Carbon is a porous substance that can absorb volatile organic compounds (VOC's) on a molecular level by converting gaseous substances to a solid form and then attracting that solid to the filter. Carbon filters contain carbon impregnated filter fabric or granulated filter media. Typically, carbon filters are made with foam or fabric layers to hold the carbon in place evenly and to provide a larger surface area for the obstruction of particles. One pound of activated carbon media contains approximately 100 acres of surface area. Carbon filters are typically referred to as "Activated". The reason is that generally carbon is stimulated with a positive charge to allow particles like smoke and VOC's to attract more easily. Carbon also has the unique feature of acting as a physical filter to remove particles. Carbon filters have the ability to aborb and trap: Carbon filters are generally used alongside HEPA filters to help increase the amount and type of impurities a purifier can reduce. Examples of Carbon Purifiers: