Fits all adult and kid's masks that have a filter pocket. A PM2.5 filter is a disposable insert that improves a mask's filtration effectiveness, especially against the smallest aerosolized particles. A certified pm2.5 filter  used with a fitted cloth pm2.5 mask can block the vast majority of airborne particulates, including up to 90% of fine aerosols. When evaluating breathable solutions suitable for everyday use, a pm2.5 filter + mask is one of the most protective systems available.