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More than just masks: Schools' ventilation systems key to keeping kids safe from COVID-19

A multi-pronged approach is important in slowing the virus and reducing outbreaks, two air experts say

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A multi-pronged approach is important in slowing the virus and reducing outbreaks, two air experts say

Published Updated

Most of America’s students have returned to classrooms, but it’s administrators and teachers who will be tested first on how well they keep their schools safe.

Keeping COVID-19 infection rates in check will require more than masks and social distancing, says William P. Bahnfleth, an engineer and a professor of architectural engineering at Penn State University.

“You can make a space safer, but not safe,” Bahnfleth said. “Multiple layers of risk mitigation are important. Get vaccinated, wear a mask, make HVAC modifications that reduce risk, stagger lunches. They all contribute to a safer space.”

Here’s a look at the multipronged approach two experts on airflow suggest schools adopt.


“Given that we're talking about putting a group of children in a room for quite a number of hours, it becomes really important to understand the difference between masks,” says Lydia Bourouiba, professor and director of Fluid Dynamics of Disease Transmission Laboratory at the Massachusetts Institute of Technology.

Bourouiba, who has advised schools around the world with a set of guidelines to help curb the spread of the virus, says masks should be worn at all times except when eating and drinking.

But a mask's fit can be just as crucial. 

“In an ideal situation, everyone would wear an N95 respirator,” says Bourouiba, but she emphasizes a double-layered mask that fits properly captures what we exhale better than a loose-fitting N95 mask.

Indoor air quality/management

Proper air management is key to preventing the spread of COVID-19.

Virus particles can build up over time, some factors are important to know: how many air changes per hour in a room, how much outdoor air is supplied and the types of filters in use.  

Check the airflow in the rooms. Since all HVAC systems vary, there is no one-size-fits-all solution. "The first thing you should do is make sure you're providing at least the minimum amount of outdoor air required by code," says Bahnfleth, who is a past president of ASHRAE, an organization that establishes guidelines for building ventilation and chair of its Epidemic Task Force. The ASHRAE task force has produced guidance for many facility types, including  schools and universities. Key elements of ASHRAE guidance are summarized in its core recommendations.

Improving the filter is also important. "Filters required by minimum standards do very little to block particles that are in the respiratory aerosol size range," says Bahnfleth. ASHRAE recommends MERV-13 filters or better if your system can handle it.

Avoid placing students near the HVAC outlets, as well as near open windows or doors. That reduces the chance students will be in the path of infected air. “Whether we are considering windows, doors or air conditioning vents, when someone is 1-2 meters (3.3 to 6.6 feet) from a strong airflow from an inlet, what they exhale is effectively distributed to the others downstream.” says Bourouiba.

Similarly, avoid placing students within 1-2 meters (3.3 to 6.6 feet) of an outlet. "If a pupil is right at the outlet [window, door, HVAC, etc.] they are receiving unfiltered air from the whole classroom over an extended period of time.”

Between classes and breaks, try to allow the room to air out by opening windows, or let the room sit empty as long as possible to allow filtration and air changes (The amount of times outside air replaces the total volume of air inside a room per hour).

The proper placement of students and improvements to the filtration are just part of the equation. The ventilation system – most often mixed air or displacement – also plays a role in the school's air quality.

In a mixed air system, if someone is expelling virus in a room, this is how the virus is flushed out:

Filtered air mixed with outside air is pushed into the room to mix with room air to dilute contaminants, at the same time air is flushed out the exhaust.

Contaminants are eventually flushed out of the room.

A disadvantage of mixed air systems is the air is mixed around the room. "Injection of fresh air is typically done with a high flow rate from the top of the room and sucked out also from the top of the room," says Bourouiba. "We know that pockets can remain poorly mixed, and build up contamination. For example, thermal plumes of individuals and devices, and obstacles can interfere with this ventilation scheme." says Bourouiba.  

Displacement ventilation takes advantage of the thermal plume or heat that humans and devices emit. It's energy efficient especially when cooling since it uses the natural force of heat rising.

Thermal plumes from occupants and other heat sources pushes warm air higher. The stratification as it's called, results in higher concentration of contaminants at the top of the room.

Without proper ventilation, if someone is infected in a classroom, contaminated air can build up and linger in the breathing zone.

With displacement ventilation cool air is injected at a lower level. "If you don't want to retrofit your whole building, there are ways to do this by injecting fresh colder air at the level of the occupant," says Bourouiba. "It can be an open window, door or cold air defusers as an add-on to existing HVAC system injected very slowly at the bottom."

The injected cool air along with the thermal plume work together to push contaminants out of the breathing zone and eventually out of the room.

“Displacement ventilation systems are usually designed to cool spaces and they may not remove contaminants as effectively when a space needs to be heated,” says Bahnfleth. “In addition, it has been shown that exhaled contaminants like viral aerosols can be trapped in the breathing zone of displacement systems.” Bahnfleth also cautions that occupant motion in a space with displacement ventilation mixes the air, which is not accounted for in most studies of its performance. “However, if there is limited disturbance of air flow patterns by movement, it can be effective.”

The good news is simple modifications can make a big difference such as adding an air cleaner or a fan blowing air out of the room. While Bahnfleth says it's best to apply multiple improvements, air cleaners are a big help. "If you combine a good quality air cleaner with the proper clean air delivery rate it's likely going to be better than upgrading the filter in your HVAC system," says Bahnfleth. 

Use plexiglass with caution

Be cautious when using plexiglass screens between students. While the screens can block sneezes and coughs, they also can trap exhalations within them.

"It's quite possible that the way (plexiglass panels) they are often positioned, they will create a region of trapped air with more concentrated, infectious aerosol," says Bahnfleth.

Bahnfleth also notes that plexiglass panels can hinder air circulation within a confined space.

"These can interfere with the performance of the ventilation system," Bahnfleth says. "The more obstructions there are in a space, the harder it will be for air to actually get to where it's supposed to go and to remove contaminants."

Hand hygiene

While the sharpest focus should be on airborne transmission, it doesn’t mean hand hygiene should be ignored, says Bourouiba.

“Children have the tendency to put their hands in their mouth, and touch everything, and we know that the virus is found  in high concentration in mucosalivary fluid secretions.” says Bourouiba.

Students should wash their hands for 20 seconds and use non-contact dispensers if possible. Clean surfaces that are frequently touched, and do not have students share pens, pencils, or crayons.

Manage interactions

Limiting interactions in classrooms and common spaces can also reduce transmission. Bourouiba suggests breaking students into pods that consistently perform activities together and not mix with other pods.

Schools can leverage extensions spaces spaces such as a library, gym and even outdoors, says Bourouiba. “At most the pod should be the size of a normal class but, if possible, it would be better to break classes into smaller pods and utilized these extension places if staffing allows.”

She also suggests staggering access to shared spaces, using multiple entrances and maintaining consistent classroom assignment/seat assignments to help with possible contact tracing.

Classroom seating arrangements

Staggering desk assignments can reduce transmission by limiting the concentration of the respiratory cloud to nearby students.  

“It’s not just distancing in any direction. Most of the time the highest risk remains between 2-3 meters (6.6 to 9.8 feet) in the breathing zone,” says Bourouiba. 

This is particularly important if an infected student is not wearing a mask. "The infectious dose would be reached more easily if they are sitting right in the breathing zone compared to the students further into a mixed space.”

There is a compromise between distance and time, says Bourouiba. “The air eventually moves around and dilutes further downstream. Masking and staggering are important for reducing the range." The longer you are in the room, that’s where filtration becomes important to reduce the overall load.” If the staggering happens to reduce the class size that's added benefit says Bahnfleth.

Bourouiba recommends those students seated furthest from the door enter the room first.

When exiting the opposite should happen: The row closest to the exit should leave first.  “When one walks, one leaves behind a wake of exhaled cloud in the immediate vicinity. Minimizing close interaction and mixing of individuals/pods/groups that would be exposed to such concentrated emissions from an infected individual from another sub-group is a goal to keep in mind whenever possible.” says Bourouiba.


Students should eat outside as much as possible and wash their hands before and after eating. If the students eat indoors, stagger times and position children side by side instead of facing each other. Also, limit shared water fountains and encourage personal water bottles. 

“Meals are a very risky activity from the point of view of transmission as masks are off,” says Bourouiba. “Ideally one should eat outdoors and even if outdoors students would be seated side by side keeping in mind directed airflow currents.”

Surface management

Bathrooms, classroom door handles, faucets, handrails, light switches, and other high-touch areas should be cleaned multiple times a day.

“If the pod system is in use, flushing the air and decontaminating the surfaces between groups would be important to effectively keep them truly separated from a transmission pathway point of view,” says Bourouiba.

The list of precautions may seem daunting, but the delta variant is more contagious for children than other variants and is currently the dominant strain in the U.S.

“We know with the new variants that the risks of transmission are higher, hence parents, teachers and administrators are naturally very concerned,” says Bourouiba.

“The vaccines have been a phenomenal success story if you look at who is hospitalized with severe forms of COVID-19,” says Bourouiba. However, “vaccination rates seem to be plateauing around 50-60% with large regional differences, and the vaccines do not seem to have interrupted the transmission cycle. Finally, children are only now beginning to be vaccinated in some regions and only for some age groups, so we are not yet at the stage of the pandemic would have all hoped to be at by now.”

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