Beyond the Hype: The Buzz on Using UV for Disinfection
By David Cyr, LC, MIES, IALD, Lilker Lighting Group Director
Using ultraviolet (UV) for disinfection, or “UV germicidal irradiation” (UVGI) is older than you may think. A patent for a germicidal low-pressure mercury source was issued in 1886, products were used for water purification as early as 1904, and UV sources have been used for decades within mechanical air-handlers and ducts to disinfect cooling coils, drain pans, and air. Recently, the germicidal properties of UV have been a hot topic for discussion.
What is UV?
Technically, UV is not light: it describes a portion of the electromagnetic spectrum outside what humans see, composed of three main wavelength bands: UV-A (or near-UV or blacklight, 315-400 nanometers (nm)), UV-B (middle-UV, 280-315 nm), and UV-C (far-UV, 200-280 nm). The human visual system is stimulated by the portion of the spectrum from 360-780 nm. This is what we call light. Beyond that range lies infrared, or heat.
Figure 1: The electromagnetic spectrum, indicating UV and human visual range; image from Consulting Specifying Engineer magazine
The sun emits a spectrum containing all types of UV, but the Earth’s atmosphere blocks UV-C radiation. A wide portion of UV has been shown to have germicidal properties, but dosage – wavelength, intensity, and exposure and duration—are key.
How does UV ’kill’ germs?
UV doesn’t technically kill germs, it inactivates them – UV damages their DNA (or RNA as is often the case in viruses) which stops them from replicating.
Figure 2: Disruption of bacterial DNA by fusion of adjacent thymine (in yellow) bases. A similar process occurs through uracil in viral RNA; image courtesy of the Lighting Research Center
Over time, the germs’ cells die and break down with photodegradation. In some microbes, UVGI can cause secondary reactions within the cell membrane that affect survival.
For germicidal applications, UV-A and UV-B require very long exposure times to be effective and work only on bacteria—not on viruses. UV-C can kill both bacteria and viruses.
Is UV harmful to people?
The harmful effects of UV-B are well known: exposure can cause skin irritation, sunburn and skin cancer. Sunlight works for UV disinfection because of its high intensity of UV-A and UV-B.
UV-C in any dosage is harmful to people. Short term exposure to UV-C can burn your corneas and skin. Depending on intensity and length of exposure, damage can be temporary or permanent. Even minor UV-C damage from which we can heal usually doesn’t hurt during exposure but can be very painful afterward. Some sources may espouse the safety of UV-C sources, but their use requires careful application and maintenance to avoiding harming people.
Figure 3 – Photobiological effects on humans; image from the Illuminating Engineering Society of North America (IESNA)
In general, if it’s not harmful to people, it’s probably not very harmful to germs. The use of high-intensity UV-C for surface disinfection is well established for hospital rooms, airplanes, and train cars (See Figures 4 and 5), but should only be used in unoccupied spaces.
Figure 4: Mobile UVGI device being tested for in use New York City subway car; image from Metropolitan Transit Authority (MTA)
Figure 5: Mobile UVGI device in use in airplane cabin; image from germfalcon.com
UV-C can be safely used for upper-air disinfection in occupied or unoccupied spaces.
Figure 6: Upper-air disinfection using UVGI in an unoccupied hospital space, image from the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE)
Currently available sources of UV-C for UVGI should not be shined in the direction of people, period. Depending on distance and intensity, harmful exposure can occur in seconds to minutes. Research with some parts of the UV spectrum (far UV-C of about 222 nm) has promise to be safer for people and tough on germs, but applications outside of the lab are at least several years away.
Effects of UV on the built environment
Beyond harmfulness to people and microorganisms, UV exposure can also cause damage to architectural surfaces, causing finishes to fade and some materials to become brittle and break down. Materials and surface treatments need to be carefully considered in environments where extensive UV treatment for disinfection is considered. We’ve all seen fabrics fade and some plastics turn yellow and hazy in sunlight. Contributing to this is the fact that UV doesn’t reflect well off most surfaces (you may have heard “UV doesn’t bounce,” technically, it does, but it’s often negligible or only off certain materials). Most finishes absorb the majority of UV instead of reflecting it. If you shine UVGI at a ceiling, it doesn’t bounce off and treat every surface in the room. Polished, uncoated or specially-treated aluminum reflects UV well, but oxidation affects this over time. Water also reflects some UV; we all know that sunburns are worse when you’re in water at the beach. If you can see shiny reflections from a UV-irradiated material, it may be hitting you too.
What are the limitations of UVGI?
UV can be great for disinfecting transparent media such as air and water since it travels through them. For solid surfaces, UV requires direct line-of-sight to work. Any surface that is in shade or is not exposed is not being disinfected. Wrinkles or folds in clothing –and in face masks—block the UV rays. In textiles, the weave of fabric also blocks UV from getting through to threads below the surface.
Many architectural lighting companies have products that can include near-UV (on the upper border of UV-A) in fixtures for disinfection. To be effective, such products basically have to be on all the time and are considered useful in healthcare applications to ’kill’ bacteria that are a major source of hospital-acquired or secondary infections.
Dosage and spectrum
In photochemistry and photobiology, the relative effectiveness of different wavelengths to produce a defined response —the “action spectrum” —is critical. Effective use of UVGI depends on the target pathogen, each potentially having different unique sensitivities to wavelength as well as dosages required for inactivation. Since UVGI generally acts on a microorganism’s DNA (or RNA), the action spectra for DNA is typically of note.
Figure 7: Germicidal UV Irradiation Action Spectrum Graph – e.coli bacteria; image from IESNA
Typical UVGI sources use mercury (Hg) to produce UV radiation, which has a wavelength emission peaking at about 254 nm. The concept of reciprocity factors into dosage: exposure can consist of high intensity for a short time or low intensity for a long time, and responses from different types of bacteria and viruses can vary. The penetration depth of UV rays is important – depending on the material properties of surfaces being disinfected, UVGI may not get through or may need more time to be effective. Also, consider that dampness may allow germs to transfer through a material. The action spectrum and dosage (intensity and duration) for inactivation of the COVID-19 virus is not yet known.
Types of UVGI sources
Sources used for UVGI typically consist of a sealed tube full of pressurized gas, mostly commonly mercury. When energized, electric current passes through the tube, exciting the vaporized gas and resulting in emission of UV radiation.
Figure 8: UVGI tubes; image from kitchenventilation.com, halton.com
This is similar to how a fluorescent tube works, except that a standard fluorescent tube has an internal phosphor coating adding red and green content to the blue glow of mercury vapor, producing white light. The tubes are part of a category of sources known as discharge lamps. UVGI sources can use mercury (Hg) or other gases—often halogen or noble gases such as xenon (Xe), Krypton (Kr), chlorine (Cl), argon (Ar), fluorine(Fl))—individually or in combination under different levels of pressure.
Developing technologies are being tested using Kr-Cl excimer lamps, sources whose gas combination produces a dominant emission at about 222 nm, which have shown to produce significantly less skin and eye damage in lab settings than conventional Hg sources at 254 nm. While not yet commercially available, they show promise as safer sources for UVGI treatment. Light-emitting diodes (LEDs) have been developed that emit UV-C in the range of 265 – 280 nm. Sources in this range exhibit increased risk of skin and eye damage over that of 254 nm Hg sources and currently have very low efficiency. Given the pace of LED development and adoption for general illumination, trends suggest that specialized LED sources may quickly progress and lead to more finely tuned wavelength emissions and increased efficiency, making them more effective for UVGI.
With regard to maintenance, UV sources deteriorate and lose effectiveness over time. For example, many conventional sources only last about one year before replacement is recommended. Also, the output of gas-filled UV sources, or discharge lamps, is affected by temperature, which must be factored into system design.
Part of a cleaning regimen
UVGI can be a part of a cleaning regimen, but should not be the only measure taken. Conventional cleaning is still the most effective way to combat germs. UV-C works well for upper-air and in-duct disinfection, but only cleans the air, not room surfaces, and it doesn’t do much if someone sneezes on you.
Mobile UV-C units can be great. Recent uses in the news include subway cars, airplanes, and hospital rooms, but again, spaces must be unoccupied when in use, dosage matters, distance between UV source and treated surface matters, and they don’t treat the ’light’ can’t see.
Figure 9: Mobile UVGI unit in an unoccupied hospital room, image from UltraViolet Devices, Inc.
Handheld UV-C devices must be used cautiously, with care taken to maintain recommended distance and exposure duration, and avoid irradiating sensitive objects, people and pets. An advantage of UVGI over chemical disinfectants is that it does not leave any surface residue after treatment. There is no drying time, no waiting period for chemicals to dissipate, and no toxicity after the treatment is complete.
To date, commercially available UV-C fixtures have not been designed with aesthetics as a key consideration. Picture a big, boxy bug zapper. It is likely that UVGI products will start to be designed to look better and blend with the architectural environment as demand increases.
How do I decide what’s best for my project?
Like the specification of other building systems and products, it’s important to take a holistic approach. Coordination between a full project team, including architect, MEP engineer, lighting designer, contractor, estimator, commissioning agent, etc. is critical to achieving a well-developed, optimized solution that works together with other systems and meets established design and performance goals. This will ensure that UVGI dollars are well spent on an effective solution relative to other disinfection methods.
Sources: Bahnfleth, W.P. (2020, April 21). Reducing Infectious Disease Transmission with UVGI. American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). In ASHRAE Learning Institute Webinar Series. Bergman, R. (2020, May 7). Sources for UVG [Webinar]. Illuminating Engineering Society of North America (IESNA). In Germicidal Ultraviolet Radiation in the Days of COVID-19 Webinar Series. Forbes, P.D. (2020, May 7). Action Spectra: Source of Guidance Regarding Photocarcinogenesis [Webinar]. Illuminating Engineering Society of North America (IESNA). In Germicidal Ultraviolet Radiation in the Days of COVID-19 Webinar Series. Guse, C. (2020, May 3). MTA to Use Ultraviolet Lights to Kill Coronavirus on NYC Subways, Buses. New York Daily News. Retrieved from https://www.nydailynews.com. Jensen, P.A. (2020, May 7). Whole Room and Other UV-C Applications [Webinar]. Illuminating Engineering Society of North America (IESNA). In Germicidal Ultraviolet Radiation in the Days of COVID-19 Webinar Series. Nardell, E. (2020, May 7). Advances in Germicidal UV Application: Reducing the Spread of COVID-19 [Webinar]. Illuminating Engineering Society of North America (IESNA). In Germicidal Ultraviolet Radiation in the Days of COVID-19 Webinar Series. Rea, M.S. & Bierman, A. (2020, May 14). Germicidal UV Lighting. Lighting Research Center (LRC), Rensselaer Polytechnic Institute (RPI). Rea, M.S., A. Bierman, & D. Carr. (2020, May 12). UV: Beyond Red, White, and Blue [Webinar]. Lighting Research Center (LRC), Rensselaer Polytechnic Institute (RPI). In Live from the LRC Webinar Series. Saputa, D. (2020, May 7). UV-C In-Duct Applications [Webinar]. Illuminating Engineering Society of North America (IESNA). In Germicidal Ultraviolet Radiation in the Days of COVID-19 Webinar Series. Sliney, D.H. (2020, May 7). GUV Photobiology Basics & Safety: An Introduction [Webinar]. Illuminating Engineering Society of North America (IESNA). In Germicidal Ultraviolet Radiation in the Days of COVID-19 Webinar Series. Vincent, R. (2020, May 7). Upper-Room UVGI [Webinar]. Illuminating Engineering Society of North America (IESNA). In Germicidal Ultraviolet Radiation in the Days of COVID-19 Webinar Series. What are UltraViolet lights in kitchen hoods used for and what outcome can I expect when using them? (2019, September 26). Kitchen Ventilation: Knowledge by Halton. Retrieved from https://kitchenventilation.com. Additional references: American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). http://www.ashrae.org. GermFalcon. http://www.germfalcon.com. UltraViolet Devices, Inc. http://www.uvdi.com.