Reducing Risk of COVID-19 Transmission: Strategies to Reopen Schools
By: N Traviss
Tags: Covid 19, Ventilation, Risk Management, Reopening Schools, Schools

By: Dr. Nora Traviss, PhD, and Dr. Ben Locwin, PhD

It’s September in New England, that time when many schools are reopening or on the verge of reopening. Meetings have been held, parent surveys received and options discussed and deliberated. There are many reports in the media that discuss various strategies how to reopen schools — reduce class size! Wear a mask! Hybrid instruction! Ventilation! Deep cleaning! But there are few resources describing how to pull all the necessary strategies together, and in what order, to reduce risk of COVID-19 transmission in schools.

This article seeks to address that gap. We think it’s important for schools to implement multilayered risk reduction — all the steps layered in order to reduce risk of COVID-19 transmission to protect kids, teachers/staff, and the broader community. One of the authors likes the Swiss Cheese metaphor here — if we layer each slice of Swiss cheese, even though the slices each have holes, all the slices layered together will work to block those holes. The other author prefers the metaphor of fortifying one’s castle — a moat is great, but a moat plus an iron gate, high walls, and fire bombs is more strategically prepared for castle defense!

First, before discussion of the strategies, we must stress that the risk reduction power is in applying all the layers listed below, in order, not in picking and choosing which steps to implement. We also must stress this article assumes that the community and region are not in the middle of a sustained outbreak, and that background community transmission is low. In New Hampshire, this generally appears to be the case. In other areas of the country, local public health data may indicate that remote learning is the appropriate option.

To summarize, our recommended strategies to re-open, in order of importance are as follows:

1. Keep distance between students — keep sufficient social distance, as much as possible and as practical between kids. This is harder with younger kids, and only slightly easier with older kids.

2. Ventilation — it is critical to supply fresh air and clean out contaminated air in the classroom.

3. Mask Use — because of talking, yelling, coughing — masks limit the emissions of particles.

4. Cohort model — keeping groups of kids together, where the teachers move versus the students move.

5. Symptom monitoring, isolation of students, contact tracing (flu will be an issue). Have that plan in place before the doors open.

6. Cleaning of frequently touched surfaces, before and after eating.

Now, let’s talk about each of these strategies in full.

#1 Distance between students

The key here is to keep students separated — as much as possible and as is feasible — from each other. The desired outcome is to reduce the number of students in classrooms, and it’s tricky to come up with the right formula. Why? The reality is that now that we know SARS-COV2 can be transmitted long distances, even hectometers. The “6 feet separation” rule that many school districts have adopted and the CDC recommends, came out of MUCH earlier thinking that the SARS-COV2 virus was emitted primarily in droplet form (so only sneezes and coughs). However, recent work by aerosol scientists, such as those who signed a letter to the World Health Organization, underlines that aerosol transmission is likely the dominant mode of transmission of COVID-19 (link: 239 Experts With One Big Claim: The Coronavirus Is Airborne — The New York Times). In other words, the virus is suspended in tiny invisible water droplets that come out of our mouths when we breathe, speak, yell or sing. More virus is emitted by more vigorous activity — singing and yelling emit a lot more aerosol particles than simply breathing. Knowing this, the 6 feet rule is now rather arbitrary since fine aerosols or tiny particles can be suspended in air for long periods of time, on the order of hours. Think of how you can see dust motes suspended in a sunbeam.

Therefore, the goal here should be reducing the number of students in indoor rooms as much as practical, because this reduces the amount of potential virus emitted, and the amount of exposure to others. The “6 feet rule” might help school administrators adjust occupancy limits, but other strategies might also help, like simply reducing each class size by half.

A parabola or U shaped classroom configuration is a great strategy to use to have sufficient distance between students AND ensure students are not breathing on each other all day. See Figure 1 below for the suggested example classroom set up for teachers. More information can be viewed at this link on how to set up a typical classroom in a U shape (plus adding ventilation in an old school building) at https://www.youtube.com/watch?v=3YStLqT2MdM&t=158s.

#2 Ventilation

Ventilation is the next important layer of risk reduction. Luckily this strategy is finally getting the attention it needs, due to the outstanding research and advocacy of scientists like Dr. Linsey Marr, Dr. Jose-Luis Jimenez, Dr. Shelly Miller and many others (all excellent follows on Twitter).

The best ventilation is 100% outdoor air, and outdoor education, but if that is not feasible then next best strategy is to encourage outdoor learning as much as possible. A strategy to consider is to start with a class indoors, for 50 minutes to 1 hour, then break for outdoor learning (allowing the room to ventilate for 15 minutes), and repeating this cycle as much as possible (with an extended lunch and recess period). For schools in rural areas, or with expanded grounds, getting outside often is an excellent idea to allow rooms to ventilate and also to provide much needed mask breaks for students.

For schools in urban areas, outdoor settings might present unique logistical challenges — but the idea is the same. Outdoor fresh air is best. A strategy may be a quick walk around the building for a mask break. One of the reasons outdoor air is better is sheer volume: diluting respiratory clouds helps to reduce risk of the hazard by reducing viral load of particulates. Also, outdoor air is exposed continuously to ultraviolet radiation, which helps to kill the virus. To this end, ultraviolet inline air treatment has been shown to be effective when put in-series within HVAC systems. It’s usually a wavelength that corresponds with ultraviolet-C (generally 254 nm, though 222 nm has potential promise of even more effectiveness); consult a professional for more information on this technique.

Schools with No HVAC and Schools with HVAC: a note on different strategies

Getting the maximum amount of outdoor fresh air into the classrooms as practically possible is a must for all schools. In older buildings with windows but no HVAC, an ideal approach is to use natural ventilation to create fresh air exchange. This literally means opening windows! In infection control settings, air exchange of 6 room volumes per hour is recommended — the idea is to get as close to this ideal as possible. This ensures effective ventilation control for many other diseases in addition to COVID-19. It may be possible to obtain this with 1 or 2 box fans, placed strategically to gently move fresh air in a cross-ventilating pattern across the classroom and outside (see Figure 1 below). Use of the box fans on setting one will allow enough air flow while not being too noisy or distracting.

In some cases, and as colder weather approaches, open windows may have to be minimized, and supplemental use of HEPA portable air cleaners will be necessary. It is still important to introduce fresh air, even if the window is open just a bit — in New Hampshire, we pride ourselves on our ability to add another layer of clothing before turning up the thermostat. But as the temperature drops, the use of HEPA filter units that have a Clean Air Delivery Rate of 300 or higher is recommended. It is also recommended to place a unit in between the teacher and the students, in the center of the room, in the U or parabola shape classroom configuration in Figure 1. A HEPA portable air cleaner can remove up to 40% of virus load and add that important layer of risk reduction (Source: http://poppendieck.com/IAQ/COVID19%20FAQ.html).

If you can’t locate a HEPA portable air cleaner — never fear. Do it yourself or DIY HEPA filter/MERV filter plus box fan configurations can also be effective. At the video link below, one of the authors (Traviss) reviews some HEPA Portable Air Cleaner basics and how to make a DIY Box Fan + Filter air cleaner for only $40.

The author (Traviss) tested the above set up, a Lasko box fan plus Home Depot FRP 10 air filter (MERV10). and found it removed 50% of the particles generated during a smoke test at a fan speed setting of 1 (lowest setting). The exhaust from a portable air cleaner (whether purchased or DIY) should point away from room occupants; ideally the exhaust flow is vertical and the HEPA unit is set up on its own desk at least 3 feet above the floor, so that the exhaust is going higher than the seated students. If the exhaust is horizontal, then the exhaust flow should be directed to a wall with no nearby students.

What if the school does have a HVAC system? This is when HVAC professionals need to be involved to evaluate the building’s system. Generally, HVAC systems are optimized to save energy costs, so outdoor air intake is minimized (only 20% fresh air intake), and air is recirculated (Source: https://www.tandfonline.com/doi/full/10.1080/23744731.2020.1794499). In a pandemic, the opposite strategy is needed. Outside air dampers should be opened to the maximum extent possible that the system can handle. Filters in the ductwork need to be at a high level of particle removal efficiency; a rating of MERV-13 on the filters is the right balance of particle removal and flow through of air through the duct work.

In many systems UV-C light can be added in the ducts for extra disinfection power. However, the air temperature and velocity in the ducts must allow enough contact time for disinfection to occur, with a general recommendation that the UV-C unit is placed in the mixed air location of the system (Source: http://www.sciencedirect.com/science/article/pii/S0360132313001649. Additionally, HVAC systems still should circulate that minimum of 20% fresh air through the rooms. Be careful of advice of using just UV-C in HVAC systems with only recirculated air — the ultraviolet light system might get overwhelmed by the sheer number of particles, unable to disinfect as expected.

Using HEPA portable air cleaners as supplemental air cleaning is also recommended in classrooms and will not interfere with HVAC operation. We recommend it in classrooms as the general HVAC infrastructure of most schools in New England is a hodge-podge of add-ons. There is no downside to using HEPA portable air cleaners which can add 2 ACH (air changes per hour) of air exchange to a room if sized appropriately.

#3 Mask use

Mask adherents such as ourselves have stayed on message regarding the need for masks since COVID-19 first arrived in the USA. This is because of the fundamental truism of mask wearing: wearing a mask helps prevent the pre-symptomatic, asymptomatic or sick person from emitting those tiny virus particles to others. It is the critical intervention to control the source of the virus. Additionally, wearing a mask provides the wearer with protection from exposure from others as well, blocking up to 50% of particles even with a simple 2 layer cotton cloth covering. If everyone in the USA wore a mask consistently and always in indoor spaces, then we could extinguish COVID-19 transmission chains across the country within 4 weeks, as the source reduction (on the presymptomatic people) and the exposure protection (to the mask wearer) would combine to help reduce transmission by 95%. Follow up contact tracing would be effective at reducing the rest of cases. This strategy has been effectively used in many other countries in Western Europe.

There are parents that don’t agree with wearing a mask for themselves or their children. Unfortunately, the CDC flip flopped initially on mask usage, which has created real confusion about the importance of masks indoors. There are politicians that are not putting science front and center, or have vehemently stoked political fires about this powerful exposure reduction tool. Americans, and especially Granite Staters, are very independent which is something to be celebrated, especially with respect to entrepreneurship and innovation. Americans tend to be individualistic, and don’t like the government telling them what to do. We get that. However, most Americans do appreciate the distinction between science and medical advice versus politics — blurring this distinction has led to the rampant COVID-19 spread in the USA now.

So while some parents might be anti-mask, and now we know why from a sociological standpoint, school administrators and teachers need to hold the line on students wearing masks. Most young people with COVID-19 are asymptomatic and presymptomatic transmitters which makes it critical they wear masks to protect their classmates, their peers with medical conditions, teachers, staff AND parents and grandparents. Teachers — know that mask wearing doesn’t have to be 100% perfect to be effective. Yes, please model for students how to wear masks properly, remove them correctly, encourage students not to touch their face, and provide positive feedback frequently. But don’t get into an endless loop of behavior management on proper masking — as long as close to 90% of the students are doing it right, that is sufficiently protective for the group.

Dozens of studies in the last 3 months have confirmed that wearing a mask is one of the most powerful tools we have to reduce transmission of the virus (Source: https://science.sciencemag.org/content/368/6498/1422). However, mask wearing MUST be used in conjunction with the other risk reduction strategies in this article. Additionally, it will soon become apparent that mask-taking breaks must be incorporated into the schedule to support everyone’s emotional, social and cognitive needs. We recommend mask breaks are planned for and supported — not an add-on. This loops back to a previous recommendation (see #2 Ventilation, above) to teach in 50 minute blocks, with time for breaks for the room to ventilate but also for kids to get a mask break outside.

#4 Cohort model for classes

Every person one comes into contact with indoors is a potential source of virus. To reduce the complexity and increased risk introduced by the potential of multiple interactions with new people each day, use of a cohort or pod model is recommended for schools. It works like this: keeping students in a specific cohort in a specific classroom for the semester while they are in school. Only teachers move between rooms. In this way, students are only interacting with a relatively small set of other students (<15), instead of adding more random student interactions by mixing classes and students. This strategy is especially critical for younger children who will find it difficult to maintain social distance and mask adherence. A cohort model reduces opportunities for the virus to find more hosts, and also makes it easier to contact trace and isolate the pod if one student comes down with symptoms. It requires a lot of planning and logistical coordination, but it can be done. It’s likely easier to do in the elementary school grades up to and including middle school.

# 5 Symptom monitoring, isolation of students, contact tracing

It is critical that schools are ready to support kids with symptoms. Ideally, children with symptoms will be kept home, but we all know that kids also get sick during school hours. Schools must have the administrative procedures and staffing personnel in place to support potential COVID cases before opening the doors. Schools must be able to address the following issues: What will happen if a child presents with a fever, severe cough, or shortness of breath? Where in the school will the child be isolated? Does the school have a nurse? If not, who is responsible? Is there a room with negative pressure for students to remain until parents are notified? If not, will the room have a HEPA Portable Air Cleaner or other way to keep the area safe?

What are the procedures for isolation for that student (minimum of at least 10 days since symptoms first appeared plus at least 1 day fever free)? Who will track that? Who will notify close contacts (classmates within 6 feet)? These emergency contact procedures for managing cases and contacting the NH DHHS or appropriate state agency will be important to have in place BEFORE opening the doors. All this must be clearly and communicated to parents in a checklist for easy understanding by all. Contact tracing must be a team effort between parents and the school.

#6 Cleaning of frequently touched surfaces and hand hygiene

People might question us — why are cleaning and hand hygiene last on the list? It is simple: because of the overwhelming evidence that COVID-19 is spread via air. Therefore, the primary risk reduction strategies we most strongly recommend are those efforts to reduce exposure to virus particles in air. This is no known documented case of transmission of COVID-19 from a surface, but we acknowledge the potential exists. It is also simply excellent public health advice to support good hygiene to prevent all virus and bacteria transmission as much as possible. Those surfaces that are frequently touched at schools, such as doorknobs, desks, bathroom faucets, shared gym equipment in high schools, etc., should be frequently cleaned. Computer sharing should be minimized or keyboards cleaned between students. Common sense is the guide here as there is no need for “hygiene theater”.

More frequent hand washing should be encouraged and especially setting up time for kids to wash their hands BEFORE eating lunch and snacks. This is standard good hygiene practice anyway, and can help reduce risks from other health concerns such as GI bugs, common colds, and food allergens.

With enhanced hand washing, we should see the added benefit of possible risk reduction in flu, colds, and gastrointestinal illnesses in kids.

COVID19 is challenging all of us — parents, teachers, community members — in a return to school. School is important for our children for their social, emotional and academic development, but during a pandemic, school districts must take steps to reduce risk of COVID transmission to protect not only the kids, but community health. For those schools who plan to physically reopen, applying these steps or risk reduction layers in order will help to keep students, teachers, staff and the wider community safe. It isn’t a pick and choose menu. All the layers are necessary. In New England, one doesn’t go skiing in February without wearing the appropriate layers to prevent frostbite; the approach to reopening schools should have that mindset, with all risk reduction layers in place.

Figure 1: The student desks are the blue boxes arranged in an upside down U shape to provide distancing and reduce the effect of students exhaling on each other. The window or windows are open at the back of the classroom, and box fans on the side moving the fresh air in a gentle cross flow to outside the building. A HEPA portable air cleaner or DIY Box Fan + Filter is placed in the center of the classroom. This figure combines multiple risk reduction layers described in the article.

Dr. Nora Traviss, PhD is an environmental health scientist and member of the New Hampshire Science & Public Health Task Force. She has professional experience in exposure assessment, environmental engineering, and risk reduction in industry, as well as extensive research experience in aerosols and toxicology. Her research has been primarily focused on aerosol particles from combustion of biofuels and their toxicological effects on human epithelial lung cells. Currently, she is using her interdisciplinary background to help improve indoor air quality at local schools and businesses and increase public awareness how to reduce spread of COVID-19. She thanks Dr. Asit Mishra and Dr. Jose-Luis Jimenez for their generosity of indoor air quality advice and encouragement on implementing DIY solutions for schools.

Dr. Ben Locwin is a healthcare executive, a member of the New Hampshire Science & Public Health Task Force, and various public health advisory panels. He has been on the front lines of the international coronavirus pandemic, including participating on various Task Forces and advisory panels. His current work has included providing oversight to various treatments for COVID-19 currently in clinical trials, as well as evaluating the upcoming vaccine candidates for safety and efficacy.