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
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 savoury. A food's flavour 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 odours or odorants.
An odour is a chemical dissolved in air, generally at a very low concentration,
which we perceive by the sense of smell or olfaction. Odours are also called ''smells,''
which can refer to both pleasant and unpleasant odours. In contrast, ''stench''
and ''stink'' are used specifically to describe an unpleasant odour. The terms fragrance,''
''scent,'' or ''aroma'' are used primarily by the air treatment companies like PURE
ved's as well as food and cosmetic companies to describe a pleasant odour. The term
''perfume'' is used to refer to fine fragrances or wearable scents.
Humans are able to distinguish over 10,000 different odour molecules. When inhaled,
these odour molecules travel into the nose and interact with odour acceptors. The
odour 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 fibres cover the circumference of the olfactory bulb. Depending
on which nerve fibres interact with or capture the odour molecules, a pattern of
activity is generated which cause the perception of a unique smell. Another theory
is that odour 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 odours are perceived as pleasant like flowers, perfumes, and cooking aromas.
Some odours are called malodours because they are perceived as unpleasant, stench
or stink. Malodours are like pleasant odours and caused by specific combinations
of chemicals. The perception of all odours 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 Centre in New York, doctors use fragrance to reduce anxiety during medical
testing. Doctors from Duke University Medical Centre are treating women in menopause
with fragrances to alleviate depression and mood swings. This use of scent to affect
mood or behaviour 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 odours provided they are not too strong. This is called olfactory adaptation
and it usually takes an hour to become adapted to an odour 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.