| http://www.w3.org/ns/prov#value | - invention, it is preferred according to the invention if the gas permeability, especially the air permeability, of the protective filter material 6 corresponds to at least 1.5 times, in particular at least 2 times, preferably at least 3 times, especially preferred at least 5 times the gas permeability, especially the air permeability of the outer material 3.For guaranteeing a good air-exchange fu
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