The aircon exhaust lever (usually found in window type aircons) can be toggled to be closed and opened; opening it will introduce fresh air to your room, which should be done when you feel that the air is stale (at the expense of a slight reduction in energy efficiency). Closing the exhaust will trap the air inside the room, only to be recirculated by the AC. You should have it closed most of the time. Only open it when the room has been closed for too long.