ASHRAE Statement on airborne transmission of SARS-COV-2
Transmission of SARS-CoV-2 through the air is sufficiently likely that airborne exposure to the virus should be controlled. Changes to building operations, including the operation of HVAC systems can reduce airborne exposures.
Is Ultraviolet Energy (UV-C, Ultraviolet Germicidal Irradiation, Germicidal Ultraviolet) Effective Against The SARS-CoV-2 Virus?
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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Whether your focus is on IAQ or reducing the probability of transmission; a strategy without an emphasis on Engineering Controls is only temporary. Making improvements to Engineering Controls is a sustainable solution, can reduce operating costs over time, and is recommended by ASHRAE and the CDC in conjunction with other health and safety protocols. We can help you navigate what solutions are right for you.
Refer to the CDC and ASHRAE for the latest IAQ updates and best practices to reduce the risk of airborne pathogen transmission.
Centers for Disease Control and Prevention
During the COVID-19 pandemic, office building employers, owners and managers, and operations specialists can take the following steps to create a safe and healthy workplace for workers and clients:
Consider taking steps to improve ventilation in the building, in consultation with an HVAC professional, based on local environmental conditions (temperature/humidity) and ongoing community transmission in the area:
Increase the percentage of outdoor air, (e.g., using economizer modes of HVAC operations) potentially as high as 100% (first verify compatibility with HVAC system capabilities for both temperature and humidity control as well as compatibility with outdoor/indoor air quality considerations).
Increase total airflow supply to occupied spaces, if possible.
Disable demand-control ventilation (DCV) controls that reduce air supply based on temperature or occupancy.
Consider using ultraviolet germicidal irradiation (UVGI) as a supplemental technique to inactivate potential airborne virus in the upper-room air of common occupied spaces, in accordance with industry guidelines.
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