Filter made entirely with natural and biodegradable materials, for the protection of the respiratory tract of patients in the medical-surgical field and process for making it. The filter has a porosity or 80-98% and pore diameters of 100-350 micrometers and the pores have a shape of channels open at the ends, which are parallel to each other. The filter is obtained by preparing an aqueous solution of chitosan and an aqueous solution of gelatin, mixing them, pouring it into a container, keeping the container closed until obtaining a hydrogel and freeze-drying. A cross-linking step takes place by (a) adding a chemical cross-linker to the mixture of the chitosan and gelatine solutions and cross-linking before freeze-drying, or (b) subjecting the freeze-drying product to a thermal dehydration treatment.