Odor - The Electronic Nose Artificial Olfaction Technology - page 86

From person-to-person, senses are variable in their range and acuity. Through
comparison with others, it was recognizing that some people can see or hear what
most of other miss, or others miss what most of us see or hear.
Odor, which refers to smells, can be utilized as a marker to identify certain
problems or sources of interest. These include air pollution, environmental con-
tamination, disease diagnostics, and as noted above, human identification in crime
investigations. Odor consists of volatile organic compounds (VOCs) that typically
have relative molecular masses between 30 and 300 g/mole [ 4 ]. Heavier molecules
do not occur as VOCs because they generally have a vapor pressure at room
temperature that is too low to be active odorants. The volatility of molecules is
determined by both their molecular weight and their intermolecular interaction,
with non-polar molecules in general being more volatile than polar ones. As a
consequence the most odorous molecules tend to have one or two polar functional
groups. More functional groups in general result in molecules that are much less
volatile.
The perception of smell consists not only of the feeling of the odors themselves
but of the experiences and emotions associated with these sensations. Smells can
evoke strong emotional reactions. In surveys on reactions to odors, responses show
that many of our olfactory likes and dislikes are based purely on emotional
associations.
4.3 Molecular Biology of Smell
Smells are with us all the time. Although the human sense of smell is lacking
compared to many animals, it is still very acute. We can recognize thousands of
different smells, and we are able to detect odors even in infinitesimal quantities.
All living organisms including humans respond to substances in their environment.
Humans have three distinct chemical senses which constitute the perception of
flavor. These are the sense of taste, the sense of smell (olfaction), and trigeminal
sense (irritation). The most significant contribution is made by the sense of smell.
In addition, the sense of taste and trigeminal sense are much simpler systems than
smell. The sense of smell has a much broader range and more power of classifi-
cation than either taste or trigeminal sense. As a result, the sense of smell is crucial
for humans and animals. It is considered one of the most ancient of senses. Smell
allows organisms with olfactory receptors to identify food, mates, predators, and
also provides not only sensual pleasure such as the odor of flowers, but also
warnings (e.g., spoiled food, chemical dangers). Therefore, it is one of the most
important systems of living organisms.
Our organs of smell consist of two patches (made up of about five or six million
yellowish cells—high up in the nasal passages) of epithelial tissue not much larger
than a postage stamp located near the top of the nasal cavity. It contains millions of
olfactory receptor cells, bipolar neurons ending in olfactory cilia. The cilia
(nonmotile) are cellular extensions bathed in mucus to which chemicals bind.