Ultraviolet energy (ultraviolet germicidal irradiation or germicidal ultraviolet) could be a powerful tool in the fight against COVID-19. ASHRAE’s position on UVC is expressed in the Position Document on Airborne Infectious Diseases: UVC air and surface disinfection is used in many different settings – residential, commercial, schools, as well as healthcare. Germicidal light (particularly 254 nm UVC produced by low pressure mercury vapor lamps, which operate near the most effective wavelength of ~265 nm) has not, to our knowledge, been tested on SARS-CoV-2, but it has been tested on an airborne coronavirus (Walker 2007). The sensitivity of that coronavirus to 254 nm was high enough that it seems like a good candidate for UV disinfection.
While UV systems are quite effective at maintaining cleanliness of HVAC coils, drain pans and other wetted surfaces, properly designed systems can be quite effective at on-the-fly inactivation of microorganisms in moving airstreams. These systems generally require more lamps so they can provide significant UV doses in a short period of time. A typical single pass inactivation efficiency is 85%, much like a good particulate filter, but systems can be designed for over 99.9% inactivation as well. Plus, a well-designed UV air disinfection system inside an HVAC system, and located by the cooling coils, can also provide the surface disinfection benefits mentioned above.
Another way to install UV is in an “upper-air” configuration. Specially designed fixtures mounted on the wall create an irradiated zone above the occupant and disinfect air in the space as air circulates naturally, mechanically or by means of the HVAC system. This sort of system has been approved for use in control of tuberculosis by CDC for nearly 20 years and there is a NIOSH guideline on how to design them.
Finally, mobile UV systems are frequently used for terminal cleaning and surface disinfection in healthcare and other spaces. Systems such as these are typically used in unoccupied spaces due to concerns of occupant exposure. All three system types may be relevant, depending on the building type and individual spaces within the building.
The design and sizing of effective ultraviolet disinfection systems can be a complex process because of the need to determine the dose delivered to a moving air stream or to an irradiated region of a room. In-duct systems are further complicated by the air handling unit and ductwork configuration and reflections from surfaces that can help achieve higher irradiance levels. Upper-air systems require adequate air mixing to work properly while paying close attention to reflective surfaces that could result in room occupants being overexposed to the UV energy. Reputable manufacturers and system designers can assist by doing the necessary calculations and designing systems specific to individual spaces.
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