What Causes Smell?
Name: May L.
Date: Tuesday, December 10, 2002
Why do some chemicals give off smell and what exactly
causes the smell?
First off, chemicals might be detected as "odors" or "odorants" (smell is
what our nose does). Odors are detected through our olfactory sense by the
olfactory "chemoreceptors"...dogs have as much as 40, 000,000 per square
centimeter. Odorants are volatile chemicals that are detected in the inhaled
air by the olfactory epithelium located in the roof of the two nasal
cavities just below and between the eyes and above the roof of the mouth.
The olfactory epithelium in humans is about 2.5 square centimeters
containing, in total, about 50 million primary sensory receptor cells. The
chemoreceptors react to various chemicals like aldehydes...benzaldehyde for
example is the odor of almonds...and can be used as an artificial flavoring
of almonds in baked goods. Others are: diallyl sulfide (onions); allyl
isothiocyanate (mustard); capsicum (hot peppers).
There is also something else involved in the sense of smell called the
"trigeminal sense" named after the 5th cranial nerve which is involved with
this sense. When we "smell" something like menthol this trigeminal sense
gets activated and we sense coolness. Some scientists believe that the
majority of odors also stimulate the trigeminal nerve.
Peter Faletra Ph.D.
Office of Science
Department of Energy
Scientists have discovered that the odor of a substance is related to the
shape and atomic composition of the molecule/s producing the odor. Of
course, for the substance to reach the nerve sensors in the nose, the
material must be volatile -- that is, it should be able to evaporate and
appear in gaseous form. For example, salt and sugar have very powerful
tastes, yet both are odorless. Neither "evaporates" easily.
On the other hand, perfumes contain a variety of compounds that do evaporate
rather easily. When the vapors reach the nasal sensors, an electrical signal
is sent to the brain. The nasal sensors have "receptor sites" that
accommodate the shape and chemistry of odor-producing molecules. Different
molecular shapes and compositions trigger responses in different sensors.
Both flavors and odors are usually very complex mixtures. Our odor sensors
are very sensitive -- often much better than any scientific odor-sensing
hardware. Even so, certain animals have vastly better senses of smell that
we humans. Consider the drug-sniffing dogs that are so often used in law
enforcement. The dogs can be trained to detect things we humans would find
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Update: June 2012