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Many HVAC-related factors could be significant with respect to the spread of COVID-19. Increased ventilation, advanced filtration, humidification and improved mechanical hygiene are being included in measures intended to reduce its spread. ASHRAE has issued general guidance for HVAC operation during the COVID-19 pandemic based on the very limited information available at this time,1 but there is considerable uncertainty over where these measures are effective. This paper summarizes what is known about the virus responsible for COVID-19 (SARS-CoV-2) and similar viruses regarding the role of HVAC in both the spread and control of infection. It also identifies critical information gaps and recommends research priorities. Scientific publications were reviewed (including 2020 preprints of COVID-19 research) and papers previously published on related viruses such as SARS (2003 pandemic).

The authors considered this information based on their experience as mechanical engineers specializing in HVAC design and operation and as industrial hygienists specializing in indoor air quality. It should be noted that this subject is rapidly evolving as efforts to control the pandemic continue. Significance of Airborne Transmission Transmission of respiratory infections through the air is classified as direct contact (within a few meters) or airborne (i.e., beyond a few meters). SARS-CoV-2 is infectious until it degrades (inactivated), but it has not been established how long the virus remains infectious in air. Because viruses generally have a minimum dose at which they cause infection and show a dose-response relationship, health risk is related to concentration in air and duration of exposure. These factors are not known for SARS-CoV-2. CDC and WHO guidelines for COVID-19 response assume that the important routes of COVID-19 transmission are direct contact with the patient, short-range droplet exposure and transfer from surfaces where aerosols have settled (fomites).2 Based on this assumption, recommended response measures by public health agencies are generally limited to social distancing, face coverings, handwashing and surface sanitizing. However, increasing evidence suggests that smaller aerosols remain suspended in the air, where they expose occupants (airborne transmission)...

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