Scientist Johann Wilhelm Ritter first discovered ultraviolet light in 1801. He determined that it was invisible electromagnetic radiation at wavelengths between 10-400 nm. In addition to UVA and UVB, which come to us from the sun, a third type, UVC, has the potential to disinfect spaces. Here are the most frequently asked questions.
Is UVC light harmful to humans?
Yes. UVC rays penetrate the skin and the eyes and can cause significant damage in just a few seconds. Similarly, the waves are absorbed by microbes and quickly destroy the genetic material and protein-based outer covering, rendering them inactive. Scientists saw this weakness as an opportunity to use UVC light to kill viruses and bacteria and disinfect both objects and the air.
How is UVC light produced?
UVC can be produced on earth by low-pressure mercury discharge lamps, sometimes referred to as mercury vapor lamps. The lamp was invented, exactly 100 years after the discovery of UV light, in 1901 by American engineer Peter Cooper Hewitt. The invention was quickly applied to industrial uses and used for water disinfection by 1910. Improved lamps, which used less mercury, were in use by the 1930s.
So, wait. We are already using UVC light to disinfect?
Yes. It’s been used for decades. It is especially good for places that are otherwise difficult to clean, like water, air, and many surfaces. Some places are using it to sanitize air within HVAC systems or in unoccupied rooms or other spaces like buses, trains, and planes. It is being used near the ceiling of even occupied hospital rooms. Along with a fan system, all of the air is exposed to the sanitizing light. One Seattle-area restaurant is reportedly using it. One Canadian biotech company has stations to allow visitors to sanitize their phone and keys upon entering their offices. Some of you may even have a UV water filter in your home. UVC light has even been used against other coronaviruses and has even been shown to kill SARS-CoV-2.
So what’s the holdup? Why isn’t UVC light being used everywhere?
There are a few barriers to putting UVC light to work disinfecting air and surfaces all over the world. The specific wavelength of UV light is very important. One set of viruses and bacteria can be resistant to one wavelength while another set is sanitized by a different wavelength. The wavelength best suited to disinfect SARS-CoV-2 is not yet known for sure.
The second challenge is that UVC light disinfects only what it sees. So if something is in shadow or some of the virus is hiding under a layer of dirt, it will not be neutralized without a significant exposure and a number of angles. Remember, at the microscopic level, even the fibers of an ordinary surgical mask can cast a shadow on a virus. For these reasons, it is difficult to know when a surface is clean. One scientist compared it to painting an invisible brush.
Finally, remember, there’s some safety concern. UV light, including UVA and UVB rays, which cause sun burns and aging, is considered a carcinogen. UV light can also be harmful to the cornea, which is part of the eye. The damage could cloud the lens or cause tissue to grow on the eye surface, which can limit vision. That’s why it can’t be used continually in most cases. And the area is only clean until a new person or object is introduced.
So what’s next?
There is some exciting research happening. A recent study has shown UVC light at 222 nm both inactivates airborne viruses (including SARS-CoV-2) and may be safe to use in occupied spaces. It has been shown to be safe for both skin and eyes in animal test subjects. However, more research is still needed to verify that people can be exposed to this wavelength over periods of time without harmful side effects. Scientists are also working to perfect the old mercury vapor lamps into more sophisticated UVC-emitting LEDs.
It should be noted that all of the research efforts spike when a new microbial threat emerges and subsides once we have all moved on to other things. So UV light research being done now will not likely be ready until our next pandemic.
In the meantime, beware the fakes. Lots of companies are rushing to provide UV lights to consumers for home use. Some are dangerous. Many others are simply useless. Some claim approval from the FDA or the EPA, but no such certification actually exists. (The FDA produced a fact sheet explaining many aspects of UV light and SARS-CoV-2, including that a device may be approved by its “Electronic Product Radiation Control Provisions” without having been shown to kill coronaviruses.) At this time, most devices that are both effective and safe cost upwards of $1,000 and are marketed to hospitals and labs. Don’t be fooled by inexpensive ($100 — $200) machines that claim to disinfect your home with UV light.
The proven cleaning methods remain alcohol and bleach wipes. The proven precautions remain keeping 6 feet from others, wearing a mask, washing your hands, and socializing outdoors and for short amounts of time.
Marie George, MD, of Southwestern Vermont Medical Center, is an infectious disease specialist.