The Centers for Disease Control and Prevention (CDC) is urging
communities to reopen schools as quickly as possible, but parents and teachers
have raised questions about the quality of ventilation in public school
classrooms to protect against the coronavirus.
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The New York Times worked with a leading engineering firm and
experts specializing in buildings systems to understand better the simple steps
schools can take to reduce exposure in the classroom.
While it is not clear exactly what level of contamination
presents the greatest risk of infection, “exposure is a function of
concentration and time,” said Joseph G. Allen, the director of the Harvard
Healthy Buildings program and an environmental health expert.
Experts agree that good ventilation is the most effective and
practical way to rid a space of contaminants. The healthy buildings program
recommends four to six air exchanges per hour in classrooms, through any
combination of ventilation and filtration.
These scenes simulate the flow of air and contaminants in a
model of a public school classroom in New York City, where officials put strict
protocols in place for reopening and in-school transmission of the virus has
been very low. Students must practice social distancing and wear masks, and
classrooms must have windows that open.
The simulation shows how ventilation and filtration can work
alongside these other precautions. The students in the simulation are wearing
masks, but their breath still circulates and mixes around the room. About 3 percent
of the air each person in this room breathes was exhaled by other people.
Even students who look healthy may be asymptomatic carriers who
can transmit the virus. And here we show how an infected student’s breath can
circulate around the classroom.
Windows closed
With all the windows closed, a room like this would lack
sufficient ventilation. That’s a problem with an airborne virus.
— The lines trace
the infected student’s warm breath as it rises and begins to disperse
contaminated respiratory aerosols throughout the room. The contaminants are
most concentrated where the lines are darkest.
— A full cross-section of
the space shows the concentration at the level where the students are
breathing, once the room has reached a peak level of contamination.
With the window closed, the contaminants accumulate in high
concentrations because they have nowhere to go.
The dense reddish fog shows a high concentration of contaminants
spreading far beyond 2 meters from the infected student. If the student were
sitting elsewhere, the pattern would be different, but the buildup in the room
would be similar.
One window open
New York City mandated that every classroom have at least one
operable window to help with ventilation, even in the winter. When we open a
window, the fresh air dilutes the contaminants as they move around the room.
The concentration remains densest near the infected student, but
the contaminants are diluted in the rest of the room. Exposure for the other
students is reduced.
We managed to achieve four total air exchanges in an hour by
opening just one window in this simulation, which was dependent on specific
weather conditions.
“Simple and inexpensive measures can make schools much safer,”
said Scott E. Frank, whose engineering firm, JB&B, assisted with these
simulations.
Air cleaner and fan in the window
To get to six air exchanges, more must be done. Two practical
and low-cost options: adding a simple air cleaner with a HEPA filter and a box
fan blowing fresh air into the room. The increased fresh air blowing into the
room and the filtered air coming from the air cleaner help to dilute the
contaminants further as they spread.
With an air cleaner and a fan, the overall concentration levels
are at their lowest. The contaminants are concentrated at the front of the
room, where the fan is blowing, and diluted everywhere else.
Though we achieved six air exchanges an hour with these
measures, there are ways to improve ventilation further in this space. Pointing
a fan out the window, not in, and placing an air cleaner in the center of the
room are best practices, along with taking other precautions.
“Improving ventilation is only one part,” said Mark Thaler, an
expert in school spaces with the design firm Gensler. “It has to stand with all
the other CDC guidelines in order to really safely reopen.”
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