A diesel particulate filter removes particulate matter from diesel exhaust by physical filtration. The most common type is a ceramic (cordierite or silicon carbide) honeycomb monolith. The structure is like an emissions catalyst substrate but with the channels blocked at alternate ends. The exhaust gases must therefore flow through the walls between the channels and the particulate matter is deposited on the walls. Other filter types are available, using sintered metal plates, foamed metal structures, fiber mats and other materials as the filtration medium. The filtration efficiencies of diesel particulate filters are > 99% for solid matter. Since diesel particulate matter has a non-solid portion, the total efficiency for DPM is lower than this, > 90%.