The brain always blocks out whatever sensory stimulus it's been exposed to for a while, it's like being in the store and hearing everybody talking around you, but just focusing in on your friend talking, all the rest is just filtered out.
People are very sensitive to ferment scent molecules, but since it's currently a part of your body anatomy, your neurons aren't as sensitive to it.
the ferment bacteria can do a bit of damage to neurons too, so sensitivity goes down a bit.