Saturday, February 10, 2018

New Twist On An Old Method To Kill The Flu Virus

Right after posting the previous article on the latest confirmation that influenza virus is airborne, I came across and article in Nature that had me thinking back to my childhood.  I remember walking into an insurance office on Main Street in our small town.  There was something strange about the environment.  Up next to the ceiling were ultraviolet lights.  The lights were shielded so that they only reflected up toward the ceiling.  I asked my parents what they were and got the answer: "They are there to kill germs."  My head was spinning from that answer: "There are germs in the air? They are up next to the ceiling? What kills the germs that are down here next to me?" Yes - I was a neurotic little kid.

Over time I learned a little about the nature of ultraviolet light, especially that it could cause eye damage if you looked right at it. As I got into the 1970s, the hippie era, and psychedelia that because less important.  There were UV lights everywhere - blacklight posters and the detergent residues in clothing phosphorescing white light after it has been activated by UV light.  In some environments everyone was bathed in UV light.

Today most Americans are aware of UV light because of sunscreen and eyeglass applications.  Long and medium wavelength (UVA and UVB) and not absorbed by the ozone layer.  It is recommended that glasses block 100% of the UVA and UVB for maximum eye protection.  That can also be designated as UV400 because they block all UV light from 280-400 nm.  The part of the UV spectrum is also important in sunscreens.  UVA penetrates the skin to a deeper level and is responsible for damaging keratinocytes, cataracts and causing premature aging.  UVB is responsible for burning and carcinogenesis.  UVA and UVB are considered both carcinogenic and carcinogenic.  Even those UV light has been known to be germicidal for over 80 years that human toxicity has limited the application.

UVC (100-280 nm) is blocked by the ozone layer and therefore is not a consideration in either eye or skin protection.  It is considered to be the part of the spectrum that is potentially germicidal and that is where the latest application begins.  In this report the authors used filtered 222-nm light sources in an experiment to see if they could inactivate aerosolized H1N1 influenza virus.  They were able to accurately measure the light dose and estimate virus inactivation using an epithelial cell model that measured infected cells by fluorescence.  The authors aerosolized the virus into a UV irradiation chamber.  The chamber had a total volume of 4.2 liters and had a characteristic particle distribution of 87% < 0.3 - 0.5 μm, 11% 0.5 - 0.7 μm, and 2% > 0.7 μm.  Those are characteristic particle distributions of airborne droplets that occur with breathing, talking, and coughing.

An air flow of 12.5 L/min through the chamber was noted and they calculated that this meant a single droplet passed through the chamber in about 20 seconds.  I think that is significant because it in unlikely in a typical building that a person would be standing in an air current moving that quickly. In other words, if the aerosolized virus can be inactivated in an airstream moving that quickly - it might have practical applications in most environments.  The authors were able to construct a dose response curve showing that at a dose of 2 mJ/cm2 viral survival is negligible.

I found this to be extremely impressive work because it clearly shows that airborne influenza virus can be inactivated using a far-UV source that is much safer to humans than previous germicidal UV sources.  Furthermore the sampling and intervention characteristics seem to be very realistic in terms of what might be encountered in public facilities.  The real question seem to be whether any commercially available air cleaner/purifiers come close to matching the characteristic of this experiment.  A preliminary search of these devices shows that the airflow characteristics are typically not listed, very few use far-UVC light sources (most use germicidal 254-nm sources shielded in the device), and none are certified in terms of how much virus they kill. They typically suggest that germicidal UV light is all that is needed for air purification.  There is also the question of whether using a device in your office at work confers any degree of protection once you leave that office and start walking down the hallways.  My speculation is that it would not, but the amount of virus generated in your office may be a significant variable.

The authors themselves suggest that if their results are confirmed far-UVC represent a significant opportunity to limit the transmission of airborne disease and that it could be widely used in medical offices and buildings as well as public areas where disease transmission is common like airports and airplanes.

I am hoping that this areas of research yields rapid results and broad implementation.

George Dawson, MD, DFAPA 


1:  David Welch, Manuela Buonanno, Veljko Grilj, Igor Shuryak, Connor Crickmore, Alan W. Bigelow, Gerhard Randers-Pehrson, Gary W. Johnson, David J. Brenner.  Far-UVC light: A new tool to control the spread of airborne-mediated microbial diseases.  Scientific Reportsvolume 8, Article number: 2752(2018).  doi:10.1038/s41598-018-21058-w

Graphics Credit:

Table 1, Figure 1, and Figure 2 are all used from reference 1 per the Creative Commons Attribution 4.0 International License.


  1. On NBC News tonight they did a story on a company that was making a robot that was using UV light to sterilize hospital rooms. It said it was more effective than other methods because it penetrates everywhere.

    1. I asked an expert if there are any commercially available air purification units that have metrics that would allow comparison with this experiment.

      The problem as noted above if you are in any hospital, clinic, or public building - the minute you leave your office you are walking into a cloud of airborne particles. The floor cleaning equipment has to be a major source of airborne contamination, even if you avoid the guy who sneezed in front of you in a hallway an hour earlier.