I'm writing a fantasy world where the good guys wear part of a mask to shield their identity and make sure they don't breath in too much pollution. I need something that isn't too heavy that they can wear (but not a handkerchief because that is similar to an idea that has already been published)
Cloth is probably your best bet to filter what is being breathed in a fantasy setting where you can't have things such as gas-masks, fine weave would get the smallest particles. You could pair it with a facial mask that covers more of the face than the mouth and nose that the cloth would, such as around the eyes. I know you said no handkerchief, but in a fantasy setting that's basically what you're down to, apart from a variation on the theme such as a scarf. To hide identity you'd probably do well with a hood or something to hide the hair.
Of course, it depends on what kind of fantasy you're writing. If it has magic involved you could have a stylized gas-mask that works through magic.
The N95 cover is exceptionally efficient because it channels air pollutants without any problems. While 95% of air toxins can be easily isolated through these veils as indicated by experts, people suffering from respiratory illnesses should not use these covers because they need to face the real consequences. The N95 screen here and there makes it hard to relax. Kids should not use the N95 veil. These covers do not provide complete protection against coronaviruses. You may find it hard to take the N95 cover and full protection is not guaranteed in the same way.
My strongest suggestion is to use the covers carefully as they are sufficient to repel 100% of the coronavirus toxins. They can do a lot without this spread of infection. The wearers in these covers have no breathing difficulties. Off-chance that it fits snugly for the face, the careful cover protects you from coronavirus.
Perhaps N95 can protect you from coronavirus but experts don't know about it because they can also cause shortness of breath.