Having good room ventilation to dilute and disperse indoor air pollutants has long been recognized, and with the COVID-19 pandemic its importance has become all the more heightened. But new experiments by indoor air researchers at Lawrence Berkeley National Laboratory (Berkeley Lab) show that certain circumstances will result in poor mixing of room air, meaning airborne contaminants may not be effectively dispersed and removed by building level ventilation.
Using CO2 as a tracer to track small respiratory aerosols that travel with air currents in a room, the Berkeley Lab team found that when overhead vents (or diffusers) are supplying heated air, it created thermally stratified conditions that block the flow of clean air down to the “breathing zone” in the middle height of the room. As a result, even when people are sitting more than 6 feet from each other, some occupants may be exposed to respiratory aerosols from others at a rate 5 to 6 times higher than if the same room were well mixed. Their study, “Measured influence of overhead HVAC on exposure to airborne contaminants from simulated speaking in a meeting and a classroom,” was published recently in the journal Indoor Air.
“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 notes that this situation would occur only in the case of heated air being supplied from the overhead diffusers. When cold or neutral air is being supplied, the researchers did not see the thermal stratification occur; instead, the room was found to be well mixed in those circumstances.
While the basic risk from overhead heating has been known for years, it had not previously been quantified under controlled but realistic conditions of a meeting or classroom. The results are important for understanding how large the risk can be when occupants are intentionally spaced for safety. “Ventilation is essential to maintaining good air quality,” said Brett Singer, the lead author of the study and head of Berkeley Lab’s Indoor Environment Group. “But if you’re heating overhead without intentionally mixing the air in the room, you will not get the full benefit of ventilation.”
Fortunately, there is a simple solution, the study found: using portable air cleaners that pull air in from below and push it out through the top. “They take care of the mixing and then they also filter the air, so they have a double benefit,” Singer said.