Adding this in at the top of my answer so you can easily find it:
In many cases it has to do with how tightly something is woven. But there are other considerations, too. The mask needs to fit without gaps and it needs to be breathable. So you're probably better off with a mask that technically blocks less but isn't going to make you woozy or doesn't bulge and gap when you exhale because air doesn't move easily through the mask. You also need to be able to put on and remove the mask safely/correctly to minimize the chance of simply spreading whatever the mask trapped to your face and hands and elsewhere and immediately wash it. That may be difficult with buffs.
Based on what I've read, as to particle size, last I saw the virus was .01 microns, but the good news is that particles that size actually tend to move around more and be easier to trap than particles of .03 microns. That's why .03 microns is used for testing filters. It's max penetration level.
All that said, wearing a mask primarily helps to stop ill (but possibly asymptomatic) people from spewing their highly infectious particles into the air by sneezing, coughing, breathing excitedly, whatever. If we all did it, it would greatly cut down on air-borne particles. It does less for the wearer him/herself, but every bit helps. So while bandannas score low at stopping incoming particles, they are more highly breathable than some other materials and they do stop outgoing droplets.
The link at the top has some guides to making and wearing a mask, including a no-sew one from a bandanna and coffee filter. The coffee filter will increase the filtering ability. 🙂
If it helps, we went through this here. I live in Western NY state, and the county officials made the mistake of announcing very general information about the first 3 confirmed cases, including vague locations. It was a mistake. Some people not in those areas didn't take proper precautions under the rationale of "Well, it's not here. It's over there." So the county stopped giving much location info. Now the most you get is which of the 4 county fire battalions the case is in, which is a huge swath. There are always going to be idiots who can't figure out what "Stay home unless it's essential, and if you have to go out, stay at least 6 feet from others" means, but it turns out that the less help you give the general populace in rationalizing why they're safe, the better. Being a mostly rural county has helped, and most people are trying to social distance on top of that. That said, it's going to hit harder here than it has. We're watching it in NYC and Buffalo (which is in an adjacent county) and people here have to travel to Buffalo for a lot of medical reasons, some which can't be put off.
What the County Health Commissioner ended up telling people was essentially, "It is here. People in your community either have it or have been places where it is or have been in contact with people who have been exposed. You need to take precautions and assume that it is everywhere."
For the most part, the Simply family is okay. We all like each other. We may have to duct tape Simply Dad to his chair because he's used to getting up at god awful o'clock in the a.m., meeting his buddies for coffee, and trotting around town as he likes as he putters on projects. This has been hard on him, but as an old guy with heart and lung problems, this virus would be a death sentence. Simply Mom and I are bigger homebodies so this is much less of a strain. Mostly we just keep on going, though I admit this website and Survivor are some of my distractions. So if you think I'm overthinking something, I probably am, and I'm so glad I can. 😂