A filtering method and a filter device (10) for removing impurities from the breathing air (24) in a room, an air raid shelter or a vehicle. According to the filtering method, the air to be filtered is driven through a carbon dioxide filter (13) by a fan (15), with the result that at least a portion of the carbon dioxide and/or mold spores and other impurities in the air are trapped in the carbon dioxide filter. After the filtering, the filter is regenerated and the carbon dioxide and/or mold spores trapped in it are removed by a technique whereby air heated by a thermal resistor (20) is passed through the filter, this air preferably consisting of the same air to be filtered.