Abstract: An air purification system for purifying recirculated air eg in an aircraft, vehicle, offices or hospitals comprises the combination of a filter section with an ultraviolet illumination section and a coating of an antimicrobial agent on surfaces in one or both of the filter and illumination sections. Air flow through the illumination section is caused to be slightly turbulent by means of a multi-bladed fan being fixed in position between the filter and illumination sections. This increases the dwell time of contaminants in the illumination section. The antimicrobial agent includes as an active ingredient a quarternary ammonium salt and preferably as the following structure.