The Science of Smell

The sense of smell (or olfaction) is our most primitive sense and is located in the same part of our brain that effects emotions, memory, and creativity. Our sense of smell allows us to identify food, mates, and danger, as well as sensual pleasures like perfume and flowers/nature. Sudden scents, like smelling salts, will jolt the mind.


The senses of smell and taste, two of the five senses identified by Aristotle, are called “chemical senses” and are sometimes regarded as one sense rather than separate senses. About 80% of what we taste is actually due to our sense of smell. Without the sense of smell, we would only be able to recognize five tastes: sweet, salty, sour, bitter, and savory. A food’s flavor can be altered by simply changing its smell, while keeping its taste the same. In fact, our sense of smell becomes stronger when we are hungry.


Aromas, scents and fragrances, good and bad smells, are all odors or odorants. An odor is a chemical dissolved in air, generally at a very low concentration, which we perceive by the sense of smell or olfaction. Odors are also called “smells,” which can refer to both pleasant and unpleasant odors. In contrast, “stench” and “stink” are used specifically to describe an unpleasant odor. The terms fragrance,” “scent,” or “aroma” are used primarily by the air treatment companies like Prolitec as well as food and cosmetic companies to describe a pleasant odor. The term “perfume” is used to refer to fine fragrances or wearable scents.

Humans are able to distinguish over 10,000 different odor molecules. When inhaled, these odor molecules travel into the nose and interact with odor eceptors. The odor receptors then transmit the information to the olfactory bulb, which is located in the brain’s limbic system. The limbic system also controls memory and emotions, and is connected to the pituitary gland and hypothalamus area that controls the release of hormones that affect our appetite, nervous system, body temperature, stress levels, and concentration.

While there is no theory that explains olfaction fully, one theory is that millions of axons or nerve fibers cover the circumference of the olfactory bulb. Depending on which nerve fibers interact with or capture the odor molecules, a pattern of activity is generated which cause the perception of a unique smell. Another theory is that odor receptor function like a key-lock system. If the airborne molecules of a certain chemical can fit into the lock, the nerve cell will respond.

Some odors are perceived as pleasant like flowers, perfumes, and cooking aromas. Some odors are called malodors because they are perceived as unpleasant, stench or stink. Malodors are like pleasant odors and caused by specific combinations of chemicals. The perception of all odors is subjective and based on cultural conditioning or emotional state.

Since the olfactory system is located in the brain, the sense of smell is closely tied to memory, mood, stress, and concentration. For example, at Memorial Sloan-Kettering Cancer Center in New York, doctors use fragrance to reduce anxiety during medical testing. Doctors from Duke University Medical Center are treating women in menopause with fragrances to alleviate depression and mood swings. This use of scent to affect mood or behavior is called aromatherapy.

Anosmia is the loss of one’s sense of smell. The inability to smell can lead to loss of appetite, libido, and depression linked to smell memories. Anosmia is sometimes an early symptom of Alzheimer’s and Parkinson’s Disease as Alzheimer’s and Parkinson’s are degenerations of systems associated with the Limbic System.

The human olfactory system adjusts over time and has trouble detecting both bad and good odors provided they are not too strong. This is called olfactory adaptation and it usually takes an hour to become adapted to an odor or scent. For example, people working in a scented environment often adapt to the scent and lose their ability to detect it even if people entering the space can readily perceive it.

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