How come I can smell things when I inhale but not when I exhale?
Because when you exhale, no exteriour scents are going into your nose. Yet, air out of your longues flows OUT. So you’re smelling your longues, if anything, when exhaling. We’re very used to that scent, which is probably why we don’t notice it. When we inhale the scent of dog shit, it DOES tend to get noticed because it deviates from the smell of clean air.
What’s your next question? “Why can’t I get a cheeseburger down my throat whilst vomiting?”
my only guess has to do with sensitivity and smell-tolerance. i’d say that the particles of whatever you breathed in mostly get absorbed by or trapped in the lungs. those that do come back out are of a much lesser concentration than came in, so you don’t smell them. that, coupled with the fact that the rest of the particles would be lung-smell (first of all, i really don’t wanna smell my own lungs) (second of all, you are probably totally used to the smell of your own lungs) means no smells.
of course, that may be total bunko and it may have everything to do with motion of the air through the nasal cavities. the only way to test is as follows:
howard, fill a balloon up with some kind of stinky nontoxic gas. yes, you could use that, but i don’t see how you could fill the balloon up. anyway, take the balloon, place it between your lips, and hold your breath (but don’t stop the airflow through your nose). release the balloon, and tell us if you can smell the stank air as it rushes out of your nose.
What would you smell exhaling? The inside of your lungs. You really want to smell that?
You probably do smell your lungs. It’s just that like any other persistant smell, your olfactory bulb is desensitized to it. This is why people with bad breath can’t really tell, and it’s up to you to offer them a mint. (Motto at my wife’s job: If someone offers you a breath mint, take it.)
Ever have such a strong burp that it goes through your nasal passages instead of your mouth? Sure you have. You do it and say, “Whoa! Taco burp!” This is basically stomach gases being exhaled past your olfactory bulb.
Also, most of what you perceive as taste is really smell. Your tongue figures out the salt/sweet/sour/bitter part of the food. The smell of the food goes up the back of your mouth and gets smelled. The combination gives you your sense of taste. This is why food seems bland to you when you have a cold. Your olfactory is blocked by mucus, so all you perceive of your food is the S/S/S/B, temperature, and texture. This is why you might salt your food more when you’re side, to give it more stimulation in your mouth.
*Coldfire: “longues” *
Fancy way to spell lungs, eh? 
Whoops! Sorry about that… I was doubting what spelling to choose, and this somehow seemed right to me. Forgive the ignorant foreigner 
With the exception of the sarcastic reply from ColdFire, I’d like to thank all of you for your replies.
It just seems to me that the vast majority of a scent, let’s say skunk molecules, is not absorbed during the second or two it is in your lungs so it will still be present when you exhale.
Wonder what the actual exchange rate is for air in your lungs. I’d be real surprised if it is even 15%.
But let’s say for the moment it is 50%. So 50% of the skunk molecules you just breathed in, now you exhale, you should still be able to smell it. I believe our nose is sensitive to something like 2 molecules per million.
I wonder if it has something to do with not being able to smell when there is a high moisture content as is with exhaled breath.
not too bring this discussion down a few maturity notches, but i don’t think you’ve got it with the humidity. in my experience, a blast from the asst smells a lot worse in a humid room than in, say, death valley. think about it- have you ever had trouble smelling a locker room?
the thing to remember is that your nose is actually absorbing particles for you to smell them. and i would also think that (since your lungs are wet and sticky) smell particulates get trapped in there real easy. mucus mucous mucuos!
True about the locker room.
As for mucus trapping smelly bits… it is all just molecules; both the smelly ones and the not so smelly ones so I don’t see that being the case.