What is odor?

We all know instinctively what good or bad odors are, but why is it so? What are the mechanics behind odor and its sources?

Odours are signals that guide our behavior, attracting us towards life and repelling from death and decay.

It is like a map drawn in thin air that helped our ancestors to survive through all odds and to make the right choices for their survival.

Today odours are still signals, as they build a very complex symbology in various areas of our life. 

There are several aspects relating to odor which involve all our activities, from sport to social relationships, so it is not strange that this problem is being targeted in many different ways since many years.

Anatomy of odor

Our nose

Technically speaking, odor is made of complex chemicals which are released in the atmosphere as a product of chemical or physical modifications. We are not going here into chemistry or physics, there are specialists who can do that very well. Examples of this are phenomena like fire or evaporation, the common point is that all these release particles in the atmosphere.

Odours and evolution

So what is the difference between good and bad odor? Actually none.

The difference is made in the evolution of our nervous system, which learned to associate odours related to decay, sickness and death to a certain type of response, and those related to life, reproduction and food to another type.

This is all there is to it, there is no difference between a good and a bad odor, other than its meaning for our survival as organisms.

Sources of odor

We are used to look at macroscopic causes when it comes to odor, with the purpose of making simple choices between what is good and what is not.

But the sources of odours are basically of two types: organic and chemical. Organics include mostly bacteria and some viruses, including the response of other organisms to these, while chemical is mainly solvents and other volatile substances. Also organic chemicals exist, derived from fossile oil and other such products.

When we look at, or rather smell, food, dumpsters, bodies, perfumes, paint, or any other substance, we are inhaling particles which react with specific sensors in our nose.

Our nose

The perception of odors, or sense of smell, is mediated by the olfactory nerve. The olfactory receptor (OR) cells are neurons present in the olfactory epithelium, which is a small patch of tissue at the back of the nasal cavity. There are millions of olfactory receptor neurons that act as sensory signaling cells. Each neuron has cilia in direct contact with the air. Odorous molecules bind to receptor proteins extending from cilia and act as a chemical stimulus, initiating electric signals that travel along the olfactory nerve's axons to the brain. When an electrical signal reaches a threshold, the neuron fires, which sends a signal traveling along the axon to the olfactory bulb, a part of the limbic system of the brain. Interpretation of the smell begins there, relating the smell to past experiences and in relation to the substance(s) inhaled. The olfactory bulb acts as a relay station connecting the nose to the olfactory cortex in the brain. Olfactory information is further processed and forwarded to the central nervous system (CNS), which controls emotions and behavior as well as basic thought processes. Odor sensation usually depends on the concentration (number of molecules) available to the olfactory receptors. A single odorant is usually recognized by many receptors. Different odorants are recognized by combinations of receptors.

The patterns of neuron signals help to identify the smell. The olfactory system does not interpret a single compound, but instead the whole odorous mix. This does not correspond to the concentration or intensity of any single constituent. Most odors consist of organic compounds, although some simple compounds not containing carbon, such as hydrogen sulfide and ammonia, are also odorants. The perception of an odor effect is a two-step process. First, there is the physiological part. This is the detection of stimuli by receptors in the nose. The stimuli are recognized by the region of the human brain which handles olfaction. Because of this, an objective and analytical measure of odor is impossible. While odor feelings are personal perceptions, individual reactions are usually related. They relate to things such as gender, age, state of health, and personal history.

Smell acuity by age and sex[edit]

The ability to identify odor varies among people and decreases with age. Studies show there are sex differences in odor discrimination, and women usually outperform men.[5]

Pregnant women have increased smell sensitivity, sometimes resulting in abnormal taste and smell perceptions, leading to food cravings or aversions.[6] The ability to taste also decreases with age as the sense of smell tends to dominate the sense of taste. Chronic smell problems are reported in small numbers for those in their mid-twenties, with numbers increasing steadily, with overall sensitivity beginning to decline in the second decade of life, and then deteriorating appreciably as age increases, especially once over 70 years of age.[7]

Smell acuity compared to other animals[edit]

For most untrained individuals, the act of smelling acquires little information concerning the specific ingredients of an odor. Their smell perception primarily offers information that elicits an emotional response.[citation needed] Experienced individuals, however, such as flavoristsand perfumers, can identify discrete chemicals in complex mixtures using only the sense of smell.

Odor perception is a primary evolutionary sense. The sense of smell can induce pleasure or subconsciously warn of danger, which may, for example, help to locate mates, find food, or detect predators. Humans have an unusually good sense of smell considering they have only 350 functional olfactory receptor genes compared to the 1,300 found in mice, for example. This is despite an apparent evolutionary decline in the sense of smell.[8][9] The human sense of smell is comparable with many animals, able to distinguish between a diverse range of odors. Studies have reported that humans can distinguish in the region of one trillion unique aromas.[10][11]

Habituation or adaptation[edit]

Odors that a person is used to, such as their own body odor, are less noticeable than uncommon odors. This is due to habituation. After continuous odor exposure, the sense of smell is fatigued, but recovers if the stimulus is removed for a time.[12] Odors can change due to environmental conditions: for example, odors tend to be more distinguishable in cool dry air.[13]

Habituation affects the ability to distinguish odors after continuous exposure. The sensitivity and ability to discriminate odors diminishes with exposure, and the brain tends to ignore continuous stimulus and focus on differences and changes in a particular sensation. When odorants are mixed, a habitual odorant is blocked. This depends on the strength of the odorants in the mixture, which can change the perception and processing of an odor. This process helps classify similar odors as well as adjust sensitivity to differences in complex stimuli.[14]

Genetic component[edit]

The primary gene sequences for thousands of olfactory receptors are known for the genomes of more than a dozen organisms. They are seven-helix-turn transmembrane proteins. But there are no known structures for any olfactory receptor. There is a conserved sequence in roughly three quarters of all ORs. This is a tripodal metal-ion binding site,[15] and Suslick has proposed that the ORs are in fact metalloproteins (most likely with zinc, copper, and manganese ions) that serve as a Lewis Acid site for the binding of many odorant molecules. In 1978, Crabtree suggested that Cu(I) is "the most likely candidate for a metallo-receptor site in olfaction" of strong-smelling volatiles. These are also good metal-coordinating ligands, such as thiols.[16] In 2012, Zhuang, Matsunami, and Block confirmed the Crabtree/Suslick proposal for the specific case of a mouse OR, MOR244-3, showing that copper is essential for detection of certain thiols and other sulfur-containing compounds. Thus, by using a chemical that binds to copper in the mouse nose, so that copper wasn’t available to the receptors, the authors showed that the mice couldn't detect the thiols without the copper. However, these authors also found that MOR244-3 lacks the specific metal ion binding site suggested by Suslick, instead showing a different motif in the EC2 domain.[17]

Evolutionary impact[edit]

Gordon Shepherd proposed that the retro-nasal route of olfaction (odorants introduced to the olfactory mucosa through the oral cavity often as food) was partially responsible for the development of human olfactory acuity. He suggested the evolutionary pressure of diversification of food sources and increased complexity of food preparation presented humans with a broader range of odorants, ultimately leading to a "richer repertoire of smells". Animals such as dogs show a greater sensitivity to odors than humans, especially in studies using short-chain compounds. Higher cognitive brain mechanisms and more olfactory brain regions enable humans to discriminate odors better than other mammals despite fewer olfactory receptor genes.[18]

Response strategies

The response of our sensors, reacting with our experience and education, gives us a full range of responses, which are grouped into the classical five responses: flee, avoid, ignore, succumb and attack.

This is what we see on the market nowadays: a full range of remedies which try to remove symptoms without resolving the underlying issues.

Moreover, being our society basically chemical, when the odor cause is being addressed this is done by exterminating it with chemical products.

What the chemicals do is twofold: kill all living sources of odor while emitting fragrance particles to cover any residual odor particle.

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