text: Fatigue or desensitization.  The sense of smell acts as a warning sign (as does sound and site).  If you are exposed to an awful smell (such as your baboon) and you choose to ignore it, your body soon tires of sending the "this stinks, lets get away message" and you literally do not "smell" it any more.  Just like having a ticking alarm clock by your bed.  The first night all you here is tick,tick,tick,but soon you don't notice until it stops. But if you are exposed to a chemical that 'numbs' your sense of smell, well then that is self explanitory. case in point. Hydrogen Sulfide, that rotten egg smell, will desensitize your sense of smell.  H2S is toxic and you die if you breath it. People (in mines for example) because they would smell H2S "Oh no lets get out of here" and then say "never mind I don't smell it anymore", and die. So if you smell or don't smell something it is because you choose not to (baboon) or you can't (h2s). There is more to smelling than just your nose.