Poorly circulated room air raises potential exposure to contaminants by
up to six times
Experiments quantify the effects of overhead heating on room air mixing
with implications for COVID-safe meetings and classrooms

Date:
September 22, 2021
Source:
DOE/Lawrence Berkeley National Laboratory
Summary:
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 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.



FULL STORY ========================================================================== 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.



==========================================================================
9 dummies in a room 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 like a person
would.

CO2 was released at mouth level to simulate small respiratory
aerosols. The temperature of the CO2 as well as the velocity of its
release were adjusted to simulate a person talking.

The experiments took place in the FLEXLAB(R), Berkeley Lab's building
simulator and test bed. "With the FLEXLAB, we were able to control every
aspect of the HVAC system, which is how we were able 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 on the study. "We were also able to have temperature
and air velocity measurements throughout the room in addition to our measurements of CO2. Those helped us verify and quantify the mixing
problem." Study limited to small aerosols only Previous studies have established that CO2 can act as a proxy for the dispersion behavior 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 millimeters,
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. "We had CO2 sensors in each corner of the room at different
heights and also at the breathing zone of each manikin." The authors
are careful to note that their study addresses only the relative risk
of poorly versus well-mixed conditions; it cannot be used directly to
predict infection risk.

"We know the chain of events that it takes to get a person exposed, 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 those are inhaled by someone else, they may or may not get infected," Delp said. "From others' studies, we know that the quantity of viruses emitted by an individual infected person can vary very widely. One person may expel millions more viruses than another infected person --
and that varies over the course of an infection and also appears to
be different for delta compared to the earlier variants. And to top
it off, the number of viruses that it takes to initiate an infection
also likely varies between people and with the sizes of the aerosols
that are inhaled. As indoor air quality scientists and engineers,
our focus is on what can be done with ventilation, filtration, and air distribution to reduce risks even when 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 DOE national laboratories focused on response to COVID-19. Other co-authors of the
study were Jovan Pantelic, Michael Sohn, and Thomas Kirchstetter.

========================================================================== Story Source: Materials provided by
DOE/Lawrence_Berkeley_National_Laboratory. Original written by Julie
Chao. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Brett C. Singer, Haoran Zhao, Chelsea V. Preble, William W. Delp,
Jovan
Pantelic, Michael D. Sohn, Thomas W. Kirchstetter. Measured
influence of overhead HVAC on exposure to airborne contaminants
from simulated speaking in a meeting and a classroom. Indoor Air,
2021; DOI: 10.1111/ ina.12917 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/09/210922121908.htm

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