We are super psyched to offer fantastic five layer activated carbon face mask filters, which fit in our 100% cotton face masks. The filters measure a little over 4.25 x 3.25". It is recommend changing your filter regularly, for example after 5-6 uses. These filters are a great design - they offer multiple layers of protection to isolate dust, enable good ventilation, and they offer even more protection against airborne substances. One filter lasts about a week, or add four for a month's worth of additional protection - you can choose in the dropdown menu.