You mean a mask which filters both on inhale and exhale? N95 masks do a decent job of that, although I'd guess there's still some leakage on exhale (not designed for that). N95s are still sold out in my area, so I got some KN95 which I use. The CDC tested some of them for effectiveness, many of them got pretty high marks: https://www.cdc.gov/niosh/npptl/respirators/testing/NonNIOSH...