Good ventilation can cut most airboarne diseases
Massachusetts Institute of Technology
Open windows and a good heating, ventilation, and air conditioning (HVAC) system are starting points for keeping classrooms safe during the Covid-19 pandemic. But they are not the last word, according to a new study from researchers at MIT.
The study shows how specific classroom configurations may affect
air quality and necessitate additional measures, beyond HVAC use or open
windows, to reduce the spread of aerosols -- those tiny, potentially
Covid-carrying particles that can stay suspended in the air for hours.
"There are sets of conditions where we found clearly there's a problem, and when you look at the predicted concentration of aerosols around other people in the room, in some cases it was much higher than what the [standard] models would say," says Leon Glicksman, an MIT architecture and engineering professor who is co-author of a new paper detailing the research.
Indeed, the study shows that some circumstances can create a
concentration of potentially problematic aerosols ranging from 50 to 150 percent
higher than the standard baseline concentration that experts regard as
"well-mixed" indoors air.
"It gets complicated, and it depends on the particular
conditions of the room," Glicksman adds.
The paper, "Patterns of SARS-CoV-2 aerosol spread in
typical classrooms," appears in advance online form in the journal Building
and Environment. The authors are Gerhard K. Rencken and Emma K. Rutherford and Glicksman, the senior
author and a professor of building technology and mechanical engineering at MIT
who has been studying air circulation issues for decades.
The battle between vertical and horizontal
SARS-Cov-2, the virus that causes Covid-19, is largely transmitted in airborne fashion via aerosols, which people exhale, and which can remain in the air for long periods of time if a room is not well-ventilated.
Many indoor settings with limited air flow, including
classrooms, could thus contain a relatively higher concentration of aerosols,
including those exhaled by infected individuals. HVAC systems and open windows
can help create "well-mixed" conditions, but in certain scenarios,
additional ventilation methods may be needed to minimize SARS-Cov-2 aerosols.
To conduct the study, the researchers used computational fluid
dynamics -- sophisticated simulations of air flow -- to examine 14 different
classroom ventilation scenarios, nine involving HVAC systems and five involving
open windows. The research team also compared their modeling to past
experimental results.
One ideal scenario involves fresh air entering a classroom near
ground level and moving steadily higher, until it exits the room through
ceiling vents. This process is aided by the fact that hot air rises, and
people's body warmth naturally generates rising "heat plumes," which
carry air toward ceiling vents, at the rate of about 0.15 meters per second.
Given ceiling ventilation, then, the aim is to create upward
vertical air movement to cycle air out of the room, while limiting horizontal
air movement, which spreads aerosols among seated students.
This is why wearing masks indoors makes sense: Masks limit the
horizontal speed of exhaled aerosols, keeping those particles near heat plumes
so the aerosols rise vertically, as the researchers observed in their
simulations. Normal exhaling creates aerosol speeds of 1 meter per second, and
coughing creates still higher speeds -- but masks keep that speed low.
"If you wear well-fitting masks, you suppress the velocity
of the [breath] exhaust to the point where the air that comes out is carried by
the plumes above the individuals," Glicksman says. "If it's a
loose-fitting mask or no mask at all, the air comes out at a high enough
horizontal velocity that it does not get captured by these rising plumes, and
rises at much lower rates."
Two problematic scenarios
But even so, the researchers found, complications can emerge. In
their set of simulations focused on closed windows and HVAC use, airflow
problems emerged in a simulated classroom in winter, with cold windows on the
side. In this case, because the cold air near the windows naturally sinks, it
disrupts the overall upward flow of classroom air, despite people's heat
plumes.
"Because of the cold air from the window, some air moves
down," Glicksman says. "What we found in the simulations is, yes, a
masked person's heat plume would rise toward the ceiling, but if a person is
close to the window, the aerosols get up to the ceiling and in some cases get
captured by that downward flow, and brought down to the breathing level in the
room. And we found the colder the window is, the larger this problem is."
In this scenario, someone infected with Covid-19 sitting near a
window would be particularly likely to spread their aerosols around. But there
are fixes for this problem: Among other things, placing heaters near cold
windows limits their impact on classroom airflow.
In the other set of simulations, involving open windows,
additional issues became evident. While open windows are good for fresh air
flow overall, the researchers did identify one problematic scenario: Horizontal
air movement from open windows aligned with seating rows creates significant
aerosol spread.
The researchers suggest a simple fix for this problem:
installing window baffles, fittings that can be set to deflect the air
downward. By doing this, the cooler fresh air from outside will enter the
classroom near the feet of its occupants, and help generate a better overall
circulation pattern.
"The advantage is, you bring the clean air in from outside
to the floor, and then [by using baffles] you have something that starts to
look like displacement ventilation, where again the warm air from individuals
will draw the air upward, and it will move toward the ceiling," Glicksman
says. "And again that's what we found when we did the simulations, the
concentration of aerosol was much lower in those cases than if you just allow
the air to come in directly horizontally."
The energy penalty
In addition to the safety implications during the pandemic,
Glicksman notes that better air flow in all classrooms has energy and
environmental consequences.
If an HVAC system alone is not creating optimal conditions
inside a classroom, the temptation might be to crank up the system full blast,
in hopes of creating greater flow. But that is both expensive and
environmentally taxing. An alternate approach is to look for classroom-specific
solutions -- like baffles or the use of high-efficiency filters in the
recirculating HVAC air supply.
"The more outside air you bring in, the lower the average
concentration of these aerosols will be," Glicksman says. "But
there's an energy penalty associated with it."
Glicksman also emphasizes that the current study examines air quality under specific circumstances. The research also took place before the more transmissible Delta variant of the Covid-19 virus became prevalent.
This
development, Glicksman observes, reinforces the importance of "reducing
the aerosol concentration level through masking and higher ventilation
rates" throughout a given classroom, and especially underscores that
"the local concentration in the breathing zone [near the heads of room
occupants] should be minimized."
And Glicksman emphasizes that it would be useful to have more
studies exploring the issues in depth.
"What we've done is a limited study for particular forms of
geometry in the classroom," Glicksman says. "It depends to some
extent on what the particular conditions are. There is no one simple recipe for
better airflow. What this really says is that we would like to see more
research done."