Experiments quantify the effects of overhead heating on room air mixing with implications for COVID-safe meetings and classrooms — ScienceDaily

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 greater. But new experiments by indoor air researchers at Lawrence Berkeley National Laboratory (Berkeley Lab) show that certain conditions will result in poor mixing of room air, meaning contaminants in the air may not be effectively dispersed and removed by building-level ventilation.

CO . to use2 as a tracer to track tiny respiratory aerosols traveling with air currents in a room, the Berkeley Lab team found that when vents (or diffusers) deliver heated air, it created thermally stratified conditions that inhibit the flow of clean air to the “breathing zone” on the middle height of the room. As a result, even when people sit more than six feet apart, some occupants may be exposed to airway aerosols from others at rates 5 to 6 times greater than if the same space were well mixed. Their study, “Measured Influence of Overhead HVAC on Air Pollution Exposure from Simulated Meeting and Classroom Speech,” was recently published in the journal Indoor air.

“When everything is well mixed, everyone is exposed to the same conditions,” said Woody Delp, an indoor air researcher at Berkeley Lab. “If it’s not mixed well, you can have potential hot spots from a COVID perspective. So if there’s one infected person in the room, instead of their expelled breath being completely dispersed and then properly diluted and removed through the HVAC system, another person sitting next to them or even across the room can receive a high concentration of the viral aerosol emitted by that infected person.”

Delp notes that this situation would only occur if heated air is supplied from the air vents. The researchers did not see the thermal stratification occur when cold or neutral air was supplied; instead, the room turned out to be well mixed under those conditions.

While the basic risk of 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 in understanding how great the risk can be when occupants are deliberately kept at a distance for safety. “Ventilation is essential to maintaining good air quality,” said Brett Singer, the study’s lead author and head of Berkeley Lab’s Indoor Environment Group. “But if you’re heating overhead without intentionally mixing the air in the room, you’re not getting the full benefit of ventilation.”

Fortunately, there is a simple solution, the study shows: using portable air purifiers that draw air in from the bottom and push it out through the top. “They do the mixing and then also filter the air, so they have a double benefit,” Singer said.

9 dummies in a room

The researchers placed eight thermal manikins (which resemble retail mannequins, but are instead used for scientific research) and had a researcher present to operate an aerosol-emission device in a 20-by-30-foot space that was previously set up as a conference room, with participants seated in a circular pattern and then reconfigured as a classroom, with one at the front of the room and eight participants facing forward. Singer noted that most previous studies examining the effects of imperfect mixing on the dispersion of pollutants used only one or two simulated inhabitants.

In this study, the manikins released heat plumes just like a person would. CO2 was released at mouth level to simulate small respiratory aerosols. The temperature of the CO2 as well as the speed of the release were adjusted to simulate a talking person.

The experiments took place in the FLEXLAB(R), Berkeley Lab’s construction simulator and test bench. “With the FLEXLAB, we were able to control every aspect of the HVAC system, allowing us to iterate on so many different conditions for the two types of occupancy configurations,” said Chelsea Preble, a research scientist at Berkeley Lab and UC Berkeley. and a co-author of the study. “We were able to take temperature and air velocity measurements throughout the room in addition to our CO measurements2. They helped us verify and quantify the mixing problem.”

Study limited to only small aerosols

Previous studies have shown that CO2 can act as a proxy for the dispersal behavior of small respiratory aerosols, or particles smaller than 5 microns. A micron is one millionth of a meter. While airway aerosols are made up of particles of a wide variety of sizes, from submicrons to millimeters, this paper focuses on the smaller particles, which usually move with the air currents. Larger particles, which behave differently, will be the subject of future analysis.

“We have the particles and the CO2 on different manikins and trying to see how these tracers and particles spread around the room,” said Haoran Zhao, a Berkeley Lab postdoctoral researcher and co-author of the study. “We had CO2 sensors in every corner of the room at different heights and also in the breathing zone of each manikin.”

The authors are careful to note that their study only addresses the relative risk of poor versus well-mixed conditions; it cannot be used directly to predict infection risk.

“We know the sequence of events it takes to expose a person, and it’s complicated and extraordinarily variable. An infected person talking and breathing expels droplets and aerosols of various sizes. But even when some of them are passed through someone else are inhaled, they may or may not become infected,” Delp said. “We know from studies by others that the amount of viruses shed by an individual infected person can vary greatly. One person can shed millions more viruses than another infected person – and that varies over the course of an infection and seems to differences for delta compared to the earlier variants.And to top it off, the number of viruses needed to cause an infection probably also varies from person to person and with the size of the aerosols that are inhaled. , our focus lies on what can be done with ventilation, filtration and air distribution to reduce risks, even if all the details of the biology are not known.”

The study was funded by the Department of Energy through the National Virtual Biotechnology Laboratory, a consortium of national DOE labs focused on the response to COVID-19. Other co-authors of the study were Jovan Pantelic, Michael Sohn and Thomas Kirchstetter.

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