A porous material on which solid particles present in air or other fluid which flows through it are largely caught and retained. Filters are made with a variety of materials: cellulose and derivatives, glass fibre, @CT07540@, synthetic plastics and fibres. Filters may be naturally porous or be made so by mechanical or other means. @MT06878@/@CT07540@ filters are prepared with highly controlled pore size in a sheet of suitable material such as polyfluoroethylene, polycarbonate or cellulose @E02219@. Nylon mesh is sometimes used for reinforcement. The pores constitute 80–85% of the filter volume commonly and several pore sizes are available for air sampling (\(0.45{-}0.8\ \unicode[Times]{x3BC}\text{m}\) are commonly employed).