Study provides suggestions for keeping classroom air fresh | MIT News
Open windows and a good heating, ventilation and air conditioning (HVAC) system are basic principles for keeping classrooms safe during the Covid-19 pandemic. But they don’t have the last word, according to a new study from researchers at MIT.
The study shows how specific classroom configurations can affect air quality and require additional measures, in addition to HVAC use or open windows, to reduce the spread of aerosols — those tiny, potentially Covid-carrying particles that can remain suspended in the air for hours.
“There are a number of circumstances where we have clearly identified that there is a problem, and if 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,” said Leon Glicksman, an MIT professor of architecture and engineering who co-authored a new paper describing the study.
Indeed, the study shows that some conditions can create a concentration of potentially problematic aerosols 50 to 150 percent higher than the standard baseline concentration that experts consider “well-mixed” indoor air.
“It gets complicated and it depends on the specific conditions of the room,” Glicksman adds.
The newspaper, “Patterns of SARS-CoV-2 aerosol diffusion in typical classrooms”, appears in advance online form in the magazine Building and environment. The authors are Gerhard K. Rencken and Emma K. Rutherford, MIT students who participated in the study through the Undergraduate Research Opportunities Program with support from the MIT Energy Initiative; Nikhilesh Ghanta, a graduate student at MIT’s Center for Computational Science and Engineering; John Kongoletos, a graduate student in the Building Technology Program at MIT and a fellow at MIT’s Tata Center; and Glicksman, the senior author and professor of building technology and mechanical engineering at MIT, who has studied air circulation problems for decades.
The battle between vertical and horizontal
SARS-Cov-2, the virus that causes Covid-19, is largely airborne through aerosols that people breathe out and can remain airborne for a long time if a room is not properly ventilated. Thus, many indoor environments with restricted airflow, including classrooms, may 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 certain scenarios may require additional ventilation methods to minimize SARS-Cov-2 aerosols.
To conduct the study, the researchers used computational fluid dynamics — advanced simulations of airflow — to examine 14 different classroom ventilation scenarios, nine with HVAC systems and five with open windows. The research team also compared their modeling with previous experimental results.
An ideal scenario involves fresh air entering a classroom near the ground floor and moving higher and higher until it leaves the room through ceiling vents. This process is aided by the fact that warm air rises, and people’s body heat naturally generates rising “heat plumes” that carry air to ceiling vents at a rate of about 0.15 meters per second.
With ceiling ventilation, the intent is to create upward vertical air movement to circulate air out of the room, while restricting horizontal air movement, distributing aerosols to seated students.
This is why wearing masks indoors makes sense: Masks limit the horizontal velocity of exhaled aerosols, keeping those particles near heat plumes, so the aerosols rise vertically, as the researchers noted in their simulations. Normal exhalation creates aerosol velocities of 1 meter per second, and coughing causes even higher velocities – but masks keep that rate down.
“If you wear well-fitting masks, you suppress the speed of the [breath] exhaust to the point where the air coming out is carried by the plumes above the individuals,” says Glicksman. “If it’s a loose-fitting mask or no mask at all, the air comes out at a high enough horizontal velocity that it isn’t caught by these rising plumes, and rises at much lower velocities.”
Two problematic scenarios
Still, the researchers found, complications can arise. In their series of simulations focusing on closed windows and HVAC use, airflow problems arose in a simulated classroom in winter, with cold windows on the side. In this case, because the cold air naturally sinks at the windows, it disrupts the overall upward flow of classroom air, despite people’s heat plumes.
“Because of the cold air out the window, some air moves down,” Glicksman says. “What we found in the simulations is that the heat plume from a masked person would rise to the ceiling, but if a person is close to the window, the aerosols rise to the ceiling and in some cases are trapped by that downward current, and to the breathing level in the room and we found that the colder the window is, the bigger this problem is.”
In this scenario, it would be particularly likely that someone who is infected with Covid-19 sitting by a window will spread their aerosols. But there are solutions to this problem: Placing heaters near cold windows, among other things, limits their impact on the airflow in the classroom.
In the other series of simulations, with open windows, additional problems became apparent. While open windows are good for fresh airflow in general, the researchers identified one problematic scenario: Horizontal air movement from open windows aligned with rows of seats creates significant dispersion of aerosols.
The researchers propose a simple solution to this problem: installing window baffles, fittings that can be adjusted to divert the air downwards. By doing this, the cooler fresh air from outside the classroom will enter at the feet of the occupants, helping to generate a better overall circulation pattern.
“The advantage is that you bring the clean air from outside to the floor, and then [by using baffles] you have something that starts to look like displacement ventilation, where the warm air from individuals will pull the air up, and it will go to the ceiling,” Glicksman says. “And again, we found that when we did the simulations, the concentration of aerosol was much lower in those cases than if you just let the air come in directly horizontally.”
Alejandra Menchaca PhD ’12, vice president and expert in engineering and ventilation at engineering firm Thornton Thomasetti, calls the research a useful step forward. The paper “provides critical new insight into one aspect of the dispersion of exhaled aerosols in indoor airflow,” said Menchaca, who was not involved in the study. “I hope the [building] the industry can use these results to advance its understanding of aerosol dispersion and the key variables – which have been largely ignored until now – that influence it.”
The energy penalty
In addition to the safety implications during the pandemic, Glicksman notes that better airflow in all classrooms has energy and environmental implications.
If an HVAC system alone doesn’t create the optimal conditions in a classroom, the temptation can be to run the system at full throttle in the hopes of creating greater throughput. But that is both expensive and environmentally harmful. An alternative approach is to look for class-specific solutions, such as baffles or the use of highly efficient filters in the recirculating HVAC air supply.
“The more outside air you bring in, the lower the average concentration of these aerosols will be,” says Glicksman. “But there’s an energy penalty associated with it.”
Glicksman also emphasizes that the current study looks at air quality under specific conditions. The investigation also took place before the more transmissible Delta variant of the Covid-19 virus took hold. This development, notes Glicksman, reinforces the importance of “reducing the aerosol concentration level through masking and higher ventilation rates” in a given classroom, and above all underlines that “the local concentration in the breathing zone [near the heads of room occupants] should be minimized.”
And Glicksman stresses that it would be helpful to have more studies that explore the issues in depth.
“What we’ve done is a limited investigation of certain forms of geometry in the classroom,” Glicksman says. “It depends to some extent on what the specific circumstances are. There is no easy recipe for better airflow. What this really says is that we would like to see more research.”