Re: Do we always need to inhale before we can smell things?

Actually, 'smell particles' don't need to reach the olfactory nerves, but 
rather they just need to reach some membranes in your nose that have 
receptors which result in activity in the olfactory nerves.  Basically, 
though, I think you've got the idea and that's not really what you're 
asking anyway.

Obviously, stuff will get to those receptors up in your nose faster if 
you're pulling air in through your nose -- which is why to smell 
something, we instinctively inhale.  But smells should still be able to 
get up into your nose if you're not breathing.  In this case, the smells 
move by 'diffusion' through the air -- same as how the smell of someone's 
stinky feet manages to cross a room even though there's no breeze.  Air 
particles are always jiggling and drifting about, and the stinky stuff 
will gradually work its way into your nose.  It just takes longer, and you 
might not be able to hold your breath that long.

You actually can smell while exhaling, but those smells from inside you 
have generally been in your nose already a while, and the smell 
receptors 'desensitize' -- that is, they don't respond to a smell after 
having been exposed to it for a while.  That's why you might notice a 
smell after first walking into a room, but then you soon don't notice it 
anymore.  This is also why you can't smell your own bad breath, which is a 
smell from 'inside' you (usually rotting food in your teeth or bacteria 
growing on the back of your tongue).  Your nose has been exposed to it all 
along, so it's desensitized.  But when you breath on someone else, it's 
new to them, and so they pass out from the stink.  So the real answer to 
your mystery about your own smells is not dilution, but desensitization.  
It works the same for smells on your outside too.  You may not notice your 
B.O. because your nose is desensitized to it, but other people will.  And 
that clearly has nothing to do with dilution of the smell from air in your 
lungs, which, if you want to see how much that is just see how far you can 
blow up a balloon with one big breath.


Here is some more discussion of how smelling works:
 http://www.angelfire.com/ms/OzConnection/How.html
 http://www.cln.org/themes/smell.html