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Won’t a portable air filter (cleaner) just move the air with contaminated aerosols around to infect more people?

    Per Dr. Poppendieck “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.”


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