New experiments by indoor air researchers at Lawrence Berkeley National Laboratory (Berkeley Lab) have shown 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. The findings of the study were published in the journal 'Indoor Air'.
Using CO2 as a tracer to track small respiratory aerosols that travel by 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.
"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 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 added.
Delp noted 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," added Singer. 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. The researchers positioned eight thermal manikins (which are like retail display mannequins but used for scientific research instead) and had a researcher present to operate an aerosol emissions device in a 20-by-30-foot room set up first like a conference room, with participants seated in a circular pattern, then reconfigured like a classroom, with one standing at the front of the room and eight participants facing forward.
Singer noted that most previous studies of the effects of imperfect mixing on contaminant dispersal used only one or two simulated occupants. In this study, the manikins released plumes of heat, much as a person would. CO2 was released at mouth level to simulate small respiratory aerosols.
Previous studies have established that CO2 can act as a proxy for the dispersion behaviour of small respiratory aerosols, or particles less than 5 microns in size. A micron is one-millionth of a meter. While respiratory aerosols are made up of particles in a vast range of sizes, from sub-micron to millimetres, this paper focuses on the smaller particles, which move mostly with the air currents.
Larger particles, which behave differently, will be the subject of a future analysis. "We released the particles and the CO2 at different manikins and tried to see how these tracers and particles spread around the room," said Haoran Zhao, a Berkeley Lab postdoctoral fellow and co-author on the study.