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 polyurethane expanded to open cells with porosity 45 pp *. The second layer is formed da a black filter element: in polyurethane expanded to open cells with porosity 60 pp *. (*pp: pores per inch) 42x29.7 cm