The purpose of this post is to take down the persistent misinformation that continues to spread about the efficacy of masks in preventing the spread of COVID-19, or for that matter, any respiratory virus. In other posts I provide scientific research-based support and explanations of how and why masks work in terms of how they interact with distancing, how heat and humidity make it better or worse, and how they are capable of filtering aerosols and droplets carrying the virus. In this post I want to explain the everyday Physics that you have already experienced as a basis for believing that masks work in these ways. Your own personal experience in Nature is sufficient for understanding how this works.
What goes up must come down is an adequate description of what people have observed in Reality for as long as there have been observers. Even animals innately know this fact of Nature. If you doubt it, go outside and throw a frisbee towards a dog and observe how well they judge the fall of that frisbee under the force of Gravity. Throw anything up in the air and watch what happens.
However, some things have the ability to hang in the air longer than others due to other effects. For example, If you spray water droplets into a sufficiently arid environment, you will notice that some of those visible droplets hang in the air longer than others. One of the reasons for this is that they immediately start evaporating into smaller droplets that have the ability to stay in the air for hours. You can’t see them once that happens though because they get too small.
Ever watch the wind blow some leaves around? Have you ever held a dandelion in your hand and blown on it to see the little white things float away? Eventually, those things will fall back to the ground, but not until the twists and turns of aerodynamics takes its full course.
Static cling. Unless you’ve never worn clothes, you’ve experienced this effect at least once in life. Friction and low humidity (or how arid it is) are the culprits. The world that we live in is full of charged particles, many of which are produced by surface friction. Ever rub a balloon on your head and then stick it to something? The reason that this happens is charged particles experiencing attractive forces. The rubbing action strips charged particles from one surface and transfers it to the other. That excess charge “wants” to find an empty seat and the oppositely charged things in Nature are ready and waiting.
One of the effects inside of the material structure of a mask is identical to what I’ve just described. Charged particles interacting with the interior surface of the mask fibers leads to attraction to the mask fibers in the exact same way as static cling. In fact, some mask materials are selected specifically for their electrostatic properties. They are designed to give aerosol particles the static cling treatment. Check out the curved path that a small stream of water takes near a charged balloon. Please note that air is also a fluid, just not as wet.
THE MAZE RUNNER
Masks made from non-woven materials have an elaborate pathway for particles being transported through them by airflow. It is essentially a really messy maze, but your typical well-ordered maze pattern is sufficient for understanding the dynamics. Consider, the basic zig zaggy rectangular maze pattern. What would happen if you rolled 100 balls into the entrance? Not much. Very few would make it past the first turn. Now imagine sending in 100 radio controlled cars with a random setting to periodically change directions. This would be more interesting because at least a few would make it deeper into the maze. Few, if any, would actually make it through the maze before the batteries ran out of power. They’d all get stuck in a corner or up against a wall somewhere in the maze. This is the fate of droplets and aerosols breathed into a mask. If they make into the maze at all, it is just a matter of time before they run into a wall and get stuck.
I have oversimplified things here for the sake of illustration, but these descriptions are basically the way it turns out as far as the Physics is concerned. Gravity is always working against droplets and aerosols particles. Electricity is working against them too. The pathway through a mask is a deadly maze for these particles given the right material structure under these forces. You would have to build a face mask out of fishing net in order to get it to fail. By the way, there are some cloth materials out there that are effectively very small versions of fishing net, and they absolutely do not work well. They should be avoided.
While we’re at it, let’s address the particle size issue that keep coming up. As seen below, masks are not struggling to catch virus particles, but rather the viruses are struggling (and losing) to get through.
For more information read Aerosol Filtration Efficiency of Common Fabrics Used in Respiratory Cloth Masks.
THIS VIDEO does an excellent job explaining all of the above.