Author: Lou Vogel, PE, LEED AP, President
Last updated: September 11, 2020
There is a great deal of discussion and research going on right now to understand what enables the transmission of the SARS-CoV-2 virus. In this multi-part series, I will work toward clarifying some of the recent findings on how the virus moves and rests in indoor spaces from the perspective of an HVAC engineer and what that means for building designers and occupants. The following series of posts will look at several facets of ventilation systems including:
- how HVAC systems might disperse, transfer, or dispel the virus concentrations;
- how the building industry is responding through improved design and enhanced filtration measures; and
- what these shifts in building design and filtration mean for the energy usage of our buildings.
In this introduction, I will define respiratory droplets and aerosols, and begin the discussion of what we are learning about the transmission of the virus through the air and how HVAC systems impact that. There are still ongoing discussions of how much of the virus gets transmitted to other people by aerosols. The World Health Organization (WHO) hasn’t changed its guidance on this yet, but it seems likely that it will eventually do so.
The WHO currently states that the “COVID-19 virus spreads primarily through droplets of saliva or discharge from the nose when an infected person coughs or sneezes.” Infectious droplets can land in the mouths or noses of people who are nearby or possibly be inhaled into the lungs. The WHO sizes up respiratory droplets as between 5 μm and 10 μm (micrometers) in size and aerosols at less than 5 μm. However, there is a continuum of airborne particle sizes; they don’t suddenly switch from one to the other. The droplets are aptly named, as they are small particles that eventually “drop” down onto a surface, mostly within 6 feet, some a little farther.
What Are Aerosols?
There is a growing consensus among researchers that aerosols also contribute to the transmission of the virus. Interestingly, droplets can dry out and become a droplet nucleus type of aerosol. Aerosols will continue floating in the air and will follow any local currents or air movements that may be present. These air currents can be caused by HVAC grilles, open windows, or any other movements in the space.
Ductwork, Filters, and HVAC System Design
As of September 10, 2020, there are no documented cases of virus transmission through a building’s ductwork into other spaces. There are case studies that suggest the flow of air in a room spreads the virus to nearby people, there are studies that have found the virus on the return air grille of HVAC systems, but there are none that show cases of the virus traveling from one room to another.
Next, I will look at how the layout of an HVAC system can affect the potential transmission of the virus and examine a case study that provides an example of the SARS-CoV-2 virus moving through indoor space.
What does that air movement mean for transmission of the virus? Does ductwork harbor droplets and aerosols? Will an enhanced filtration system help reduce exposure to SARS-CoV-2 virus in indoor spaces?
Modes of transmission of virus causing COVID-19: implications for IPC precaution recommendations: Scientific brief. March 29, 2020. World Health Organization.