20–30 equivalent air changes per hour without directly exposing occupants to UV light, so it is currently a reasonable alternative. However, in small spaces and spaces with low ceilings, upper room UV is not an option, so far-UV might be the only possibility to get a very high disinfection rate and provide a high level of protection.
In my opinion, for personal residential use, far-UV can be considered, but should only be used in high-risk situations and should be accompanied with ventilation (open windows) and filtration to mitigate air pollution from far-UV. When used in this way, the benefits likely outweigh the risks.
For commercial use, ASHRAE 62.1–2022.5.9.1 requires that “air cleaning devices shall be listed and labeled in accordance with UL 2998 (zero ozone)”. ASHRAE 241–2023.A.1.4.1.1 also requires testing for formaldehyde and particulate matter. I personally believe that all air cleaning equipment should now be required to comply with ASHRAE 241 before being allowed. That same standard can be applied to far-UV in commercial spaces. To my knowledge, there are currently no far-UV lamps that have UL 2998 certification. Consequently, no far-UV devices would currently be allowed in commercial spaces based on ASHRAE 62.1.
Why don’t we have it everywhere?
While far-UV can be used effectively and within recommended threshold limit values (TLVs), this is still a relatively new technology. Technologies like upper room UV and HEPA filters have been around for over 80 years. We’ve been using ventilation for centuries, but far-UV has only been used in the past couple of decades. It has great potential, but as discussed, there is still a lot we don’t know. It currently is also pretty expensive, for example many of the 15 W fixtures by Ushio (the most well known company) are around $2000-$3000 and would likely require replacement every year or two. Because of the unknowns and the fact that we have a safe and effective alternative with upper room UV, the Ontario Society of Professional Engineers Indoor Air Quality Advisory group does not recommend against far-UV. However, they recommend that there should be more data about its use before widespread implementation. The CDC also recommends the use of upper room UV in occupied spaces, but advises caution with the use of far-UV and that it should be treated as an emerging technology (see FAQ #7).
