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Name: May L.
Status: student
Age: 14
Location: N/A
Country: N/A
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.
Assistant Director
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 completely odorless.

ProfHoff 539

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