Disclosed is an HEPA air filter for a respiration ventilator requiring fewer filter changes. The air filter comprises a housing with an air inlet and a coaxial air outlet, and a HEPA filter material in the path of air flow from the inlet to the outlet. The pleated filter material is formed as a tubular body positioned coaxial with the inlet and the outlet, frustoconical in shape and it tapers from the air outlet towards the air inlet. The tubular body is located at one of its ends around the air outlet and being blanked at its end opposite the air outlet so that the blanked end faces the air inlet and the air flow is inwardly through the tubular body. The air filter further incorporates an electrostatically charged material positioned at the blanked end of the tubular body and externally thereof to treat inlet air to the housing before it passes through the filter material. The electrostatically charged material is in the form of a flat pad seated on the blanked end of the tubular body and being positioned directly facing the inlet and perpendicular to the direction of air flow through the inlet and is of larger cross-sectional size than the air inlet.
