by Ruchira Paul
We don't have accurate standards for measuring smell. No yard stick like the wavelength of light, nor any criterion like scale, pitch or timbre that measure sound, exists for odor perception. We don't even have very good words to describe smells. Yet like sight and sound our sense of smell is a powerful component of our experience and memory.
The exact mechanism of olfaction is still being debated. Odors are defined by our reaction to them. Adjectives like overpowering, fruity, spicy, pungent, appetizing, stale, putrid etc. are used to describe smells. We have formulaic expressions such as bouquet, aroma, fragrance or stench which can encompass a whole host of different smells, unlike a precise word like “blue.” The best we can do sometimes is to compare a new smell to another familiar one. We resort to evocative similes such as “the restaurant had a dank odor like a musty cellar,” “her hair smelt of green apples,” “he reeked of alcohol and tobacco,” or “the garbage is stinking of rotten eggs.” We use abstract concepts for imagined odors – a smell like death, the acrid smell of fear, smelling victory. Impossible though it may be to put in words, we are constantly reacting to our environment through our sense of smell. Recording an odor in the brain can also trigger other physiological reactions – increase or decrease in appetite, nausea, feeling of cleanliness, relaxation, sexual arousal. We even smell danger. In the case of fire we often smell it burning before we see its light or feel its heat. Most of the time we are able to get a fair idea of what an odor may be like from its description, provided we are familiar with the reference through prior experience.
Smells are powerful reminders of our past experiences. It has been established that the olfactory memory stays with us long after visual memory has faded. Vivid childhood memories are very often intimately associated with distinct scents. We clearly remember festive days, our mother's proximity and mundane household routines from remembered smells. Most of us who are parents, forever recall the happy smell of an amalgamation of baby powder and regurgitated milk which dominated our senses when our children were infants. A particular scent in one place reminds us of another one far away. Many of us are reminded of the homes we have lived in and the places we have visited through odors. I have never encountered the very special fragrance of wet earth after the first monsoon rains break the oppressive monotony of the scorching summer heat, anywhere outside India. A pleasant smell does not necessarily evoke pleasant memories and neither is a foul odor always a reminder of an unpleasant one. A friend of mine could not stand the perfume of sandalwood; it reminded her of the day in childhood when her grandfather died and the house was redolent with the scent of sandalwood incense. On the other hand, the rather noxious reek produced by the intermingling vapors of benzene, acetone and acetic acid, the stock odor of all chemistry labs, to this day floods my mind with pleasing nostalgia for the earnest hours spent as a student and teacher of chemistry many moons ago.
The reason our sense of smell is somewhat of an enigma and why it is often termed the “forgotten sense,” is that unlike sight and sound which require physical stimuli, smell (as also taste) is a chemical sense. Our olfactory system reacts to tiny smell bearing molecules in the environment which waft into our nostrils.
The mechanism of olfaction is described by a pathway that leads from the external olfactory organ to the brain. The most widely accepted explanation for the first step when an odorant molecule is recognized by an organism is that it happens via a ligand-receptor action. The idea is that a molecule of a specific shape and composition fits a specific receptor in the nose (or the antenna in case of insects) setting forth a chain of transmission that ends in the brain resulting in the sensation of a unique smell. Another idea, the Quantum Vibration Theory of Scent, rears its head from time to time as a competing explanation. Most biologists tend to dismiss the vibration theory as fashionable junk science. A recent paper in PLoS ONE claims anew that while the structure of a molecule is indeed key to defining smell, it is not by the conventional receptor-ligand binding that we perceive odors. Instead, olfactory receptors detect the vibrations of a molecule to decide which substance smells (and how) and which doesn't. The renewed claim is being bolstered by substituting ordinary hydrogen atoms with its the heavier isotope deuterium in odorants. The substitution does not change the molecular structure but does effect molecular vibration and supposedly also alters the scent. According to the researchers, people as well as flies are able to tell the difference. But not everyone is persuaded by the evidence. I have not read the original paper but the report in Scientific American does not convincingly close the case for either theory.
In examining the nature of scent, we find some common characteristics but not a predictable formula. As the article points out, hydrogen atoms, sometimes lots of them, are present in most odoriferous substances. But hydrogen itself is odorless. What determines whether a substance is fragrant, stinking or neutral smelling depends on what else is present and how it binds hydrogen. Most strong odors – fragrance as well as stench – have organic origins. But smaller inorganic molecules can also be smelly. Sulfur, one of the scientists interviewed points out, is a common culprit in stench production. The small sulfur compound, hydrogen sulfide produces one of the most commonly recognized foul odors usually described as the “rotten egg” smell. It is formed during the decomposition of organic matter in oxygen poor settings such as swamps, garbage dumps and in the intestinal tracts of animals. It is also present in volcanos and it most certainly would be a major component of the atmosphere of the fire and brimstone spewing hell of religious mythology. It is a simple molecule in which one sulfur atom binds two hydrogens.
Here is what Tim Jacob, a supporter of the vibration theory of smell says about sulfur.
Meanwhile, smell biologist Tim Jacob of Cardiff University in Wales, says that rotten egg smell is a good example of the vibration theory's appeal. Sulfur is a chemical hallmark of rotting organic material—something that is dangerous for us to eat. And molecules containing sulfur almost always smell horrible to us, he says—just as should be the case if evolution worked properly to favor our survival.
Let us also look at oxygen and water, two odorless substances that are vital for our survival. The structures of water and hydrogen sulfide are very similar – see image above. But pure water is odorless (as also tasteless). Sulfur follows oxygen in period VI of the Periodic Table. Both have six electrons in their outermost shells and show similar properties in binding other elements. Based on the structure alone, it is okay to wonder why water doesn't smell. The reason of course has to do with its ubiquitous presence. We are water based living forms; water is present in our tissues, organs, bodily fluids and it lines our nasal passage. It makes sense then that water doesn't trigger a smell sensation in us. It is tempting to imagine that had water not suffused our system, it may have been perceived as a smelly substance like hydrogen sulfide. On the other hand, hydrogen peroxide, a molecule with one more oxygen atom than water but with a different molecular structure does have an odor. So the mere presence or absence of an element in a molecule is not a reliable predictor of odor. Consider oxygen itself, the breath of life on earth. In its odorless diatomic form it is present in the air and in our lungs. The other pure form of oxygen is ozone. It consists of three oxygen atoms and has a distinct odor. Its name derives from the Greek word ozein meaning “to smell.”
Part of the limbic system, olfaction is intimately connected to our emotions and behavior. So, whether we detect scents by the ligand-receptor method or as the author put it, by “ringing a molecule like a bell,” purveyors of goods and services know the connection and will continue to manipulate our sense of smell to profit from it. The perfume industry will promote new scents promising freshness, energy and sex appeal. Real estate agents will urge homeowners to bake bread and simmer potpourri to ensure a quicker sale of their property. And the casino industry and dance clubs will always know when to introduce an aroma into the room's air to energize tired customers and mess with their sense of decision making and inhibitions.
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Ruchira Paul lives in Houston and was born in New Delhi, India. An erstwhile chemistry teacher (once upon a time) and blogger (2005 – 2012), she is currently a person of leisure and a long-time fan of 3 Quarks Daily.