U-M scientists give tips about how teachers can keep classrooms safe from COVID-19

Reviewed by Emily Henderson, B.Sc.Sep 14 2022

Since the Centers for Disease Control and Prevention acknowledged SARS-CoV-2 was airborne in spring 2020, University of Michigan aerosol scientists Andrew Ault and Kerri Pratt have advised public school systems on how to mitigate the transmission of COVID-19 in the classroom.

That’s partly because social distancing isn’t enough of a mitigation strategy. Have you ever smelled food burning in a kitchen or cigarette smoke in restaurants before indoor smoking was banned? Distance doesn’t automatically mean particles in the air aren’t reaching you, say the researchers.

Now, as students return to school, Ault and Pratt, both associate professors in the U-M Department of Chemistry, give tips about how teachers can keep their classrooms safer for themselves and their students. The key, they say? Masks, open windows, and HEPA filters, including a do-it-yourself air filter made out of furnace filters taped around a box fan.

Can you describe how airborne transmission of COVID works?

Airborne transmission means that a virus is spread by aerosols, which are tiny particles we exhale into the air when we are breathing and talking. The louder we are talking or harder we are breathing, the more aerosols we emit. When someone has COVID, those aerosols contain virus (SARS-CoV-2), which can infect others that breathe the aerosols in. These aerosols are so small (100 times smaller than the diameter of a human hair) that they can stay in the air for hours.

What are some ways people can visualize airborne transmission via aerosols?

You can think of aerosol transmission like cigarette smoke, which is similar in size to exhaled aerosol. Smoke travels across rooms and builds up in poorly ventilated spaces. When restaurants and bars previously allowed smoking, it didn't matter if you were one table away or across the restaurant, everyone was breathing in the smoke.

How do aerosols move throughout enclosed spaces, such as classrooms?

Classrooms are just like any other indoor space. If infected kids or adults are in the room, they will exhale aerosols with the coronavirus in them, and these aerosols will hang in the air, like smoke. The best things to do to prevent others from breathing in those virus aerosols are to wear masks and improve ventilation through filtration and opening windows.

What are some affordable ways teachers and school administrators can keep classrooms more safe for students?

There are a number of low-cost steps that schools can take to clean the air and reduce transmission of COVID-19.

1. All students and adults should wear masks indoors at all times, since respiratory aerosols are released through breathing. Masks reduce the amount of aerosol released to the room by someone infected (including those that are asymptomatic) and also reduce the amount of aerosol breathed in by someone else (reducing the chance of infection). Mask fit is important, with the mask sealing against your face without gaps (i.e., not leaking) for best protection. KF94, KN95 and N95 masks are readily available and provide an even higher level of protection than cloth and surgical masks by removing greater than 90% of aerosols exhaled by others when you breathe in.
2. Eat outdoors, and in inclement weather, have students eat in individual, well-ventilated classrooms, rather than large cafeterias, to reduce the number of students exposed to an infected individual.
3. Improve ventilation, which can be as cheap as opening a window. Steps beyond just opening a window are to use fans to bring fresh air in and push dirty air out. Improving ventilation has the added side benefit of improving cognitive function by decreasing carbon dioxide buildup.
4. Lastly, a cool do-it-yourself option is to make a Corsi-Rosenthal Cube, which is a box fan with 5 MERV-13 furnace filters duct taped together and can be made for less than $100. Research shows that these filters decrease aerosol levels, and they are being used in numerous schools nationwide. There are also commercial HEPA air filtration units available that cost more. There should be at least one HEPA air filter/cube in each classroom and multiple filters/cubes in any larger indoor spaces. Portable HEPA air filters supplement HVAC MERV-13 filters by filtering air immediately around students.

Do not buy ionizers, ozone generators or any products claiming to use ions/chemicals to remove virus particles. We, in the indoor air community, have been trying to raise the alarm about this, but unfortunately many individuals and school districts have wasted a lot of money on products that do not effectively remove virus aerosols, but do introduce other harmful gases. HEPA and MERV-13 level filtration is the best option for aerosol removal, as used in hospitals.

How can teachers or school administrators monitor indoor air quality and know if a room may have high aerosol concentrations?

A carbon dioxide monitor (about $250) will tell you how much exhaled air has built up in a room. With good ventilation, carbon dioxide concentrations indoors and outdoors should be similar (about 420 ppm). If the carbon dioxide levels reach higher than 800 ppm, that means that the room is poorly ventilated and that you are rebreathing the air that someone else has exhaled. This calls for the need for increased ventilation (opening a window to mix in outdoor air) and adding an air filtration unit to remove exhaled aerosol.

How will what we’ve learned about the transmission of COVID through aerosols impact how we handle other diseases such as the flu or the common cold?

What we’ve learned about aerosol transmission of COVID can greatly improve how we handle many other airborne respiratory diseases, like influenza, and improve our health overall. By improving indoor ventilation, we will reduce respiratory disease transmission, reduce exposure to air pollution and allergens like pollen, and improve cognitive function (by reducing exposure to high levels of carbon dioxide). In many countries even before COVID-19, it is customary to wear a mask when sick to prevent infecting others.

We have received public health guidance to social distance—what does this mean in the context of airborne transmission?

Just like standing next to a smoker, there is more exhaled aerosol close to a person, such that social distancing reduces exposure, particularly to droplets. However, aerosols travel across rooms, much farther than 6 feet, and build up in poorly ventilated spaces. Another way to think of it is—if you are in a pool and someone pees in it, you don't want to stay in the pool, even if you are more than 6 feet away from them. This is why masking and ventilation indoors are so important, in addition to social distancing.

What’s the difference between exhaled droplets and aerosols?

Droplets are emitted during coughing and sneezing and are approximately the diameter of a human hair (100 times larger than aerosols). Droplets stay in the air for only a few seconds while traveling up to 6 feet of distance. Since the early 1900s, most doctors thought that infectious diseases spread primarily via droplets and surface contact. This was in part because droplets and surfaces are easier to measure.

Over the course of this pandemic, we’ve come to realize that this thinking was outdated. Aerosols can stay in the air for hours in poorly ventilated indoor spaces and can infect people across rooms that breathe in the previously exhaled aerosols. There is now overwhelming evidence that aerosols are how COVID-19 is transmitted.

What is the riskiest part of the school day for COVID transmission?

The riskiest part of the school day is anytime students are indoors and unmasked together. For students wearing masks, this risk occurs when eating lunch or snack. When multiple cohorts (classrooms) of students are together in a cafeteria/lunchroom, this increases the number of students exposed to an infected individual, since the aerosols travel across the entire room, far beyond 3-6 feet. Outdoor eating is the best solution, but with inclement weather, the next best solution is for small cohorts to eat in well-ventilated classrooms to reduce the number of students exposed to each other, to prevent outbreaks.