Some people, including myself, have had a smell that only they can smell. It could smell like dog-poop, old cheese, ammonia
, a burning smell, etc. The smell comes on, often, as a result of a cold or sinus infection. Some, not all, of these smells are caused by an anaerobic bacteria thriving in the upper sinuses, and I was able to remove mine with a Flip-Turn Sinus Flush, and many other people have had success with it as well.