How do odors work?
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Smell (Olfaction)
There are five human senses that help us interact and perceive the world around us. Of the five, taste and smell rely primary on chemical interaction with key receptors known as chemoreception; with taste being based on the physical interaction, and smell being based on remote interactions at some varying distance. From an abstract perspective olfaction can be a simple concept, being that receptors at the top of the nasal passage receive chemicals transferred through the air to relay a chemical response to your brain identifying the odor or scent. In practical research and experimentation the sense of smell is much more complicated. In each naval passage there is an approximately 2.5cm2 span containing approximately 50 million receptors. Each of these cells is linked through a single synapse directly to the brain, making it the human sense with the most interaction with the brain. The intensity of the perceived odor is related to the number of receptors used and the degree of excitement seen in the olfactory cells, whereby complex odors result from concurrent stimulation of two types of receptors.
Where does it come from?
Why does it smell so bad?
Odors
As olfaction is a remote sense, odors can only be sensed in the presence of a gaseous mixture and therefore all odor causing materials must produce vapors or molecules that mix or dissolve within the air. Substances that release no vapor, such as ionic salts are also odorless, leading to the conclusion that only volatile chemicals or molecules that that can be absorbed by the nose can be sensed. There are seven broad classifications for odors with innumerous subcategories relevant to food and cosmetic industries.
Although a greatly simplified method of representing complex odors, in 1916 Hans Henning, a German psychologist, created the Henning's odor prism, whereby all other complex odors derive from the primary odor classes. These primary odor classes can be broken down to more common or understandable odors whereby ethereal odors relate to fruity, nutty, or earthy smells; putrid odors relate to rotten, or foul smelling odors resulting from decomposing materials such as meat, manure, or urine. Further, resinous odors are best seen in woody, sappy, or leathery odors; whereby spicy, burned, and fragrant odors are as they sound.
Since 1916 these principles have been expanded upon to include the now understood seven primary odors: ethereal, camphoraceous, musky, floral, minty, pungent, and putrid; whereby all other complex odors are a derivation of these. Complex odors, for example jasmine blossom (a floral odor), contains more than 90 odiferous compounds all of which comprise the jasmine odor. Odor intensity correlates heavily with the odortype; putrid odors being by far the strongest at 0.1ppm (ppm - parts per million), floral odors at 300ppm, and pungent odors at 50,000ppm to create equal odor intensity.
Chemical Structure of Odors
The chemical composition or structure of odors varies greatly depending on the odor class, and even varies substantially within classes resulting in unique or different smells within each class. For example, when looking within the minty category in the fragrance class; there are both R-carvone and S-carvone molecules that result in two distinct minty odors. The difference is subtle, with the hydrogen atom near the asterisk is below the double bonded carbon in one molecule and above in the other - as seen in figure 4. Ultimately there are nearly unlimited complex odors, with a few common examples seen in the table to the right.
