You might think that all there is to a mask are billions of very tiny holes, small enough to admit air, but block particles. This is wrong.
Before the electron microscope was invented in the 1930’s, biologists already knew of infectious agents that could pass through a block of unglazed porcelain. Hence the term, seldom heard these days, filterable virus.
So catching a virus that can pass through porcelain with something that looks like fabric is quite a trick. The secret: electrostatic charge. If on a dry day, you’ve ever had to unpack a box containing packing peanuts, and found that the peanuts tend to stick to you — that’s electrostatic charge.
It turns out that common, nonconducting materials can be so charged. Synthetic fibers can be placed between high voltage plates. When heated, the fibers become electrets, behaving analogous to magnets, acquiring permanent electric polarization. An electret has a + side and a – side. There are excess electrons on the – side, and a shortage of electrons on the + side. (Don’t be confused by the + and -. It’s an old convention.)
Electret materials are found in most microphones, many furnace filters, and high efficiency masks. Electrets attract and grip most small particles, even if they are small enough to pass through the pores of the mask.
But electrets have a shelf life. The charge in the fibers equalizes in time, at which point the mask behaves like it is made of plain fabric.
This is also why a high efficiency mask cannot be washed. On contact with water, the electret charge leaks around and equalizes. The excess electrons on the – side fill the vacancies on the + side. After washing, the mask is about as good as your scarf.
Your scarf is not totally useless, but it’s not enough for the exposure of a clinical setting.