Not medical or professional advice. See full disclaimer near page bottom.
What if there were light rays that disinfect air and surfaces without harming humans? Sounds like science fiction? Amazingly, the technology already exists: special ultraviolet rays known as 222nm far-UVC. This may be a golden opportunity to help us beat this pandemic, prevent future pandemics, to reopen the country, to restore normalcy, and to protect our loved ones. It may reduce transmission of not just coronaviruses but other pathogens that hitch rides in the air and on surfaces.
As reported by the Wall Street Journal and Columbia University, emerging science is uncovering far-UVC’s astonishing efficacy and safety. Produced by krypton chloride excimer lamps, far-UVC rays with wavelengths around 222nm were experimentally shown to kill a variety of pathogens (including coronavirus strains) but are blocked by stratum corneum of skin and teary layer of eyes. Its broad spectrum efficacy is no surprise — its cousin conventional 254nm UVC disinfection has been an industry standard for decades. What is surprising is its safety. Its tolerable dose on skin and eyes appears to be 10–100x higher than conventional UVC in mouse studies.
The advent of 222nm far-UVC may be as profound as the discovery of antibiotics.
Science
Effective disinfection of air and surfaces (including coronaviruses)
Studies suggest far-UVC is as or even more effective than conventional 254nm UVC at inactivating various pathogens including human coronaviruses [Ushio; Columbia 1,2]. Efficacy is pathogen dependent and measured in terms of the dose needed to achieve a specified reduction (e.g. 99%) of that pathogen. Far-UVC dose [mJ/cm2] equals intensity [mW/cm2] (optical power per unit area) at the target surface multiplied by irradiated time [s]. Most microorganisms roughly follow a logarithmic (log) kill curve. For example, if 1 mJ/cm2 inactivates 90% (1 log kill) of a bacteria species, then 2 mJ/cm2 would approximately 99% (2 log kill). Thus, most inactivation happens at the beginning with exponentially diminishing returns with increasing time and accumulated dose.
Safe on skin and eyes at germicidal doses
Studies suggest that far-UVC poses minimal threat to skin and eyes while being lethal to unshielded pathogens [Ushio; Columbia 1,2]. Safety is measured in terms of threshold limit value [mW/cm2], denoting the maximum dose that humans should be exposed to within a time interval (e.g. a day). These exposure limits are recommended by the American Conference of Governmental Industrial Hygienists (ACGIH®) or the requirements of IEC 62471. The ACGIH TLV around 222nm is 20 mW/cm2 (hasn’t been recently updated), whereas for around 254nm it’s 3.1 mW/cm2. In light of studies in the last 5 years, the TLV for 222nm is likely to be revised up significantly, perhaps as high as 200 mW/cm2. More long term human safety studies are needed. However, the current conservative TLV of 20 is still more than 10x the 1-log-kill dose of many pathogens, suggesting a safe and effective role for far-UVC even within current regulations.
Engineering
Far-UVC source: bandpass filtered krypton chloride excimer lamp
A KrCl excimer lamp produces a broad spectra containing energy at multiple wavelengths. We only want its far-UVC output, which can be defined as the power of emissions within the 200nm-230nm range. Other emissions at longer UVC wavelengths (e.g. 250nm-280nm) as well as UVA and UVB ranges should be filtered out because they do damage skin and eyes. This is achieved via a bandpass filter coating on the quartz glass. Ushio’s products have this, but many other OEM excimers do not!
Buyers caveat: look for optical power, not electrical power
An excimer’s wattage rating is a terrible standard because it doesn’t correspond with the total far-UVC output power — the prime determinant of efficacy. In fact, a typical KrCl excimer lamp converts anywhere from 0.1% to 10% of electrical power to far-UVC output.
Optical intensity depends on angle and diminishes with distance
UV dosing at a point in space depends on its distance and angle from the source. At more than a meter away the intensity decays approximately 1/r² . Many lamps also have a reflector making the output directional, which is maximum orthogonally facing the aperture and decreasing at oblique angles. The beam angle quantifies this angular range, typically spanning the half maximum intensity angles.
Example
A far-UVC lamp with electrical rating of 20W claims optical intensity of 0.1mW/cm2 at 0.5m in the 200nm-230nm range. A coronavirus strain shows 90% reduction (1 log kill) at a dose of 1mJ/cm2. The daily threshold limit value (TLV) for humans at 222nm UV is 20 mJ/cm2 but can potentially be as high as 200mJ/cm2 as suggested by animal studies. Please calculate the following:
- Far-UV intensity at 2m (6ft) away in front of the lamp
- Time needed for 99% reduction in the coronavirus strain in an aerosol particle 2m (6ft) away
- How long can a person stand in front of the lamp at 2m away before TLV is reached
- Total far-UVC optical power
- Power efficiency in converting electrical energy to far-UVC photons
Solution
- Far field irradiance falls off with r^-2
0.1mW/cm2 x (2m / 0.5m)^-2 = 0.06 mW/cm2 - 99% is 2 log kill
2 x 1mJ/cm2 / 0.06 mW/cm2 = 320s - 20mJ/cm2 / 0.06 mW/cm2 = 3200s
200mJ/cm2 / 0.06 mW/cm2 = 32000s
At germicidal doses one can be around the lamp for almost an hour or (if animal safety studies translate to humans) as much as 10 hours daily. - Integrate the intensity over a sphere. As a rough estimate, we only use the peak intensity and account for the beam angle of 120deg by dividing by 3. If we knew the lamp’s angular radiation function, we can evaluate the integral more exactly.
0.1mW/cm2 x (4 x pi x (0.5m)²) / 3 = 1W - 1W/20W = 5%
Power conversion is inefficient. It also varies significantly between manufacturers so the total electrical wattage is a terrible measure.
Economics
As of winter 2021 germicidal far-UVC is expensive: ~$1000 for a 10W room lamp. Upstream in the supply chain, far-UVC excimer light sources have traditionally been specialty industrial and scientific products, with niche suppliers and limited volume. Sterilray was among the first startups couple years ago to integrate excimer light sources in full assembly germicidal lamps . It saw exploding demand during the pandemic but the excimer supply hasn’t caught up.
Making far-UVC available en masse hinges upon scaling up supply of krypton chloride excimer light sources. This is easy and fast if we invest in the technology now! Excimer manufacturing is practically identical to manufacturing fluorescent bulbs! Simply replace the gas mixture, use quartz glass, tweak the electrodes. Krypton gas is used in very small quantities during manufacturing and widely available. The rest of the lamp including the power supply and mechanical assembly are trivial to manufacture at scale. Of course it takes a few months and some capital to retool assembly lines — though substantially easier compared to drug development.
References
[WSJ] Mims, Christopher. “What It Will Take to Make the Indoors Feel Safe Again.” https://www.wsj.com/articles/what-it-will-take-to-make-the-indoors-feel-safe-again-11591966803
[Columbia 1] Buonanno, M., Welch, D., Shuryak, I. et al. Far-UVC light (222 nm) efficiently and safely inactivates airborne human coronaviruses. Sci Rep 10, 10285 (2020). https://www.nature.com/articles/s41598-020-67211-2
[Columbia 2] Welch, D., Buonanno, M., Grilj, V. et al. Far-UVC light: A new tool to control the spread of airborne-mediated microbial diseases. Sci Rep 8, 2752 (2018). https://www.nature.com/articles/s41598-018-21058-w
[Ushio] Care222® Filtered Far UV-C Excimer Lamp Module. https://www.ushio.com/files/specifications/care222-filtered-far-uv-c-excimer-lamp-module.pdf
Disclaimer
Not medical or professional advice. The author(s) strive for the best but can’t make guarantees of accuracy. Read and use the information presented at your own risk and discretion. The author(s) are involved in developing far-UVC.
