Why is it that I can only detect smells if I breathe, and not when I hold my breath?

Question

At school we're told that the receptors for smell (olfactory receptors) are present in the Schneiderian membrane. But I've noticed that I'm only able to detect smells only while I breathe and not when I hold my breath.

I even tried an experiment to confirm my observations and the results were positive. I placed an open bottle of really strong perfume fairly close to my nose, and lightly inhaled. I could easily detect the smell. But as soon as I hold my breath I'm not able to smell it anymore, even though I placed the bottle right under my nose I wasn't able to smell anything. Immediately I moved the bottle away, walked into another room and took a breath, and the smell of perfume was so strong it made me choke.

I guess this 'experiment' of mine contradicts what we've been taught. That Olfactory receptors present in the nasal passage simply pick up traces of molecules (in this case, perfume) and this is perceived as smell, since by keeping the bottle right under my nose the perfume would've already diffused into the air in large quantities and would've already entered the nasal passage, yet I was unable to detect it.

So my question stands, 'Why is it that I can only detect smells if I breathe, and not when I hold my breath?'

Answer 1

The olfactory epithelium is located in the highest part of the nasal cavities, overlying the cribriform plate of the ethmoid bone. In order for you to smell something, the odoriferous molecules have to travel the length of the nostrils (which are a pair of very narrow passages which also happen to be filled with hair) and up though the full height of the nasal cavities, having to pass three sets of baffles (known as the nasal conchae) on the way.

When you take a breath, your lungs and inspiratory muscles form a suction pump which pulls air through the nostrils up into the nasal cavities; some of this air, along with any odoriferous substances contained therein, is swirled up to the top of the nasal cavities, where the olfactory epithelium is located.

In contrast, when you're holding your breath, there is no significant air movement through the nasal cavities, and even airflow contained entirely within the nasal cavities is considerably obstructed (thanx to the aforementioned hairs and baffles); therefore, for any odoriferous substance (such as the perfume you're probably sick of smelling by now) to reach the olfactory epithelium, it has to diffuse all the way from the tips of the nostrils to the top of the nasal cavities on its own. This is a much slower process, and, therefore, it takes rather a while to notice anything smelly when you're holding your breath - even if it's, literally, right under your nose.