There are a number of dedicated meters available; however, a wide range of scale is normally required, e.g., a range from 0.1 to 100 microwatts per square centimeter (μW·cm-2).25 Safety readings require the lower range, and efficacy requires a range up to at least 10 mW·cm-2. A common practice is to have two calibrated meters: one in reserve and for reference. The two instruments should be periodically compared. The user should retain the manufacturer’s instructions, including a description of the meter, its safe use, and maintenance and calibration of it. Some healthcare facilities contract with a full, outside maintenance contractor that uses calibrated meters and correctly and safely replaces burned-out lamps. Some users retain a simple, less precise meter for staff to use, but the installer uses a professional meter.

There are very sophisticated programs to calculate the lamp sizes and in-air dose requirements in terms of energy required for space and radiant fluence (joules per square meter, J/m2) across a cross-section of a UV-C beam, but there is a much simpler evidence-based dose that has been developed over many years for TB control, typically specified as about 17 mW of 254-nm lamp-emission radiant power per cubic meter (m3) of space to disinfect air.13 Although this sounds too simplistic to be true, since air in any room is always moving and mixing, one can correctly assume that all air will be treated—the better the air mixing, the sooner this will happen. Studies at the Harvard School of Public Health13,14 and elsewhere show log units of reduction equivalent to 24 ACH to achieve 80% reduction of transmission. Of course, 100% reduction is not possible, because of the multiple modes of transmission. To disinfect surfaces, this depends on the type of surface and its cleanliness; recommended exposures vary from 200 to 1,000 J/m2 (20 to 100 mJ/cm2).

Lamp technologies include continuously emitting low- and medium-pressure mercury lamps, as well as pulsed xenon arc lamps. Studies have shown that these technologies—continuously emitting or pulsed—are comparably effective for disinfection. Pulsed sources may be more practical if rapid disinfection is required. 21 Light emitting diodes (LEDs) and krypton-chlorine excimer lamps, which emit in narrow bands in the germicidal range (UV-C), are emerging technologies.

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 spread by 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.

This is important, but difficult to answer in a simple fashion 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.

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 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 official concern for airborne transmission. The possibility that inhaled virus may result in more-severe lung damage than acquisition by other routes—for example, via the mouth, nose, or eye—is currently being investigated.

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. 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 mold 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. In fighting a war, which this is seen to be, a single weapon is never used; rather, multiple weapons in the armamentarium are exploited. There is no reason not to make full use of GUV with appropriate precautions in this “war” against 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 it is essential that individuals be protected to prevent UV hazards to the eyes and skin. 

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 micrometer (μm, one-millionth of a meter) in size, and bacteria are typically 0.5 to 5 μm.

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), have been shown to be 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. 

Germicidal UV (GUV) refers to using ultraviolet radiant energy to inactivate bacteria, mold 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 of the public’s concern 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.