There is a wealth of information available about UV light, much of which can be lengthy and hard to digest. We wanted to bring you the basics in a quick 7-minute read. This blog post provides a quick overview, from definitions to light sources, germicidal UV applications, and safety concerns.

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WHAT IS UV, GUV, UVGI?

The discussion and research surrounding ultraviolet (UV) light in relation to disinfection has been a recent hot topic due to the COVID-19 pandemic. However, the origins of the study of light to kill bacteria date back all the way to 1877 when physiologist Arthur Downes and scientist Thomas P. Blunt performed an experiment exposing solutions that would typically produce bacteria to sunlight. They found that the intensity, duration, and wavelength of the sunlight all had an impact on the growth of bacteria1,2.

Further research has also demonstrated that not all UV light is created equal. UV light ranges from 100nm to 400nm in wavelength and the properties of the light vary significantly along that wavelength. Wavelengths closer to the low end of the UV spectrum are known as UV-C (100 – 280nm). They are shorter and have higher photon frequency and energy. Conversely, the upper end of the UV spectrum is known as UV-A (315 – 400 nm) and has longer wavelengths with lower photon frequency and energy. Wavelengths in the middle of the spectrum are known as UV-B (280 – 315 nm).

When we think about UV light, many of us think about the sun. However, the sun only emits about 10% UV wavelengths, 95% of which is UV-A light and 5% of which is UV-B light3. However, it is the UV-C spectrum which had been shown to be most effective in inactivating bacteria, mold spores, fungi, or viruses, especially in the range of 240-280nm4,6. This use of the UV radiant energy has been coined germicidal ultraviolet (GUV). When this is applied in a specific location, it is referred to as ultraviolet germicidal irradiation or UVGII6.

So, how does it work?  In short, the spectral sensitivity of DNA is most responsive to UV-C wavelengths, meaning that they can break DNA so that the cells can no longer replicate. If you want to read in greater depth about the photobiological action spectrum and its impact on DNA and RNA, the CIE has published a detailed paper here.

WHAT LIGHT SOURCES ARE USED FOR UVGI?

Low pressure mercury lamps have been around since 1901. These lamps are very similar to a fluorescent lamp, but they differ in two respects:  (1) they are made of either fused quartz or a special glass that transmits 253.7 nm and (2) they don’t have a phosphor coating, which blocks out UV wavelengths. They are relatively inexpensive and efficient, with 85% of their radiant power in the 253.7nm range (near the peak of germicidal efficacy)5. These lamps are frequently seen used in in-duct applications, upper air disinfection, as well as some newer products advertised for surface disinfection.

Medium pressure mercury lamps also emit in the 253.7nm range but are less efficient compared with low pressure mercury lamps, as they have a broader spectrum range with some light in the UV-B spectrum. They do, however, have much higher output power which makes them a good source for water disinfection5.

Xenon arc lamps for UVGI applications typically use pulsed light, which greatly increases the density of wavelengths in the UV-C range, though they also emit a broad spectrum of light. They are more practical for applications where rapid disinfection is needed in unoccupied spaces7.

Krypton-chlorine excimer lamps are a more recent trend in UVGI because of this study from Columbia University. They emit in the range of 205 – 230nm. A couple studies have shown promise for a high deactivation rate of some bacteria and viruses as well as reduced impact on human skin and eyes compared with the 253.7nm mercury lamps. However, there are limited products on the market as further study is still needed6.

LED sources are being developed that can emit in the UV-C region around 265-270nm6, though consumers and specifiers should be extremely cautious with these products as several make claims without the data or studies to back them up. The IES photobiology committee anticipates more LED products will be introduced into the market in the future.

TYPES OF UVGI

Upper-room GUV is primarily used for air disinfection. It is typically used in rooms with higher ceilings (at least 10-ft) and uses a UV-C fixture mounted high on the wall (above 7-ft) to irradiate the air above the occupied space. The UV-C emissions are intended to stay above head height to minimize human exposure and safety concerns. It is most effective when combined with mechanical air mixing via fans or HVAC.

In duct applications, with UV-C lamps mounted in HVAC ductwork or inside air handling units, is most effective for filtering air after it leaves a space and, therefore, is not an effective method of irradiation of air within a room. This type of GUV is also used for surface disinfection of the cooling coil and drain pan. This helps prevent the development of biofilm over time8.

Portable units or autonomous robots are currently being used is some applications for surface disinfection in unoccupied spaces. These units can be more effective compared with a fixed light source as they move around a room covering surfaces that might not be reached from a single-fixed position. However, surface disinfection does not replace the need for manual cleaning as UV-C may not penetrate residues and will not disinfect shadowed areas or surfaces beneath an object or covering6.

IS UVGI SAFE FOR HUMANS? MATERIALS?

When it comes to human safety, the short answer is that GUV lamp emissions can pose a safety and health risk. The primary concern is for potential hazards to the eyes and skin. It is important to follow safety rules for GUV lamps/light sources.

That said, the safety risks vary depending on the light source, wavelengths emitted, and type and length of exposure. UV-B emissions pose the greatest risk for the skin due to deeper skin penetration. UV-C still poses a risk although it does not penetrate the skin as deeply as the UV-B range, but is more likely to result in mild temporary surface conditions such as erythema (or sunburn)6.

Like with humans, UV can have a harmful effect on materials, including plastics, paints, air filters, and plants. It has been shown that the shorter wavelengths (UV-C), which are more efficient at inactivating bacteria, also are more likely to accelerate aging of materials6.

WRAPPING THINGS UP

Though UVGI has been around for a long time, there are a lot of new products being introduced to the market due to the COVID-19 pandemic. Be sure any product you consider has adequate testing, peer-reviewed studies and data, and spectral power distribution charts available. And finally, remember UVGI is not for all applications. Be smart about where, why, and how you use it.

Sources

  1. DOWNES, A., BLUNT, T. The Influence of Light upon the Development of BacteriaNature16, 218 (1877). Available from: https://www.nature.com/articles/016218a0.pdf
  2. Hockberger, Philip E. The discovery of the damaging effect of sunlight on bacteria. Journal of Photochemistry and Photobiology B: Biology 58, 185 (2000). Available from: https://www.researchgate.net/publication/12097536_The_discovery_of_the_damaging_effect_of_sunlight_on_bacteria
  3. IARC Working Group on the Evaluation of Carcinogenic Risk to Humans. Radiation. Lyon (FR): International Agency for Research on Cancer; 2012. (IARC Monographs on the Evaluation of Carcinogenic Risks to Humans, No. 100D.) SOLAR AND ULTRAVIOLET RADIATION.Available from: https://www.ncbi.nlm.nih.gov/books/NBK304366/
  4. Gupta, Asheesh et al. “Ultraviolet Radiation in Wound Care: Sterilization and Stimulation.” Advances in wound care 2,8 (2013): 422-437. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3797459/
  5. CIE Technical Committee 6-35. Technical Report CIE 155:2003 (2003). Available from: http://files.cie.co.at/cie155-2003%20(free%20copy%20March%202020).pdf
  6. IES Photobiology Committee. IES Committee Report CR-2-20-V1a (2020). Available from: IES https://media.ies.org/docs/standards/IES%20CR-2-20-V1a-20200507.pdf
  7. Wang T, MacGregor SJ, Anderson JG, Woolsey GA. Pulsed ultra-violet inactivation spectrum of Escherichia coli. Water Res. 2005;39(13):2921-5. Available from: https://www.sciencedirect.com/science/article/abs/pii/S0043135405002125?via%3Dihub
  8. Martin Jr., Dunn, Freihaut, Bahnfleth, Lau, Nedeljkovic-Davidovic. Ultraviolet Germicidal Irradiation Current Best Practices (2008). Available from: https://www.ashrae.org/file%20library/technical%20resources/covid-19/martin.pdf