An apparatus for filtering particulate matter from a gas. The apparatus includes at least one tube with a substantially axially located ionizer structure, and a fan for propelling the gas through the at least one tube. The ionizer structure includes a flat blade extending axially along at least a substantial part of the tube and having a saw tooth shape with a high number of sharp teeth placed regularly along the blade edges. The blade is twisted about its own longitudinal centerline to provide rotation for a gas stream flowing along the tube.