Thoughts on Light Disinfection – The Timing of Hype

Posted: September 20, 2020 in Light and Health, Uncategorized
Marketing vs. Reality

There is now a great deal of noise from marketers promoting the use of light as a disinfection tool. The current COVID19 Pandemic has fueled this effort and created a vehicle for many to roll out claims and campaigns that have varying degrees of relevance.

If you listen casually, you would be led to believe:

  1. That all light disinfecting products are effective against a viral outbreak
  2. That LEDs are the primary choice and driver behind all light disinfection
  3. That LEDs are effective in deactivating viral contamination
  4. That there is hard scientific proof of the numerous claims being made.
The Differences in Contamination Types

To sort through all of the hype and well timed marketing campaigning, one must understand that there are two basic types of contamination being addressed:

  1. Bacteria, which are single cell living organisms that are transmitted through the air, from surfaces and from person-to-person contact. These contaminates can live in water, dirt, food, plants, the human body, etc. Bacteria are very large compared to a virus, with the smallest being around 0.4 micron. Bacteria can move, make food, live and multiply within an infected host, often confined to a single localized area, where they make a home, and reproduce. The bacteria and the toxins (waste) they produce is what makes one sick. Bacteria are what medical professionals prescribe anti-biotics to help the body to fight off and kill, by strengthening the immune system at the time of infection. One can “Kill” a bacteria, as it is a living cell.
  2. Viruses are non living cells made from proteins and DNA/RNA, that floats through the air, or is carried through fluids on surfaces. When the virus enters a body through casual means, the virus (about .02 to .25 microns), it simply flows into the host system, until it is collected by a cell that causes the outer shell of the virus to open, releasing the DNA/RNA inside. This re-programs the cell to reproduce the virus, which eventually explodes the cell, shedding the new virus packets, which make their way through and eventually out of the host, to be seeded into other hosts around the primary. Virus’s cannot be “killed”, as they are not living. However, Viruses can be de-activated by exposing them to agents or environmental conditions that cause their structure to fail.
Implications of Light, Surfaces, Human Exposure

Hard surfaces that are easily exposed to UVA, UVB, and UVC light can be effective for disinfection, or reducing growth over time. However, porous surfaces, surfaces with dirt or other covering contamination, are less easily addressed. UVC light, for instance, cannot penetrate very deeply into a surface, and since very short wavelength light is absorbed by most surfaces, there is no indirect reflected component, meaning that the only surface that UVC can be effective on are those directly irradiated.

Exposure to UV light has varying degrees of impact on the human body. UVA (320 to 400nm) light can penetrate deep into skin, causing aging and other dermal irritation, and can damage the eye, cause cataracts, burn the retina, etc. UVB (280 to 320nm) is what causes sunburn and can cause retinal damage, as well as dermal cancers. UVC (200 to 280nm) can cause damage to DNA in exposed cells. However, the shorter the wavelength, the shallower the penetration, so UVC exposure is not as damaging to skin, although it can cause photokeratitis at the longer wavelength end of the spectrum (<225nm).

For surfaces, The shorter the wavelength of light, the more damage the light will do to surfaces. Short wavelength UV light (<300nm) will fade dies used in fabrics, surfaces and paint, break down plastics, reduce the integrity of natural materials (cotton, wool, leather, rubber, etc.).

For purposes of discussion here, exposure to UVA light should be controlled and limited to avoid causing skin damage, and potential harm to the eyes. The longer the wavelength used, the better – however, this will reduce the effectiveness for disinfection, which will require a higher dose level through intensity or time, or both, so their may be a point of diminishing return between protecting occupants and reducing infections.

Exposure to UVB light should be avoided completely, as this is the region that causes the greatest harm to humans. There is literally no form of UVB radiation that could be considered acceptable for human exposure at levels necessary for disinfecting surfaces.

Exposure to UVC requires consideration of wavelength. At one end (230nm to 280nm UVB margin), the same precautions against human exposure should be taken as one would with UVB. However, at the 200 to 225nm range, exposure is reasonably safe.

How Bacteria Contamination Responds to Light

Bacteria are very sensitive to light. Short wavelength energy, below 415nm to 222nm, will “kill” the living bacterial cells, if exposed to a high enough intensity, for enough time. The longer the wavelength, the greater the intensity and time required, but generally, light disinfection does work, with a few limitations. The primary issue is exposure levels and time, and shadowing effects, coupled with distance. The light must be of high enough intensity, delivered long enough, to produce a decontamination effect. In general, UVA is not used as the primary source of decontamination, but a supplement to other processes (chemical), to increase efficiency and lengthen time between treatments.

How Viruses Respond to Light

One of the reasons viruses are so infectious, is that within interior, artificially illuminated spaces, there is little that will cause them to break down, giving them a long effective period on surfaces and in the air. Outdoors, these cells break down quickly, from exposure to UVB and UVA light from the sun. Viruses are not effectively deactivated by UVA irradiation (>320nm), as was shown in several studies, such as ‘Inactivation of the coronavirus… SARS-CV’ 2004. , even at wavelengths as short as 365nm. Longer wavelength light can be expected to be even less effective. UVC light is an effective means for deactivating viruses in an amount of time practical of application as a disinfection method. However, UVC light does not transmit through most transparent or translucent materials, nor does it reflect from most surfaces, so this means that the only surfaces that will be disinfected are those directly irradiated.

Since very short wavelength UVC radiation (<230nm) is relatively safe for human exposure, it is entirely possible to use it in occupied spaces. Gary Trott, in an interview with Craig Delouie, ‘Acuity’s Gary Trott on Filtered Far UV-C Disinfection’ covered this very well, when describing the use of filtered UVC light sources in disinfection practice against viruses. There are other studies supporting this, like the ‘Far-UVC light 222nm) efficiently and safely inactivates airborne human coronoviruses’, 2020.

The use of any UVC light can be used to decontaminate air as it circulates overhead. As long as it is not directed at occupants of the space, the limits on wavelength are not as critical.

LEDs in the Picture

For UVA, there are many LED products and sources available for generating the wavelength and irradiation energy required to disinfect or supplement other disinfection protocols.

For UVC, at this time, 222nm is not a wavelength that can be provided by LEDs. At this moment, the shortest LED wavelength commercially available is 275nm, which should never be used when human occupants are present. Experimental products are down to 255nm so far, but not yet available. It will likely be some time before an LED product will be available for 222nm application. This means that light sources used for the shorter wavelength require some filtering, generate some heat, and use more energy.

Another effective strategy for deactivating viral contamination, is heat. Temperatures above 65C (150F) also deactivate the virus. This may be a more effective strategy for loose items and object disinfection than light, as heat suffers none of the shadow/reflection/absorption issues, and penetration of porous surfaces that light does. For fabrics, the combination of heat and peroxide gas seems to be the protocol of choice, since light cannot penetrate into contaminated fabric layers effectively.


Regardless of the timing of marketing by various degrees of scrupulous product manufacturers, the difference between products effective against bacterial and viral contamination are literally night and day. What works for bacteria is not effective against viruses. However, the short wavelength or filtered UVC does work against both, but may not be safe for human exposure.

You can readily assume that if the product is LED based, and noted as safe for human exposure, that it is likely in the <400nm range, which is not going to have any effect on COVID contamination. If the LED based product is advertised as effective against COVID viral contamination, you can assume it is not safe for human exposure. If it is stated as “filtered UVS” with a stated emission of 222nm, it is both effective against COVID viral contamination, as well as most bacteria.

In other words, be careful when reading manufacturer marketing claims, know what the product can an cannot do, by investigating the promises made.

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