External Gross Particle Filter for left side of FreeStyle® Comfort® Portable Oxygen Concentrator Unit. Air enters the FreeStyle® Comfort® through air intake filters located on the front, bottom of the oxygen concentrator. These filters prevent dust and other large particles in the air from entering the unit. At least once per week, the air intake filters should be cleaned. Your oxygen provider may advise you to clean them more frequently, depending upon the operating conditions.