Abstract: An air filter (1) particularly for use in a ventilator device for assisting respiratory function comprises: (i) a housing (3) with an air inlet (4) and an air outlet (6), and (ii) a filter material (8) in the path of air flow from the inlet (4) to the outlet (6). The filter material (8) is formed as a tubular body located at one of its ends around the air outlet (6) and being blanked (13) at its end opposite the air outlet (6). The air filter (1) further incorporates an electrostatically charged material (10) positioned at the blanked end (13) of the tubular body (8) to treat inlet air to the housing (3). This improves the life of the filter.