Proper ventilation has always been stated as a key tenant for healthy indoor air quality, and recent research done by the Lawrence Berkeley National Laboratory (Berkeley Lab) shows how much of a difference it can make. It also highlights the negative impact HVAC systems can have on occupant health when set up in certain configurations.
In the study, researchers focused on a specific delivery method of HVAC systems where hot air is supplied to a room through overhead diffusers. Using CO2 as a tracer, the researchers tracked the path of respiratory aerosols in these settings, simulating a typical classroom.
Through testing, the team discovered that the overhead vents worked to create thermally stratified zones in the middle height of a room. What this means is that clean air located above these zones could not circulate into the normal breathing zone of the room’s occupants.
As a result, even with people seated more than six feet apart from one another, some occupants may still be exposed to aerosols at a rate of five to six times higher than a properly ventilated room.
Considerations to be Taken
While the risks of this method of HVAC heating delivery have been known for some time, the researchers note the study finally puts the risk into quantifiable numbers. It’s also noteworthy for showcasing how large the risk remains, even when occupants are spaced for safety.
Fortunately, the study found that facility managers can use portable air cleaners that push air out from the top to help with flow. “They take care of the mixing and then they also filter the air, so they have a double benefit,” said Brett Singer, the lead author of the study and head of Berkeley Lab’s Indoor Environment Group.
“When everything’s well mixed, everybody’s exposed to the same conditions,” said Berkeley Lab indoor air researcher Woody Delp. “When it’s not well mixed, you can have, from a COVID perspective, potential hot spots. So, if there’s one infected individual in the room, instead of having their expelled breath fully dispersed and then then properly diluted and removed by the HVAC system, another person sitting next to them or even across the room could get a high concentration of that infected person’s emitted viral aerosol.”
Delp goes on to note that this situation only pertains to this specific set-up with overhead heating. When cold or neutral air is used instead, the isolated pockets did not develop, and instead, the air was found to have sufficient circulation occurring. He also went on to note that the study only addresses the risk of poorly circulated air conditions. It cannot be used to predict infection risk.
Their study, “Measured influence of overhead HVAC on exposure to airborne contaminants from simulated speaking in a meeting and a classroom,” can be explored further by following the provided link.