Why do we only smell odors when we breath in?

The Big M:

How come when we breathe in, you can smell odors, but when you breathe out through your nose you cannot? Do the odors we smell while breathing in disappear in some odor disposal plant, or do the odor sensors only work in one direction?

-- Ramjet of El Cajon

The same highly skilled medical professional who unraveled the mysteries of gleeking tells us that olfactory fatigue accounts for some of this smell thing. When you suck in odor molecules and they zap past the odor sensors way at the back of your nose, those sensors become momentarily fatigued and aren't as sharp until they have time to recover. Ever notice how, if you stand in dog poop for a minute or so, you can't smell it anymore? That's olfactory fatigue. Ever wonder how a lady reeking of perfume can stand the stench? Her olfactory receptors have crapped out to that particular scent. She can't smell it anymore and figures no one else can either, and what's the point of perfume, anyway, if no one can smell it, so she pours on more. Also, the air we inhale gets mixed with air already in the lungs, the scent gets diluted, and is exhaled in several breaths, not just one.