What to know about how UV light kills coronavirus

As more data about the spread of the novel coronavirus SARS-CoV-2 is compiled and tested, scientists have begun to state that Covid-19 spreads primarily through the air. Responses to curbing the pandemic will be shaped by these findings, such as the use of masks and social distancing. In indoor areas where the protection from masks and space to distance may be stretched to the limit, air purification is one more method that can reduce the risk of transmission.

There are a few different technologies out there that claim to remove the virus by passing air through a purifier of some kind. Some of these systems use a UV light to kill Covid-19. Let’s take a look at how effective of a strategy this might be.


What is UV light?

Much of what we cover in this next section is also covered in our post on how UV light purifiers work and our post on how they’re different from PECO.

UV light is an invisible form of light like infrared light. Since sunlight is composed of all spectra of light, the most likely place any of us will encounter UV light is outside. It is classified into three different bands- UV-A, UV-B, and UV-C, each with different exposure recommendations from the CDC:

  • UV-A light. Not absorbed by the ozone layer. Excessive exposure can cause skin aging. UV-A is emitted by common consumer items, such as tanning beds and decorative “black light” that makes posters glow fluorescently.
  • UV-B light. Partially absorbed by the ozone layer. It is thought to be mostly responsible for sunburn by causing direct DNA damage, and exposure also carries a risk of cancer. UV-B also stimulates vitamin D and serotonin production, though you only need a few minutes of sun each week. Pet reptiles who live indoors usually need a UV-B light in their cages to stay healthy.
  • UV-C light. Completely absorbed by the ozone layer. UV-C light does not penetrate far into the skin, but still carries enough energy to cause skin and particularly eye damage. In addition to being absorbed by ozone, UV-C is also absorbed by oxygen, and can convert oxygen into ozone. 

At ground level, we don’t have to worry much about UV-C. You’ve probably noticed that sunglasses and sunscreen advertise protection from UV-A and UV-B, because that’s what gets through the ozone layer. However, special bulbs and LEDs emit UV-C, and it can be used to disinfect by destroying the genetic material of viruses. Though it doesn’t penetrate our skin, UV-C can penetrate the extremely thin outer coatings of most microbes to sterilize them, effectively rendering them non-infectious.

Molekule’s PECO technology uses UV-A to power the catalyst. The inventor of PECO, Dr. Yogi Goswami, deliberately designed the catalyst to use UV-A precisely because there was no trace of ozone. Using UV-C to power the catalyst, as many other photocatalytic air purifiers do, adds the needless risk of ozone polluting the very air the purifier is cleaning.


UV-C disinfection on surfaces

When journalists, government agencies, or science publications mention using UV light to sterilize, they are almost certainly referring to UV-C unless they say otherwise. Many articles and papers discuss “far UV” which might be able to kill pathogens without harming us, but far UV is still in the UV-C band. 

Using light to disinfect has a few advantages compared to harsh chemicals. Chemicals can harm the very thing they are meant to clean. Using cleaning chemicals on electronics, for example, can damage their protective coatings. Alcohol is recommended by the CDC to disinfect, but can dissolve paint, plastic, or other labeling. UV light will disinfect without any chemical dissolution. In addition, chemicals can linger in the air or continually evaporate off of the surface they are used on. While some ozone may need to be vented after use of UV-C to disinfect, once the light is off the UV is gone.

It’s important to remember that while UV-C is effective in killing pathogenic organisms that linger on surfaces, it requires sufficient time and a close enough distance to do so. Also, tiny spaces or cracks in a surface can shield microbes from the light. Experienced professionals who use UV light to disinfect healthcare settings are trained in its use so they get into every nook and cranny, but as consumers we should be careful when using any UV sterilization devices. A company that sells UV-based devices should have research that demonstrates their effectiveness.


UV-C disinfection in the air

Using UV-C to purify the air is trickier since microbes have to spend some time in front of the light before they will die. This approach makes sense when preventing mold growth on air conditioning coils or inside ducting where any mold growth would be exposed to the light for an extended amount of time. Though we know that the very bright UV in sunlight will reduce viable viruses in the air, it doesn’t work the same indoors.

Reducing the indoor risk of viral infection with UV is challenging since the virus needs a sufficient dose of UV to be killed as it speeds through the purifier. The EPA notes that microbes must be exposed to the light “on the order of minutes and hours rather than the few seconds typical of most UVGI air cleaners.” The CDC only recommends UV disinfection in ductwork or pointed at the ceiling in infectious wards, and cautiously warns they are no replacement for an air filter. 

Few air purifiers are UV-only, so almost all come along with at minimum a fabric filter like HEPA paired with some carbon. Often the UV is shining on the exit-side of the fabric filter so any viruses are killed. 

Be skeptical if a consumer air purifier claims to use UV to protect from viruses. If the manufacturer just states that UV is included without any data to back it up, then it’s probably not worth buying.


Reducing viral exposure

UV light seems to be best utilized on mold, bacteria, and their spores where they tend to linger or try to colonize surfaces. Using UV to kill coronavirus in the air hasn’t been shown to be more effective than the simple mechanical filtration already in use.

Active systems like Molekule’s PECO technology meet FDA criteria for reducing viruses in the air in healthcare settings. In laboratory testing, 99.9994% of the viruses that landed on our filters were sterilized. Be sure that if you’re going to use an air purifier to reduce your risk, that the manufacturer provides performance information on how well it does so.

Learn more about how to deal with the current pandemic situation in our How to Live in a World with Covid-19 blog, or our What is the Best Mask? blog.

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