Double layer filter element, the first of which has le medium micro porosity that retains impurities picoarse ù, while the second is equipped with a very fine micro porosity that retains the microscopic pulviscle, pithe insidious one. TECHNICAL CHARACTERISTICS Filter elements in self-extinguishing polyurethane sponge, formed da two hot-coupled layers with a special process that does not alter le technical filtration characteristics. The first layer is formed da a red filter element: in open cell expanded polyurethane with porosity 45 pp* The second layer is formed da a black filter element: polyurethane foam with open cells with porosity 60 pp* (*pp: pores per inch) 21x29.7 cm