A filter will help prevent microscopic particles, like those spread from Covid-19, from getting through your mask. Finding the right filter can be tricky and you should exercise caution.
The Center for Disease Control and Prevention recommends using a coffee filter. Studies have show its efficacy. Another experiment found that two layers of paper towels could alone stop between 23 and 33 percent of 0.3-micron particles.
Others have tried air filters or vacuum bags, but while those may be effective, they're also risky. Some make it hard to breath. Others include harmful fibers. And most folks don't need the level of filtration these materials provide.
Read the CDC's guidance on how to use a cloth face covering to help slow the spread of Covid-19.