From the nature of our olfactory system, we can deduced that to be smelled, material must have certain properties.
First it must be volatile. A substance such as onion soup cooking, for example, is highly odorous because it continuously gives off vapor that can reach the nose. On the other hand, a substance such as iron is completely odorless because it is not volatile and it does not evaporate molecules into the air.
Second, an odor bearing molecule must be soluble in water even if only to an infinitesimal degree. If it is not water soluble, it will be barred from reaching nerve endings by the watery film that covers their surface.
The third common property of odor bearing substances is that they are soluble in lipids or fatty substances, which enables them to penetrate the nerve endings through the lipid layer that forms part of the surface membrane of every cell.
The final common property is that the odor bearing molecules must be customarily absent from the nasal tissue. Individuals can become acclimated to odors. The baker doesn’t smell the baking bread, for example, and the barista at the coffee shop doesn’t smell the brewing coffee.
Odor is most apparent when both temperature and relative humidity are high