Guest post by Leah Scott

As long as the threat and reality of pandemic-level super viruses continue to wreak havoc, the topic of germicidal lighting for indoor disinfection is bound to come up. Before blasting surfaces with ultraviolet (UV) light, however, there are some important things to consider and a few facts.

First, viruses cannot really be ‘killed’ by light because they are not really alive, rather they must be deactivated or suppressed. Therapeutics are developed to suppress viruses inside a living host but to truly stop viruses from being transmitted through surface-to-human or airborne cross-contamination, requires a different application. In a recent webinar from the International Association of Lighting Design (IALD), Center director Dr. Robert Karlicek, Jr., explains the process of ‘deactivating’ the RNA or DNA (a protein structure that acts as the organism’s messaging system) of viruses, which can be done by using specific UV irradiation applications.

UVC radiation, which can deactivate viruses, can also be very harmful to living things, not just the ‘bad viruses’ being deactivated. It can also damage surfaces resulting in accelerated degradation of materials such as plastics, wood, or fabrics over a long exposure period. With rapid progress on the development of powerful UVC LEDs, new UVC system form factors, and potential for pulsed operation may offer advantages for UVGI system operation not easily supported by UV lamp technologies, making customized UVGI applications, device power, environmental impact, and lifetime limitations less of an issue.

With the growing interest around coronaviruses (i.e. SARS CoV2 and COVID19), interest in UVGI has also substantially renewed. To address some of the most pressing and common questions, Karlicek answers the top 10 most asked questions around UVGI for germicidal disinfection:

1. Can UVC radiation kill SARS CoV 2?
Yes, several groups have demonstrated that UVC radiation can quickly deactivate the SARS CoV 2 virus that causes COVID-19. To be effective, however, the delivery dose needs to be high enough and the required dose depends on several environmental factors (surface or airborne, relative humidity, and other environmental factors) that impact the delivery of UVC radiation to the virus’ RNA. The higher the dose, the faster the process, and the greater the percentage of virus deactivated.

2. Is 405 nm disinfection technology effective for deactivating viruses (including SARS CoV)?
The short answer is no. While there is some research that shows an effect of 405 nm light (as well as UVA radiation), the mechanism requires the presence of other molecules that absorb at 405 nm or UVA radiation to generate reactive oxygen compounds, that, in turn, attack the virus. It is a slow, concerted process that is not worth considering for virus deactivation systems.

3. Can UVC LEDs be used for germicidal applications?
Absolutely, and there is considerable published evidence for the effectiveness of UVC LEDs in germicidal applications including SARS CoV 2. Also, the output power, reliability and cost-effectiveness of UVC LED solutions are continually improving with continuing research on the design and manufacturing of UVC LEDs. Note that both UVC mercury lamps (254 nm) and UVC LEDs have lifetimes that are considerably shorter than LEDs used in solid-state lighting, so that should be factored into the design of GUV systems using either technology.

4. Is Far UVC (222 nm) effective in deactivating microbes and viruses?
Yes, 222 nm excimer lamps are effective in deactivating microbes and viruses. Like any UVC radiation, the required 222 nm dose for a given desired deactivation will depend on the pathogen and the environmental conditions. (See Question 1)

5. Is Far UVC germicidal radiation safe for humans?
A significant amount of research on 222 nm excimer lamp (so-called Far UVC) disinfection impact on possible damage to animal tissue (skin and eyes) suggests that it may be safe for humans. While this work is encouraging, it is the opinion of some in UV industry groups and several photo-biologists that more research is needed, in particular with long term exposure. The safety argument is simple: at these UVC wavelengths, the radiation cannot penetrate human skin or eyes to cause damage to human DNA. Of course, this presumes that all human skin exposed to 222 nm radiation is intact (not abraded wounded or diseased, etc.). For the time being, it is advisable to use 222 nm systems only when humans are not present.

6. Do UVC sources produce ozone?
Not if properly selected. Some mercury lamps are manufactured specifically for generating ozone by using specialized quartz tubing transparent to radiation below 200 nm, so care must be exercised in selecting mercury lamps for use in UVGI applications that cannot generate ozone, but still emit 254 nm radiation efficiently.

7. How can UVC sources be used safely around people?
So long as humans are not in the direct line for exposure to the UVC radiation there should be no issues. Ordinarily, UVC systems are used only when people are not present or in disinfecting air (either inside of ductwork of HVAC systems or with specially designed optics to irradiate the upper portions of room-air) with little or no radiation to the people below. Some UVC lamps come with presence detection systems that turn off when persons approach, but these systems will have to have very low false-negative error rates (turning on when the system falsely thinks that people are not present).

8. Does UVC radiation damage other materials that it irradiates?
Many types of materials will slowly degrade with extended exposure to UVC materials. The type and extent of damage depend on the materials exposed. If prolonged UVC exposure of interior surfaces in the built environment becomes commonplace, more research on the development of UVC resistant paints, fabrics, carpeting, and furniture surfaces will be needed to ensure an adequate service life of the exposed surfaces.

9. How do you measure UVC irradiance and dose?
UVC irradiance is measured in watts per unit area (typically W/cm² in the US) and dose is calculated by multiplying the irradiance by the exposure time in seconds to get energy per unit area (typically joules(J)/cm²). Care needs to be used in selecting and using UVC radiometers as they are typically accurate over different ranges of UVC wavelengths, and may or may not accurately measure UVC irradiance coming into the sensor from an angle, and periodic re-calibration of these types of radiometers is important to ensure that adequate germicidal dose.

10. What regulations or agencies regulate the application of UVC sources for germicidal application?
Various agencies have oversight of some aspects of UVGI systems and applications. The FDA’s Q&A report on UVC disinfection is one and also describes when FDA has regulatory responsibilities for UVC devices sold as medical equipment. For UVC devices sold as electronic equipment, regulatory control is less clear.