June 30, 2020 – updated March 29, 2021
While scientists continue to investigate the theory that SARS-CoV-2, the virus behind the COVID-19 disease, can be transmitted through HVAC systems, ASHRAE has stated that the “Transmission of SARS-CoV-2 through the air is sufficiently likely that airborne exposure to the virus should be controlled.” (Note: On March 29, 2021, ASHRAE revised its statement to say “Airborne transmission of SARS-CoV-2 is significant and should be controlled. Changes to building operations, including the operation of heating, ventilating, and air-conditioning systems, can reduce airborne exposures.”)1 The latest guidance from the ASHRAE Epidemic Task Force’s Commercial Guide published on August 17, 2020 recommends different strategies to mitigate any airborne transmission of SARS-CoV-2 including the use of high-efficiency filtration, ultraviolet germicidal irradiation (UVGI), humidity control, and dilution through increased outdoor air (OA) ventilation.2 Additionally, in the August 17 Commercial Guide ASHRAE acknowledges the energy penalty associated with an OA strategy, and recommends pursuing a minimum of OA as required by Standard 62.1 plus high-efficiency filtration when concerned about the energy costs associated with maximum OA ventilation. This white paper will detail how different strategies lead to the same outcome, but at widely varying costs.
In this paper we will analyze the relative efficiency and cost of various OA ventilation and filtration approaches to reduce the risk of airborne transmission of SARS-CoV-2 through bioaerosols in commercial buildings.3 Bioaerosols are airborne particles smaller than 5 microns that contain viruses. The cost/benefit risk model presented in this paper can be used by HVAC engineers and building owners to best design cost-effective HVAC systems that reduce the potential airborne transmission of SARS-CoV-2 in commercial buildings.
- For commercial buildings, improved filtration from MERV 7 (or lower) to MERV 13 (or higher) significantly reduces the risk of airborne transmission of SARS-CoV-2.
- Increasing OA ventilation adds material cost without significantly reducing the risk of airborne transmission. Furthermore, it makes it harder to control humidity, and increases the intake of particulate matter from polluted outside air.
- The strategy that both minimizes risks and reduces costs is to design for improved filtration and use ASHRAE’s Indoor Air Quality Procedure (IAQP) with sorbent-based air cleaning to reduce OA requirements and offset the cost of improved filtration. For example, in an 100,000 ft2 office building in NYC, by reducing rather than increasing 10,000 CFM of OA (using IAQP with sorbent-based air cleaning and advanced filtration) $500,000 can be saved on combined first cost and 20-year operating cost without increasing the risk of airborne transmission.