This post is an attempt to provide background to help you sort through the barrage of contradictory information circulating about masks for respiratory protection. First, some background on me. I studied the physics and practice of filtration in college in the contexts of air-pollution control, water purification, manufacturing, and industrial hygiene. (Yes, that was a long time ago.) In industrial hygiene, we specifically studied the use of various kinds of respirator masks to protect people from fine particles and droplets in the air. In my work, I wore various types of respirator masks for protection against various hazardous materials, including responding to emergency situations with potential for immediately-deadly exposure to highly-toxic materials. So I know a little about masks and filtration, and take the subject seriously. This does NOT make me an expert on using masks to protect a population from an infectious virus, but it does enable me to explain some basic principles of filtration and respiratory protection. Some basic factors include pore-size, resistance, thickness, and bypass.
I’ll start with bypass. If the air goes around the filter, not through, then the filter can’t catch it. If the mask does not seal tightly to your face on all sides, then it doesn’t matter much how many layers or what kind of material it is made from. (There is an exception to this that I’ll address later.)
Resistance is how much pressure it takes to push the air through the filter. The greater the resistance, the more air is likely to bypass and the less it is likely to actually be filtered. Generally, smaller pore size and more thickness are associated with greater resistance. The thicker and finer the filter material of the mask, the more tightly the mask must adhere to the face to be effective.
Thicker filter material and smaller pores are generally associated with more efficient removal of fine particulates and droplets. This, of course, assumes the air actually goes through the filter and not around it. Filters can remove particles much smaller than their pore size because in passing through the filter the particles tend to bump into the fibers and stick.
For extremely-fine particles, the viscosity of air can keep them bound to the air molecules surrounding them. (This is an oversimplification, but will work for this discussion.) Such extremely-fine particles tend to stay with the air when passing through filters and are extremely difficult to capture. Any filter fine enough to capture them will require a lot of pressure to push the air through—a lot more pressure than is practical for most mask wearers.
Now, for that exception I promised. Unlike the extremely-fine particles, larger particles that readily separate from the air flow can travel ballistically. For example, the particles released by a cough or sneeze can continue for a few inches in an approximately-straight path and be captured by a mask even if the air expelled with them is deflected around the mask. This appears to be the primary value of masks in a pandemic situation.
Posted January 29, 2021
Revised January 31, 2021: changed one word to reduce possible misunderstanding.