UV Disinfection Confusion

The pandemic situation has generated a lot of interest and activity in UV disinfection, which has been keeping me busy. Whether it’s for masks, air, surfaces or whatever, there are lots of things getting posted and promoted for using UV. There seem to be an overwhelming number of devices and designs being suggested or sold online. Unfortunately there are also a lot of misconceptions, errors and possibly fraudulent claims being promoted. I’m not going to try and address each and every device (there are too many!), but I can provide some basic ideas that one should know or ask about when considering UV devices. If the supplier can’t readily provide answers or details, then something is possibly wrong. Here are a few key confusing points:

It uses UV to disinfect: That’s a starting point, but what type of UV? Like visible light, UV light comes in different “colours” (you can’t see UV, so I use the term “colours” loosely). There is UVA (tanning UV from sunlight or tanning lamps), UVB (also from sunlight and some lamps), and UVC (germicidal UV from lamps, but NOT from sunlight). The UVA and UVB versions are largely useless for rapid disinfection, so only UVC should be considered. Unfortunately, some UVA devices like nail polish hardening lamps or flashlights are being promoted, and they won’t realistically work for disinfection or killing germs/viruses. Waste of money. (It’s fairly easy to tell the difference between UVA/UVB and UVC lamps if you know how. The UVC bulbs are clear glass, and the others are coated.)

UV can kill coronavirus: true, if it is UVC. UVA and UVB, not so much. It’s not surprising that UVC kills coronavirus or any other germ for that matter. UVC affects the DNA and RNA in cells, and since all pathogens contain DNA and/or RNA, then it should work. Human cells also contain DNA, which is why exposure to UVC is not a good idea, as it damages them too.

UV can kill 99.9% of germs: this is a common claim, and among the most problematic. The key word here is “can” which is not the same as “will“. UVC will only kill 99.9% of germs under very specific conditions, primarily related to the “dose” of UVC that hits the germ. This “dose” is related to three important things: 1) the lamp power, 2) the distance from the lamp to the disinfection target, and 3) the time that the UVC is applied. Often the advertisements don’t mention any of this, or it is buried in the fine print. If the system is not used in exactly the right way, you may not get anywhere near 99.9% disinfection. Suppliers should provide instructions with these details so that you can ensure you get the disinfection level you are looking for.

Laboratory proven: this can be nice information to have, if you can get it from a supplier. However, I’ve seen lab tests that don’t prove anything about the actual intended application, because the tests weren’t designed to do that. They might show 99.99% disinfection of some pathogen, under conditions that don’t exist when you actually use or install the device. Interpreting lab results takes some expertise unfortunately.

Scale matters: this is especially true for air cleaners that employ UVC. The device might get 99.9% disinfection of air going through it, but if the device is too small for the room then it won’t be doing much useful disinfection overall. As an example, think about putting a single desktop air cleaner in a hockey arena (for example the PPG Paints Arena in Pittsburgh). Would you really expect it to have any measurable impact on the air quality in that huge volume? The same idea goes for putting a low power lamp in a huge duct for HVAC, or a single ceiling mounted UVC lamp in a large room. The point is, the UVC lamp design needs to be carefully matched to the scale of the intended use, as well as other variables like occupancy and air recirculation rates. Unfortunately, some of the pictures I see in news items seem to be more for show than a properly designed and scaled UVC system.

The key message is that selecting and specifying a UVC disinfection system takes some technical experience and insight, otherwise it may not work and will be a waste of time and money. If equipment suppliers aren’t willing to help out with this, and won’t provide technical details or good independent lab studies to prove their claims, then that’s a bad sign.

2 thoughts on “UV Disinfection Confusion

  1. Hello Bill,

    I have stumbled upon an interesting article where you were featured.


    I am working on a project that involves UVC light to kills Sars-Cov-2, MRSA, e.coli (we ran tests), and the like and I was wondering if we can use your expertise and knowledge in this domain.

    We have some results that show efficacy of 4-6 log reduction in 20-30 seconds of exposure.

    How much expertise do you have in UV-C 265-275nm area.
    We target this wavelength as it shows the best efficacy in killing these viruses.

    Looking forward to hearing from you.

    Thank you for your reply and have a great day

    Best Regards,

    Mike (Mahluf)


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