Ventilation in the COVID Era – Airborne transmission of SARS-CoV-2

Author: Lou Vogel, PE, LEED AP, President
Published: October 5, 2020
Last updated: October 5, 2020

As expected on Friday, September 18, the World Health Organization (WHO) changed its recommendations regarding airborne transmission of SARS-CoV-2 (the virus that causes COVID-19) and now confirms that it is an airborne illness that can be transmitted by aerosols, not just respiratory droplets. As defined in Part I of this series of posts, aerosols are small particles that, unlike droplets that will land on a surface, continue floating in the air and will follow any air movements that may be present.

And still, the most important method to reduce transmission is to stop it at its source; wear the mask, and don’t go out if you aren’t feeling well. 

There is a new study (published August 28, 2020, from April 2020 measurements) that circumstantially points to a lack of outdoor air contributing to an outbreak in a nursing home in the Netherlands. One of the HVAC units in the ward that had the outbreak was set up with CO2 control of the outside air damper. This is a very common energy saving measure that only provides the maximum code required outside air when there are enough people in the space to trigger the CO2 sensor. We generally call this Demand Controlled Ventilation (DCV) and it can save substantial amounts of energy in large occupancy spaces that are not always full of people. In times of low occupancy, when the sensors do not call for ventilation, the fallback should be set to a minimum amount based on the size of the room, which was likely not the case in this nursing home ward.  

In this case, it says that the setting was 1000 ppm of CO2. This is higher than usual, and it is entirely possible that the outside air damper would never open up, depending on the size of the room and number of people. Currently, the atmosphere is listed as being 414 ppm (it is going up, according to So, the air in the room would progressively have more concentration of virus if there was an infected person in the space and no windows open. Another factor that should be considered is whether or not the CO2 sensor was properly calibrated so that it was reading correctly.   

The other 6 wards in the building did not have an outbreak and had no positive test results. Evidently, the other 6 wards did not have the DCV control and the other HVAC units in each space were providing outside air, though it’s hard to be sure from the write-up. 

There’s still no evidence that the virus traveled in ductwork, but a complete lack of fresh air or good filtration may have contributed to the high transmission rates. In this case, it can also be considered that if the HVAC unit had a good filter (MERV 13 or higher) it likely would have reduced the concentration of virus in the air.   

For future posts, I will go into what impact your HVAC system layout has on reducing aerosols in your building. As new information is released, I will continue to respond here and help navigate new guidelines set forth by the CDC and ASHRAE as they relate to building ventilation. If there are any questions you have, feel free to ask here

How COVID 19 Spreads. September 21, 2020. National Center for Immunization and Respiratory Diseases (NCIRD), Division of Viral Diseases and the Center for Disease Control and Preventions.