I'm pretty sure that viruses are a lot smaller than 2.5 microns, so if you want to take all possible precautions why not just wear an N95 mask which does filter viruses? The type of mask you seem to be describing is designed to block air particulates (pollution), not viruses. Yes it will clearly have some protection as it will filter droplets, but COVID is also transmitted through the air, so a mask needs to block viruses as well.