Some notes on “far UV-C” light

Can we use a new kind of UV-C light in occupied spaces?

In 1903, Niels Finsen was awarded the Nobel Prize for using UV-C light for disinfection, originally for lupus. Since then, UV-C has been used in tackling Tuberculosis and MRSA in hospitals.

Recent interest in UV-C for COVID-19 has involved disinfecting air (since there is potential for the virus to transmit via aerosols), and also for fast surface disinfection.

Historically, UV-C sources have mainly been 254nm lamps, a wavelength very good at destroying DNA/RNA. However, 254nm is not safe to use where people can see it, because it can hurt your cornea. While this kind of light will not make you blind, it hurts: it is described as a gritty feeling right after, with intense pain after that, until your cornea has a chance to regrow.

“Far” UV-C

Recently, work by David Brenner and colleagues at Columbia has included “far UV-C” light from 207–222nm, with 222nm the most promising.

• Far UV-C (222nm and below) is different than the 254nm tubes that most people are familiar with, mainly because it has the potential to be used to disinfect surfaces in occupied spaces. 254nm should never be used this way because of corneal damage.
• Extremely “far” UV-C makes ozone, which must be ventilated for safety. However, 222nm is better than 207nm because it does not create ozone, so increased ventilation is unnecessary if you filter wavelengths below 210nm.
• The potential for corneal damage using 222nm has not been fully tested, so it is not yet approved for use in occupied spaces. The current belief is that 222nm will not affect the cornea, and some installations are already being tried. Also, because far UV-C is absorbed before reaching the lens, it is very unlikely to cause lens/retina damage either.
• LEDs are not expected to be useful at 222nm, because the quantum efficiency of LEDs is extremely low in these ranges. Current LEDs in the UV-C range are typically 260–280nm.
• Today, only excimer sources can be used for 222nm. I do not know how to compare the cost of these sources to mainstream 254nm tubes, but it would be interesting to know.

Some additional notes about UV-C

• Many worry about skin cancer. However, skin cancer is mostly in the UV-B range, and so UV-C products do not pose a known skin cancer risk. The reason for this is that they don’t penetrate the outer “dead layer” of the skin, the epidermis. As wavelengths approach 300nm (e.g., some LEDs), we should worry more about this.
• Wall surfaces are not very reflective to UV-C, so any surfaces that are in shadow (requiring a “bounce” of light) cannot be disinfected using UV-C technology.
• Some people market UV-A LEDs as “safe” disinfection lamps. These sources can kill some kinds of bacteria, but they have no effect on viruses. Make sure to know the difference. If it’s not UV-C, it’s not going to inactivate viruses.
• Due to low reflection from walls and ceiling surfaces, “upper-room UV-C” can be used for occupied spaces, even with 254nm. In this installation method, louvers or reflectors are used to keep UV-C above eye level, and fans are used to circulate air. This method has a long history, and is used to safely “clean the air” and disinfect aerosols. Only a tiny amount of UV-C reflects off the ceiling, so it doesn’t hurt your eyes.
• To disinfect surfaces, a different installation must be used. Existing UV-C can be used to clean surfaces when rooms are not occupied. Also, occupied spaces may be able to be disinfected with “far UV” using the new 222nm lamps.
• However, due to issues with shadowing, surfaces that face away from a light source may still be an issue, and longevity of plastics and other materials may also be an issue as well.

For more information about UV-C disinfection (with actual experts), refer to the IES FAQ here.