Abstract: An electrostatic filter for separating particles suspended in air has an electrically conducting elongate separation tube and an elongate electrode within the tube connected to a high-voltage direct current source to produce an electric charge current from the electrode to the tube. The tube has air-inlet openings distributed over the surface thereof and is covered with a layer of air-permeable material. The layer of air-permeable material is dimensioned and said air-inlet openings are dimensioned and spaced over the surface of the separation tube to to provide an air resistance such that substantially the same pressure drop of the air passing through the layer and the air-inlet openings to the interior of the separation tube along the whole length of the separation tube covered by the layer is created.