Filtering medium based on activated carbon which is characterized in that it comprises three superposed layers, respectively an inner layer and two outer layers, the inner layer consisting of 80 to 95% by dry weight of activated carbon, the balance for 100% consisting of organic and/or inorganic chemical fibres, the first outer layer comprisingfrom 45 to 95% by dry weight of organic and/or inorganic chemical fibres, the balance for 100% consisting of activated carbon and/or of a material having a density of less than 0.9, the second outer layer comprising from 5 to 25% by dry weight of activated carbon, the balance for 100% consisting of organic and/or inorganic chemical fibres, and in that the weight of the inner layer is between 40 and 200 g/m and the weight of the outer layers is between 10 and 100 g/m .