Filter - ist HEPA good enough?
A NASA study from 1996
A lot of people engage in projects working on masks to protect themselves or others from COVID-19. There is lots of discussion about what kind of filtermaterial to use.
The filtermaterial you want to use might depend on the type of risk you want to minimise. If you want others to be protected from you coughing at them a different kind of mask might be sufficient than protecting you from inhaling virus.
Well known HEPA Filters filters particles down to 0.3 micrometer. A study done by NASA shows if it can protect you from even smaller objects: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20170005166.pdf
NASA scientists did find: They do have some capability, but it depends on travel-speed of the smaller objects. For details READ the study.