Smells and scents are terms used for odorants (or aroma compounds) that can be either pleasant or unpleasant. Other terms for odorants tend to have more specific meanings. For example, enjoyable smells are also called fragrances, perfumes, and aromas, whereas disagreeable smells are termed stench, stink, and reek. Nonetheless, all smells are odorants, which are the chemical stimuli of the olfactory system. Today, scientists have grouped odorants into seven primary odor types: (1) camphoraceous, (2) ethereal, (3) floral, (4) musky, (5) pepperminty, (6) pungent, and (7) putrid. These seven categories of odors, however, may not be complete as science continues to study the sense of smell in humans and animals.
Anatomy and Physiology
The sense of smell is mediated by a volatilized odorant (either in air or liquid) that binds to olfactory sensory neurons (also called olfactory receptor neurons). These neurons are located in the olfactory epithelium in the posterior portion of the nasal cavity. Scientific studies have shown that there are millions of olfactory receptor neurons in the olfactory epithelium. Each neuron has cilia that protrude from the olfactory epithelium into the airway of the nasal cavity. This direct contact with air allows the volatized odorants to bind on to the receptors located in the ends of the cilia. Once a neuron binds its specific odorant, an action potential is generated and travels through the neuron’s axon to the brain for perception and identification. The axons of the olfactory sensory neurons make up the first cranial nerve (CN I) or the olfactory nerve.
Aroma compounds are chemical compounds that have a smell or scent that can be volatilized so as to enter the posterior portion of the nasal cavity. This means that most aroma compounds have a molecular weight that is less than 300 and are generally classified by chemical structure, such as alcohols, aldehydes, amines, aromatic, esters, ketones, lactones, linear- and cyclic-terpenes, and thiols.
Camphoraceous smells are often found in mothballs, medicated ointments (e.g., Vicks® VapoRub™), and rosemary leaves. Camphor is the main ingredient in the above items and it is found in the camphor laurel (a large evergreen tree), which is native to Asia. Camphor is a terpenoid (similar to cyclic-terpenes) and is a waxy, flammable solid. Because of its strong smell, most insects stay away from items with camphor, making it a successful bug repellent.
Ether-like or cleaning fluid smells are called ethereal. These smells are often found in essential oils (e.g., lavender, eucalyptus, and sandalwood) as well as dry-cleaning solution. Most of these kinds of smells are types of ethers.
Floral smells are those scents from flowers, such as roses, lilies, and lilacs. Some floral odorants may have a sweet smell. Many perfumes in cosmetics and laundry powders use a commercial compound, Lilial, which has a floral scent. Lilial is a synthetic aldehyde and in rare cases can develop as an allergen for some people.
Musky smells are scents derived from glandular secretions from animals, particularly the musk deer, and plants that emit similar smells. Musky odorants have been used for the base notes of perfumes and colognes since ancient Greek times. Today, a synthetic form of musk is used called muscone, which is a ketone.
Pepperminty smells are scents derived from mint plants, such as watermint, spearmint, and peppermint (which is a hybrid plant, Mentha x piperita). Peppermints are used in many gums and candies as the scent has been shown to freshen breath. Peppermint is a terpenoid (similar to terpenes) and its oil has been used topically for muscle pain.
Pungent smells are those that are strong and sharp, such as vinegar. In food science, pungency is a reference for spiciness from mild to hot. Capsaicin and other spices such as piperine mediate a spicy sensation by stimulating the free nerve endings of the trigeminal nerve.
Putrid smells are those that are associated with decomposition of proteins. Usually it is the smell of a decomposing body of a dead animal or rotten eggs. It is a very strong and unpleasant smell that can cause some people to vomit.
Jennifer L. Hellier
See also: Anosmia; Olfactory System; Pregnancy and Sense of Smell
Agapakis, Christina. (2011). What does this smell like? Wine snobbery made easy. Scientific American. Retrieved from http://blogs.scientificamerican.com/oscillator/what-does-this-smell-like-wine-snobbery-made-easy/
Gilbert, Avery N. (2014). What the nose knows: The science of scent in everyday life. New York, NY: Crown Publishing Group.
Schoenfeld, Thomas A., & Thomas A. Clenland. (2005). The anatomical logic of smell. Trends in Neurosciences, 28(11), 620—627.