Frequently Asked Questions about COVID19 in Classrooms
I read on the internet that...
As schools and teachers prepare for the return of some form of in-person instruction many teachers and administrators have lots of questions on how to return safely. The objective of this page is to help answer some of the most common questions teachers and administrators may have. I have chosen to answer questions based on talking to my sister (a teacher), and questions I have seen in Twitter and Facebook comments.
What do I need to know about aerosol transmission? Our strategies to combat COVID19 are focused on the three main routes of SARS-COV-2 (the one that causes COVID19) exposure: fomites, droplets and aerosols. Many of the questions I have seen surround the aerosol route. When discussing the aerosol route there are two important things to remember.
First, when we breathe, sing, or talk loudly we produce a continuum of aerosols (an aerosol is general term for a liquid, solid or mixture in the air) with diameters from 0.01 microns up to 100 microns. Aerosol greater than roughly 1 micron can contain viable SARS-COV-2 virus and those smaller than 50 microns can stay airborne indoors for minutes to hours.
Second, our exposure to aerosols containing SARS-COV-2 is a function of aerosol concentration and time. In a classroom we can't control the time we are present, but we can control the concentration of the aerosol. If we reduce the concentration of relevant aerosols in the room, we reduce our exposure to the virus, and we reduce the associated risk of contracting COVID19.
How can I teach safely in a classroom? Like all indoor environments we can never be completely safe from airborne SARS-COV-2 in a classroom. Rather, we want to think about transmission as a risk reduction problem. Consider the analogy of driving a car: driving a car is one of the most dangerous things we do. However, we have implemented numerous risk reduction strategies to make our commutes safer: traffic laws, seatbelts, crumple zones, airbags, anti-lock brakes, blind spot monitoring, and more. New cars are constantly getting safer in order to reduce our risk while driving.
We need to address our COVID19 risk in a classroom in the same manner. We will never be completely safe, but we can take a number of actions to make it as safe as possible: ensure low community transmission, mandate mask wearing, increase room ventilation, open windows, install MERV 13 filters in HVAC systems, install portable air cleaners, reduce number of students per classroom, reduce hours spent in a classroom, wash hands and clean surfaces. One step alone is likely not enough. More steps are better. We must acknowledge not all steps will be possible everywhere, but we should take as many as we can in as many classrooms as we can.
Do portable HEPA air cleaners remove the virus from the air? The SARS-COV-2 virus is about 0.12 microns in diameter. HEPA filters are rated to remove 99.97% of 0.3 micron particle. (The fundamental physics of particle and aerosol removal by HEPA filters is similar). So won't the virus get through the filter? There are two reasons the virus is unlikely to get through a properly installed HEPA filter.
First, the virus is rarely excreted from our mouth as a singular virus particle. It is often in aerosols that contain the virus, water, and salts that are naturally in our mouth. Hence, a viable SARS-2-COV virus in the air is likely part of a aerosol that is at least 1 micron in diameter.
Second, due to the fundamental physics of particle motion, the hardest sized particle to remove is approximately 0.3 micron particles. Below that size, particles start running into filter materials via Brownian motion (random diffusive motion). Above that size, particles start running into filter materials via inertial forces related to their velocity. In fact, the recent study in preprint that was the first to demonstrate viable SARS-COV-2 in the air, also demonstrated that a HEPA filter attached to a sampling device was the only case where no viable virus was detected in the air (Table 2). Hence, the 99.97% of 0.3 micron particles rating for HEPA filters is a conservative rating for other particles sizes, both larger and smaller.
I just bought a portable HEPA air cleaner. Where should I put it in the classroom? To be effective, portable air cleaners need to get as much of the air in the room through the filter as quickly as possible. Think about trying to beat an egg in a square pan. If you try to beat the egg with your mixing device in the corner or on an edge, the egg in the far side of the pan will not get stirred very quickly. However, if you place the beating device in the middle of the pan, the egg will mix faster (but still not perfect in the corners).
Likewise, in a classroom, you want to place an air cleaner as close to the middle of the room as possible, elevated up off the ground. This will maximize the amount of air moving through the HEPA filter. Make sure you place the air cleaner in a location where it won't fall onto someone and secure the cord to prevent any tripping hazards.
Most portable air cleaners draw air in from the sides and exit on the top. Many Do-It-Yourself filters made with box fans and filter furnaces exhaust air to the side. These DIY designs should not be placed on the floor as the exiting air may resuspend particles from the floor that may contain viruses.
Operate your portable HEPA cleaner on its highest setting. The higher the setting, the more air that moves through the filter and lower the virus concentrations will be in the air. Hence, highest setting equals lowest risk for people in the classroom. Lower settings maybe desirable due to fan noise, but be aware they will increase risk as less air is moving through the filter.
Finally, HEPA filters typically need replacement every six to twelve months. Prefilters may need replacement every three months. Wear a mask and gloves when changing the filter and place the filter in a sealed plastic bag for disposal.
We put a HEPA air cleaner in our classroom. Do I have to wear a mask in the classroom? Yes. Think about a used diaper in a room. What is the best way to keep the room from stinking? It is not to open a window or put in a filter. It is to remove the diaper from the room. This is one of the fundamental pillars of indoor air quality: source reduction.
Wearing a mask, even a cloth mask that isn't perfectly fitted, will reduce the number of aerosols that get into the air from a contagious individual. When we talk, we expel more particles that are airborne than when we are just breathing. When we talk loud or sing, we expel even more. Hence, even the teacher should wear a mask. Masks with clear mouth shields should be sought if people who need to lip read are present.
The goal is a layered risk reduction. We need to take as many actions as we can (increase ventilation, HEPA air cleaner, masks, etc.) to reduce the risk as none will completely eliminate it. Wearing masks will reduce average airborne particle load (1 micron) in the classroom by roughly 30%.
Won't a portable air filter just move the air with contaminated aerosols around to infect more people? I actually love this question because it shows the person has thought about what we intuitively understand about our indoor environment. We generally don't feel the air move indoors. Humans can sense air movement down to about 0.1 m/s (0.3 ft/s). Below that, we generally consider the air to be "still." Hence, there is a natural assumption that the air on the other side of the classroom won't reach me. But most of us have also observed particles moving in a sun beam in a room we feel is "still." So we intuitively think air is still, even through we have observed it moving.
In fact, in most indoor environments air is moving around 0.05-0.1 m/s (0.15-0.3 ft/sec). This means that if an airborne particle traveled in a straight line it could travel the length of a classroom in roughly three to five minutes. That speed is without taking into account inputs that enhance air movement (a person coughing, a person walking, the heat rising from every person, HVAC airflows, settling, thermal gradients due to sunlight).
You already know that this is true, just think about how long it takes you to smell a cigarette that someone is smoking from across the room. A room is much like a mixing bowl full of ingredients for making cookies. Once we add a new ingredient to the batter and start mixing, some of the new ingredient will stick to the edge of the bowl (like the corners of a room), but most of it will evenly mix throughout the batter relatively quickly.
But what about plumes forming when a portable air cleaner is on? Well, there will be plumes of aerosol containing viruses moving throughout the room without any filtering device present due to the all the things in a room that enhance airflow. Remember air is not still, even if you can't feel it moving. Airborne viruses are already moving around the classroom in a time frame that is faster than the length of a one hour class. So even without a portable air filter, everyone in the classroom will likely be exposed to the airborne viruses by the end of the class.
A portable air cleaner will slightly increase the air velocity and mixing in the room (although you won't likely feel air going into a portable air cleaner unless you are less than ~0.5 m or 1.5 ft from the intake), while also reducing the average aerosol containing virus load in the room by 10 % - 40 % (1 micron) over the duration of the class.
So will portable air cleaners in a classroom make the airborne virus problem worse? Probably not since the room is fairly well mixed in the time frame of a class even without an added fan (air cleaner) and the air cleaner will reduce the average concentration of aerosols that one breaths in. Will portable air cleaners keep me safe from COVID19 while teaching? We can never be completely safe while not in isolation. However, portable air cleaners are one layer of a risk reduction approach to make teaching in the time of airborne viruses safer.
Stay up to date. Research is still ongoing to identify new methods and refine existing methods to help make our classrooms safer. Continue to look for those results from valid, scientific studies and scientists with this area of expertise. I recommend following Linsey Marr, Jose-Luis Jimenez, Shelly Miller, and Rich Corsi on Twitter for updated information.