UVGI devices can take many shapes and sizes.  They can also be mounted in various places. Depending on the manufacturers’ claims, these devices may be regulated as pesticide devices by the EPA and require efficacy data.  In addition, the site where the product is manufactured may also require registration.  See the EPA’s Pesticide Devices: A Guide for Consumers for additional information. Upper-room UVGI Upper-room (or upper-air) UVGI uses specially designed UVGI fixtures mounted on walls or ceilings to create a treatment zone of ultraviolet (UV) energy that is focused up and away from people. These fixtures treat air as it circulates from mechanical ventilation, ceiling fans, or natural air movement. The advantage of upper-room UVGI is that it treats the air closer to and above people who are in the room. Since the 1980s, UVGI systems have been widely used for control of tuberculosis (TB). The CDC guidance Environmental Control for Tuberculosis: Basic Upper-Room Ultraviolet Germicidal Irradiation Guidelines for Healthcare Settings provides information on appropriate UVGI system design, related safe operation, and maintenance. Based on data from other human coronaviruses, a UVGI system designed to protect against the spread of TB should be effective at inactivating SARS-CoV-2 and therefore prevent spread. While small spaces may require a single UVGI fixture, most UVGI systems usually require multiple UV fixtures to be effective. For example, a rectangular-shaped waiting room with 10–30 occupants will require 2–3 upper-air UVGI fixtures. As part of system installation, care must be taken to control the amount of UV energy directed or reflected into the lower occupied space. Reputable UVGI manufacturers or experienced UVGI system designers will take the necessary measurements and make any required adjustments to prevent harmful UV exposures to people in the space. Potential Application: Can be used as an additional layer of protection to treat the air (kill germs) in indoor spaces; most useful in spaces that host large gatherings or where the risk of disease transmission is high. In-duct UVGI In-duct UVGI systems are installed within a heating, ventilation, and air conditioning (HVAC) system. These systems are designed to serve one of two purposes: 1) Coil treatment UVGI keeps HVAC coils, drain pans, and wetted surfaces free of microbial growth. These devices produce relatively low levels of UV energy. This energy is continually delivered 24 hours a day, which is why they are effective. Coil treatment UVGI devices are not designed for treating the air and should not be installed for this purpose. Potential Application: Can be used to reduce HVAC maintenance and improve operational efficiency within large, commercial HVAC systems or residential HVAC systems; not recommended for inactivating airborne pathogens. 2) Air treatment UVGI systems can be effective at applying intense UV energy to inactivate airborne pathogens as they flow within the HVAC duct. HVAC air treatment UVGI systems generally require more powerful UV lamps or a greater number of lamps, or both, to provide the necessary UVGI required to inactivate pathogens in a short period of time. Air treatment systems are often placed immediately downstream of the HVAC coils. This location keeps the coil, drain pan, and wetted surfaces free of microbial growth and also treats the moving air. Potential Application: Can be used inside any HVAC system to reduce the concentration of infectious airborne pathogens. Whole-room UVGI Whole-room UVGI (commonly referred to as Far-UV) uses specially designed UVGI fixtures mounted on walls or ceilings to create a treatment zone of ultraviolet (UV) energy that extends throughout an occupied space. While standard UVGI fixtures emit UV energy at a wavelength around 254 nanometers (nm), far-UV devices use different lamps to emit UV energy at a wavelength around 222 nm. Aside from the wavelength, a major difference between the two technologies is that standard UVGI systems are generally designed to avoid exposing people to the UV energy, while many far-UV devices are marketed as safe for exposing people and their direct environment to UV energy.  Recent research has indicated 222 nm energy is much safer for humans than once thought. In fact, the American Conference of Governmental Industrial Hygienists (ACGIH) recently increased their Threshold Limit Values (TLVs) 7-fold for eyes and over 20-fold for skin exposed to 222 nm energy. This increase was in response to data showing 222 nm energy does not penetrate the tear layer of the eye or the layer of dead skin (stratum corneum) that protect living skin beneath. Research studies also indicate that far-UV wavelengths can effectively inactivate microorganisms, including human coronaviruses, when appropriate UV doses are applied under experimental conditions. However, there are still some questions about how effective 222 nm energy can be in real occupied spaces against human-generated pathogens when UV exposures are controlled to safe limits. Far-UV is a promising technology that may well prove to be effective at treating air and surfaces, without some of the safety precautions required for standard UVGI. Due to the potential promise this technology represents, there are substantial private and public research activities underway to further validate claims of safety and efficacy.  In the near term, whole-room UVGI is best viewed as new and emerging technology. Consumers considering an emerging technology such as Far-UV should read FAQ #8 on emerging technologies below. Potential Application: Air and surface treatment in occupied indoor environments.