The airway can be a source of bad breath, commonly known as sinus breath. From the air filled sinuses, through the throat and into the lungs, there are plenty of opportunities for bad breath. Mouth breathing can make this worse. A simple solution that can help is flushing saline through your nostrils and into your sinuses once or twice a day. You can make your own salt water or buy little squirt bottles of saline at the drug store. Also, if you have tonsils (or pockets where your tonsils used to be) make sure you are not collecting debris, which can smell pretty awful.