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The 2014 outbreak of the Ebola virus and the recent spread of the novel coronavirus disease 2019 (COVID-19) has renewed interest in germicidal ultraviolet (GUV) lamps for disinfection. UV radiant energy was first used for disinfecting surfaces in 1877,1,2 for water in 1910,3 and for air in 1935.4 GUV’s use in recent decades has been largely limited in the U.S. to water treatment facilities and hidden (shielded) in heating and air-conditioning ductwork, or used in biological laboratories. GUV is being used in many countries to control the airborne transmission of tuberculosis (TB). In addition, some U.S. healthcare facilities are now using autonomous mobile units (“robots”) to add enhanced hygiene to patient rooms in order to reduce hospital-acquired infections. More widespread use of GUV is often limited by safety concerns, but these are manageable and minor compared to potential infection prevention. Most of the public is not aware of its unique value in the disinfection of air and contaminated surfaces. The frequently asked questions (FAQs) about GUV addressed in this document fall into these categories:
1. Basic questions
2. Medical and healthcare
3. Disinfecting room air with GUV
4. Disinfecting surfaces, masks, and instruments
5. GUV safety
6. Lamp technologies
7. GUV applications

1.0 Basic Questions

1.1 What is germicidal UV, and what is UVGI?
Germicidal UV (GUV) refers to using ultraviolet radiant energy to inactivate bacteria, mould spores, fungi or viruses. When the process is applied in a given location, it has generally been referred to as ultraviolet germicidal irradiation (UVGI). Because the public is concerned about ionizing radiation (e.g., X-rays and gamma rays), the term GUV avoids needless concerns about a link with that type of radiation. Another non-technical term is germicidal light, although “light” is technically only visible radiation.

1.2 Is all ultraviolet considered germicidal ultraviolet (GUV)?
No. Germicidal ultraviolet (GUV) – refers to short-wavelength ultraviolet “light” (radiant energy) that has been shown to kill bacteria and spores and to inactivate viruses. Wavelengths in the photobiological ultraviolet spectral band known as the “UV-C,” from 200 to 280 nanometers (nm), are the most effective for disinfection, although longer, less energetic UV can also disinfect if applied in much greater doses. UV-C wavelengths comprise photons (particles of light) that are the most energetic in the optical spectrum (comprising UV, visible, and infrared) and therefore are the most photochemically active. (See Figure 1.)

Figure 1-1. The ultraviolet portion of the electromagnetic spectrum.

1.3 Can UV-C kill viruses as well as bacteria?
Yes, UV-C kills living bacteria, but viruses are technically not living organisms; thus, we should correctly say “inactivate viruses.” Individual, energetic UV-C photons photochemically interact with the RNA and DNA molecules in a virus or bacterium to render these microbes non-infectious. This all happens on the microscopic level. Viruses are less than one micrometre (µm, one-millionth of a meter) in size, and bacteria are typically 0.5 to 5 µm.

1.4 Can UV-C effectively inactivate the SARS-CoV-2 virus, responsible for COVID-19?
Yes, if the virus is directly illuminated by UV-C at the effective dose level. UV-C can play an effective role with other methods of disinfection, but individuals must be protected to prevent UV hazards to the eyes and skin as elaborated in Section 4. UV-C should not be used to disinfect the hands!

1.5 Can near-ultraviolet (UV-A) lamps, such as UV insect traps, be used for GUV?
No. UV-A and longer (visible) wavelengths do not have germicidal effective emission wavelengths to inactivate viruses. Their relative disinfection capability is very minimal on the order of 1,000 times less effective in terms of fluence rate than the low-pressure mercury germicidal lamp. There have been only very special applications of wavelengths in the UV-A and violet (e.g., 405 nm), which require very high doses not practical in an occupied environment and were not recommended for viral sterilization. The trace amount of UV-B that is emitted from some white-light fluorescent lamps probably has similar efficacy.
Light-emitting diodes (LEDs) have been available for some time in the UV-A region. The advantage of UV-A or visible-light LEDs would be that they can easily be incorporated into LED-based luminaires, and there might be no need for protective gear. However, the efficacy of violet or UV-A energy that is not harmful to the skin or eyes is minimal.

1.6 What about UV-B lamps for GUV?
UV-B (280 to 315 nm), particularly the shorter wavelengths near 300 nm and below, can be relatively effective as a germicidal source, but in accidental exposures, there is a significantly higher risk for severe sunburn and even delayed effects for both skin and eyes, because UV-B penetrates the skin more deeply.

1.7 Does the ultraviolet in sunlight have a GUV effect?
Yes, particularly in the late spring and early summer when the sun is high in the sky and the UV index is high. At a UV Index of 10, the duration to achieve at least a three-log kill off bacteria (99.9% killed) is estimated as less than one hour.5

2.0 Medical and Healthcare Questions

2.1 How is the COVID-19 virus spread?
The official position of the World Health Organization (WHO) is that this virus is spread by contact with large respiratory droplets, directly or indirectly by touching contaminated surfaces and then touching the eyes, nose, or mouth. However, research is underway to determine the degree of airborne spread— meaning virus in particles so small that they remain suspended in the air. Such aerosol results from the evaporation of larger respiratory particles generated by coughs, sneezes, ordinary speech, singing, and possibly by faulty plumbing systems, as occurred with the severe acute respiratory syndrome (SARS) virus. How much of the virus responsible for COVID-19 is spread by the airborne route is not clear, but recommendations for healthcare workers to use fitted respirators, not surgical masks, reveal an official concern for airborne transmission. The possibility that inhaled virus may result in more severe lung damage than an acquisition by other routes—for example, via the mouth, nose, or eye—is currently being investigated.

2.2 How long do virus particles and bacteria remain airborne?
This is important, but difficult to answer simply and it depends on how the microbes were made airborne, e.g., from a sneeze or cough, or by being blown up from surfaces or dusted off clothes. The smallest particles (1- to 5-µm droplet nuclei) can remain airborne much longer than cough droplets—for many minutes or even hours.

2.3 How can airborne spread viruses be reduced?
Diagnosis of infectious cases and their isolation is a critical intervention, but transmission from asymptomatic persons is believed to play an important role in community transmission. In the U.S., the Centers for Disease Control and Prevention (CDC) has recommended that everyone wear non-medical face covers to reduce the spread of respiratory droplets, both large and small. Healthcare workers should wear well-fitted respirators designed to exclude airborne particles, in addition to following all contact precautions. For the airborne component, ventilation, social distancing, and other means of air disinfection are expected to have a role. Natural ventilation outdoors and in homes can be highly effective where conditions are optimal in terms of airflow and temperature. Mechanical ventilation can be effective, but 6 to 12 air changes per hour (ACH) are recommended in general for air disinfection or dilution.
Upper-room GUV air disinfection is a primary means of safe and highly effective air disinfection, provided it is planned, installed, commissioned, and maintained according to current international standards. A knowledgeable consultant is recommended. Room air cleaners, disinfecting air through HEPA filters, in-duct UV lamps, or other methods seem attractive, but their clean-air delivery rate, when converted to room ACH, is often trivial—no more than 1 or 2 added ACH. GUV in-duct air disinfection is a secondary approach to treating any recirculated air.


2.4 How does GUV work to disinfect air?
Commonly used GUV lamps generate predominantly 254-nm UV radiant energy, which is close to the peak germicidal wavelengths of 265 to 270 nm – both in the UV-C range, compared to the longer- wavelength ultraviolet (UV-A and UV-B) in sunlight. GUV radiant energy damages nucleic acids (DNA and RNA) by causing mutations that prevent replication, thus leading to the death of virtually all bacteria and inactivation of all viruses–both DNA and RNA types. Bacteria and viruses vary somewhat in UV susceptibility, with environmental organisms, fungal spores, and mycobacteria being relatively harder to kill than more rapidly replicating and non-environmental microbes and most bacteria. But even fungi are effectively killed with high-dose UV, which is used, for example, to treat fungal contamination of air conditioning systems. GUV can be most effectively used to disinfect air in the upper room where ceiling height permits, but can also be used in ventilation ducts and room air cleaners, as noted. As explained below, upper-room GUV is considered the most effective application for room air disinfection, where feasible.

2.5 Has GUV been useful in medical treatment facilities?
Yes. Some hospitals have used portable GUV fixtures to disinfect air and surfaces in unoccupied, locked rooms as a supplemental control measure to reduce the spread of healthcare-associated infections.6 However, well-controlled studies of efficacy are very difficult to conduct and therefore lacking. Medical treatment facilities are using GUV in three primary ways: 1) upper-room GUV fixtures with air mixing, for controlling airborne pathogens in an occupied space; 2) mobile GUV units, to disinfect high-touch surfaces; and 3) GUV in HVAC air handling units, to treat recirculated air and to reduce mould growth on cooling coils. Autonomous (“robot”) systems have been used in some U.S. hospitals and were used in the People’s Republic of China in response to COVID-19.7 In fighting a war, which this is seen to be, a single weapon is never used; rather, multiple weapons in the armamentarium are exploited.8 There is no reason not to make full use of GUV with appropriate precautions in this “war” against COVID-19.

2.6 Can whole-room UV-C effectively inactivate the SARS-CoV-2 virus responsible for COVID- 19?
While UV-C could be a secondary infection control measure for disinfecting potential germ-carrying deposits on accessible (not-shadowed) surfaces, its great value would be in disinfecting air in areas where this may be a concern (e.g., intensive care wards, hospital intake facilities [or tents]). Upper-air GUV is the safest, most effective application of UV-C. In special locations, where viral transmission is highly likely, whole-room UVGI (from suspended fixtures directing UV-C downward) could be applied, provided strict precautions can be followed. Any persons remaining in the space being disinfected from overhead and side UV-C lamps wear protective clothing and eye protection, or exposure to harmful UV must occur. Whole-room GUV has been safely applied in unoccupied rooms where entry is forbidden during the UVGI.

2.7 Does the CDC recommend GUV in healthcare facilities?
In the U.S., the Centers for Disease Control and Prevention (CDC) has provided guidelines for the use of UVGI lamps in upper rooms and air handling units (AHUs) as a supplemental control measure for air disinfection.9,10,11

2.8 How do research scientists determine efficacy for killing or deactivation of different microorganisms and viruses?
The most fundamental concept in photobiology is the action spectrum (or relative response) for a given effect. Although there is a standardized germicidal action spectrum in the IES Handbook,12 its was based on the inactivation of E. coli bacteria, and action spectra for spores, other bacteria, and different viruses can vary. This standardized action spectrum extends from 235 nm to 313 nm and peaks at approximately 265 nm. A wavelength of 254 nm has a relative efficacy of 0.85; by contrast, 313 nm in the UV-B has a relative efficacy of only 0.01.

Germicidal effectiveness is proportional to the exposure dose (radiant exposure, typically in millijoules* per square centimetre, mJ/cm2, or joules per square meter, J/m2), which is the product of the dose-rate (irradiance, typically in mW/cm2 or W/m2) and time (from 1 μs to several hours). A nonlinear relationship exists between UV exposure and germicidal efficacy. For example, if a certain UV exposure kills 90% of a bacterial population (frequently referred to as “one-log kill”), doubling the exposure time or intensity can kill only 90% of the residual 10%, for an overall germicidal efficacy of 99% (“two-log kill”). Likewise, a 50% decrease in dose or exposure time decreases germicidal efficacy only from 99% to 90%.

Humidity can reduce the effectiveness of germicidal UV radiation. There is a reference dose to attain a survival of 37%; however, in practice a GUV dose of interest is 3 or 4 log-kills, corresponding to 99.9% or 99.99% inactivation, respectively. To be effective in practice, achieving two log-kills (99% inactivation) is frequently accepted.

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