Three Keys to Reducing Airborne Covid-19 in Elementary Schools

How can we make the return to kindergarten or elementary school safer during the pandemic? Ventilate, filter and humidify to optimal levels.

Based on what we know about about the coronavirus, no child will be 100 percent safe going back into an enclosed classroom in the U.S., even if PPEs and social distancing rules are strictly followed. Enclosed spaces without proper ventilation create ideal conditions for coronavirus spread.

Three Keys to Reducing Airborne Covid-19 in Elementary Schools

In fact, the World Health Organization has just been accused in a letter signed by 200 scientists of downplaying coronavirus infection risks due to airborne virus particles. Linsey Marr, a virus specialist and one of the scientists who signed the letter, told the New York Times that "the WHO had relied on studies from hospitals that suggested low levels of virus in the air. This underestimated the risk, she said, because in most buildings “the air-exchange rate is usually much lower, allowing virus to accumulate in the air.”

For a deeper dive, this article in The Atlantic looks at studies of how the virus spread indoors in Japan, and has some good insights.

The same is true of most school classrooms. They're not designed for rapid air changes. That needs to be remedied before we put children at risk.

Why? Because of the way the Covid-19 virus infection works. Particles need to achieve a critical mass before successfully infecting a host, a scenario most likely when air is static and dry, and particles are being introduced repeatedly. And, while masks and good hygiene definitely reduce transmission rates, airborne control of virus spread is at least as important, especially for young children, who may not be capable of consistent social isolation. These key healthy air qualities include humidity levels, speed or air exchange, filters and virus-killing lights.

Although no mechanical system can guarantee reducing infection rates to absolute zero, every layer of defense helps. If masks, eye gear and social distance lower infection rates to less than 3 percent in health care settings (i.e. hospital rooms--as suggested by a synthesis of research above), adjusting the way classroom air functions can chip away at the remaining risk.

A new study just came out looking at ventilation strategies enacted at schools over the past year. Here's a quote from one of the research survey respondents:

"After increasing fresh air ventilation through the pandemic (as well as other mitigation strategies), we found that there was zero spread in our school through the entire previous school year and a half despite very high community spread."

The report has lots of interesting observations made by participants. For example, confidence in standalone filtration was low: "Participants worried that standalone filtration units were potentially wasteful “junk” and without long-term benefit. District respondents felt that they could achieve enough infection control through HVAC. Most put their energy into making their HVAC systems better and didn’t feel the need to duplicate efforts with standalone filtration."

Here are three building science adjustments that school administrators must ensure are in place prior to the school semester this fall.

1. Ventilate. Because static air kills.

A key finding in coronavirus research is that when Covid-19 first infects the human body, It’s usually not one particle. Rather, the virus is implanted in a person's nose or eyes from a buildup of airborne particles after several minutes near an infected person.


Ventilator Location Matters. A new study from the Univ. of Minnesota found that viral particles are filtered most effectively when a return vent is near the speaker (such as a teacher). Of equal importance, they found that if ventilation speed is not optimized, filters only get about 10 percent of infected droplets. The rest end up on classroom walls. This result should be studied more closely. The ventilation system used by the researchers for classroom simulation appears to have been one where inlet and outlet vents are close together, part of the same unit, although the volume of air used in the study, approx. 1600 cfm, is about what would be recommended by CDC-based guidelines (below).

The CDC has created guidelines for hospital ventilation based on AIA standards, that could be applied immediately to school classrooms. For an ER Waiting Room, for example, the minimum air exchange with the outdoors is 2 times per hour. When the outdoor air is brought in, stale air is exhausted to the outdoors too. But this is just part of the equation. Along with that this complete air exchange, they recommend 12 additional exchanges of indoor air. What that means is that air inside the room should recirculate through a filter system a dozen times over the same period. We will look at filtration below, but for now, let’s focus on the ventilation component.

Let’s assume our sample classroom has an older heating system, such as the familiar radiator-style hot water boiler (in a cold climate) and cannot be retrofitted easily to bring include enough fresh air from outdoors. Its dimensions are 40 ft. long, 30 ft. wide, with 8-ft. ceilings, amounting to 9,600 cubic feet of airspace.

The easiest way to increase the overall ventilation is to add a standalone heat recovery ventilator (HRV). This can be mounted in a closet, inside the ceiling, or even on the wall. To size it, divide the cubic feet of space by 60, and multiply by 2 (air changes per hour). This gives you the minimum cubic feet per minute (CFM) requirement for the space. So our classroom requires 9600/60 x 2, or 320 cfm of airflow to accomplish two air changes per hour.

This is a larger volume of airflow than most residential HRV units, but you can find models with this capacity, or simply install two separate units from companies such as Broan or Panasonic. There is also a standalone unit called the Super V that looks promising, moving up to 2000 cfm of air through its filtration system.

The reason you are installing a heat recovery ventilator (or energy recovery ventilator in hot climates) rather than simply a one-way fan, is because if you simply blow air out of the room, additional makeup air will be pulled in from somewhere else, and this might include another classroom or office space. This technology also saves energy and maintains classroom comfort.

Now that you have the diffusion aspect of ventilation installed, let’s move on to filtration.

2. Filter: Because virus particles are small.

Next on the classroom risk reduction list: Air recirculation that includes filtration and virus-killing technology.

Removing airborne Covid-19 particles completely is possible only with either HEPA or MERV 16 or better filters, due to the extremely small size of virions (.06 to .14 microns). However, these filters restrict airflow and are hard to find, reserved for hospital operating rooms with special filtration in place. Your next best option may be a MERV 13 filter, which will catch larger droplet particles. These filters should not be relied upon as a 100% solution, however.

The good news is that based on our classroom target ventilation level of 12 recirculating exchanges per hour, free floating particles will have numerous passes at the filter media. The bad news is that these ultra-fine MERV filters may need frequent replacement, as they capture much larger particles of dust and dander and lose airflow. Also, keep in mind that the number of coronavirus particles in a classroom is not fixed. Any contagious child will keep producing new virions all day.

Another important piece of filtration technology is UV Lighting. Although these lights are unsafe for human exposure, they kill coronavirus, and can be installed inside the ductwork of the recirculating air filter. They can be installed at the air handler (see below) or inside the ductwork. One caveat: UV lights may be less effective if air is moving too quickly past them. More research is needed on how fast the UV lights can actually destroy virus particles.

Now how do you achieve the substantial airflow of 12 air changes per hour? Use the same formula we did for the HRV above: 9600/60 x 12. That’s a whopping 1,960 cfm. One way to achieve this would be to add an air handlera self-contained unit that includes a powerful blower fan and options for high-performance MERV filtration without losing flow rate. Trane makes one called the Hyperion, for example, which is capable of heating or cooling, that will do the job. It also allows you to add a UV light inside the double cabinet.

Another way to achieve the necessary air changes, if the school is equipped with a forced air heating/cooling system, is to add the filtration components to the existing equipment. The risk with this approach is that if the filtration is not perfect, you may expose every person in the building to whatever virus particles get through. In that scenario, a single contagious person in one classroom could theoretically infect the whole school.

Editor's Note. Some indoor air experts have suggested that recirculation systems should not be used at all in public buildings to control virus spread. This is certainly true of any system that has not upgraded its filter media. My suggestions for recycled air filtration using MERV or HEPA filters above are based on CDC guidelines for hospitals, but you could also take a more conservative approach and use HRV/ERV technology exclusively, with no recycling of air. You would need to boost the HRV sizing for each classroom to achieve 12 or so air changes per hour, however.

3. Humidify: Because virus particles fly when it’s dry.

Research by Dr. Stephanie Taylor suggests that when indoor air is dry, airborne drops of water and flakes of skin that contain virions and bacteria stay airborne longer and travel farther, and tend to be resilient enough to remain infectious. Humidity should be kept between 40 percent and 60 percent. Anything lower than 40 percent and viral risks increase dramatically.

Relating this finding directly to schools: “A team at the Mayo Clinic humidified half of the classrooms in a preschool and left the other half alone over three months during the winter. Influenza-related absenteeism in the humidified classrooms was two-thirds lower than in the standard classrooms.”--Source

How much water does it take to humidify a classroom? That depends on several factors, including the number of air changes per hour ventilating the space and the temperature and humidity of outdoor exchange air. So, for example, for a cold climate classroom (outdoor temperature 0°F, desired indoor temp 70°F, at 50% relative humidity) which is 9,600 cubic feet. Let’s assume, based on good ventilation practices (see below), you are changing the air in the room (with ventilation) three times per hour, so you’re looking at a formula like this:


This calculation yields 14 lbs. of water per hr. One gallon of water weighs 8.34 lbs. So you would need about 1.67 gallons of water per hour introduced to this space to achieve a virus-unfriendly humidity level of 50 percent. Note that the more often you exchange the air in the room, the greater the volume of water you need to introduce to the space. If you choose the best case scenario of exchanging indoor and outdoor air completely 12 times per hour, you will need to add 6.68 gallons of water per hour on a cold winter day.

Defensive, Not Fail-Proof

I’ve only addressed one aspect of how to manage Covid-19 indoors—the airborne version of the virus. Regular cleaning of surfaces, non-sharing of toys, learning aids and laptops, and many other protocols must also be followed to reduce contagion risk. No defensive measure can completely defuse the coronavirus threat, but this article is intended to give school administrators an idea of what’s involved in making a classroom relatively safe in the age of the Covid-19 pandemic. Let’s hope they either take this advice, or teach kids online until the danger has passed.


Update 7-13-2020:

Concern about schools reopening in the worsening conditions of the Pandemic are mounting, as highlighted in this article from the Guardian. Teachers are resigning rather than risk unsafe classroom conditions. The damage from politicizing this issue is likely to increase the harm exponentially. Airborne spread of Covid-19 is beginning to reach the public consciousness. Excerpt:

"...consider my brother-in-law, a public school teacher married to my sister, who is immuno-suppressed after intensive cancer treatment. He looks at classroom plans for the fall and sees almost nothing to protect him from the aerosol spread of the virus. Once winter comes, air will recirculate among closed classrooms. Schoolchildren are hard to manage in normal times, and they cough on whatever is close. Given that many more children will be facing crises at home as parents and grandparents lose their jobs or their health or both, behavior will be even harder to manage."

In other news, Carrier today announced the launch of its OptiClean TM 1500-cfm, Dual-Mode Air Scrubber Negative Air Machine, with specific emphasis on cleaning the air in schools. At 1,500 cfm, with HEPA filtration, it meets the necessary criteria to clean one classroom.

Editor's Note: Please feel free to share this research with your local school superintendent, particularly if you live in a district that seems to be rushing school openings without the necessary due diligence.

Additional Reading:

"We Need to Talk About Ventilation" from The Atlantic.

UPDATE 4-22: A new article in Scientific American details some of the latest findings about Covid-19 transmission and ventilation. It's an important read. Here is an excerpt:

"Places with rapid rates of ventilation and filtration—such as some subways—are also much lower risk. The Bay Area Rapid Transport (BART) system in San Francisco Bay, for example, filters the air more than 50 times an hour with “virus-trapping MERV-14 air filters” inside each car. An Italian study of schools found that classrooms with ventilation systems that exchanged air six times per hour reduced infections by more than 80 percent, but many classrooms in the U.S. fail to meet this standard. Corsi characterized current public health recommendations of four to six air exchanges per hour as “a little bit anemic … we can do better.” He recommends owners or managers of crowded indoor spaces, such as classrooms, offices and bars, aim to filter or ventilate with fresh air at rates approaching 12 air exchanges per hour to reduce risks down to the level of an airborne isolation room in a hospital. Not all venues have the resources to do this, but the benefits increase with greater filtration rates, so the closer to this ideal, the better. In places with inadequate ventilation, consider bringing a portable high-efficiency particulate air (HEPA) purifier—or building your own using box fans and high-quality HVAC (heating, ventilating and air-conditioning."

Contact Matt Power with with technical corrections, updates or suggestions. Scientific knowledge is still incomplete, and we will update this article as needed.