support nonwoven for the filter configuration the spunbond layer can in particular be formed in such a way that it corresponds to a so called microdenier spunbond nonwoven layer in accordance with ep 0 960 645 fig3 shows a four layer configuration in which the two exterior layers 31 and 34 are formed as spunbond layers these two layers can also particularly correspond to a microdenier spunbond nonwoven layer in accordance with ep 0 960 645 layers 31 and 34 do not have to be identical however a meltblown nonwoven layer 32 follows as the second layer in the airflow direction this meltblown layer can in principle be formed in such a way that it corresponds to a so called high bulk meltblown in accordance with ep 0 960 645 in the present case however the meltblown fibres are coated with chitosan this was achieved by means of quenching the extruded polymer with water spray water quenching during the meltblown manufacture whereby chitosan in a water soluble form was added to the water the chitosan is then bound to the meltblown filaments consequently achieving the antibacterial effect in this nonwoven layer the next to last layer in the flow direction is in turn a meltblown nonwoven layer 33 which is formed however as a fine filter layer as is the layer 22 in fig2 the filaments of the layer 33 can also be coated with chitosan analogously to the layer 32 fig4 shows the interior layer 41 as a drylaid nonwoven layer this nonwoven layer can in particular be carded or airlaid bamboo pulp fibres bamboo staple fibres and or chitosan fibres are used.