Filter medium

A filter medium for an air filter, comprising:

• a particle filtration layer of a non-woven or woven fabric;
• at least three active layers, comprising:
• a first catalytic active layer comprising:
• catalytic activated carbon particles impregnated with first impregnation material of potassium carbonate for targeted removal of acidic gases (pH<7), the a first catalytic active layer arranged at a flow face of the particle filtration layer;
• and
• a first adhesive bonding and cross-linking the catalytic activated carbon particles together, the adhesive bonding the first catalytic active layer to the particle filtration layer;
• a second active layer arranged on an opposite side of the first catalytic active layer relative to the particle filtration layer, the second active layer comprising:
• catalytic activated carbon particles impregnated with a second impregnation material, the second impregnation material selected from the set consisting of:
• hydrogen sulfide, sulfur dioxide, nitrogen dioxides, phosphoric acids, sulfuric acid, or combinations of these, for targeted removal of alkaline gases (pH>7);
• and
• a second adhesive bonding and cross-linking the activated carbon particles of the second active layer together;
• a third active layer arranged on an opposite side of the second active layer relative to the first catalytic active layer, the third active layer comprising:
• catalytic activated carbon particles impregnated with a third impregnation material comprising ethylene urea and phosphoric acid for removing ammonia and formaldehyde;
• a third adhesive bonding and cross-linking the activated carbon particles of the third active layer together, and
• wherein the at least three active layers differ from one another.