A smell is a sensation triggered by airborne chemical compounds called odorants, which are small, volatile organic molecules. The question of “where smells go” has two distinct answers: the physical fate of the odorant molecule in the environment, and the neurological fate of the signal it generates in the brain. The physical disappearance of a scent involves the dissipation and breakdown of the molecules themselves, while the sensation lingers within our neural pathways and memory centers.
The Environmental Fate of Odor Molecules
The physical existence of a smell ends when the airborne odorant molecules are either scattered or chemically altered. The primary mechanism for the fading of a scent in an open area is dilution and dispersion. As a chemical plume travels from its source, its molecules spread out, rapidly decreasing in concentration until they fall below the human detection threshold.
Another means of dissipation is the chemical breakdown of the odorants in the atmosphere. Ozone, a naturally occurring and highly reactive molecule, acts as a strong oxidizing agent that chemically alters the structure of organic odor molecules. This oxidation process breaks the compounds down into smaller, often odorless, components, effectively neutralizing the original scent. Ultraviolet (UV) light also contributes to this process, providing the energy needed to drive photolytic reactions that break down certain volatile organic compounds.
Odor molecules can also disappear from the air by a process called adsorption, where they stick to solid surfaces. Fabrics, carpets, and other materials with a high surface area readily attract and retain odorants. These surfaces act as a “sink,” trapping the molecules until changes in temperature or humidity cause them to be released again.
Detection: From Molecule to Electrical Signal
The process begins when odorant molecules are inhaled and travel to the olfactory epithelium, a patch of specialized tissue high in the nasal cavity. To reach the sensory apparatus, the airborne molecules must first dissolve into the thin layer of mucus that coats the epithelium. This mucus layer contains proteins that help shuttle the hydrophobic odorants to the receptors.
The actual detection occurs when an odorant binds to a specific olfactory receptor protein located on the cilia of an olfactory receptor neuron. Humans possess approximately 400 functional types of these receptor proteins. Binding to the receptor triggers a signal transduction cascade inside the cell, converting the chemical presence into an electrical impulse.
This cascade involves the activation of a G-protein, which leads to the production of cyclic AMP (cAMP). The increase in cAMP opens ion channels, causing an influx of positively charged ions that depolarize the neuron. The resulting electrical signal then travels along the neuron’s axon toward the brain, carrying the information about the detected odor. Once the signal is sent, the odorant molecule is either cleared by enzymes or diffuses away, allowing the receptor to reset and prepare for the next stimulus.
Interpretation and Memory: Where the Sensation Lingers
The electrical signal generated in the nose travels along the olfactory nerve to the olfactory bulb, a structure located just above the nasal cavity. From the olfactory bulb, the signal takes a unique pathway compared to all other senses, which route their information through the thalamus before reaching the cortex. The olfactory signal bypasses the thalamus and projects directly to the piriform cortex, which is the primary olfactory processing area.
The most significant aspect of this pathway is the direct, rapid connections that the olfactory bulb makes with the limbic system. The limbic system includes the amygdala, associated with emotion, and the hippocampus, involved in memory formation and retrieval. This anatomical proximity explains why smells often trigger involuntary, vivid emotional memories, a phenomenon called the “Proust effect”.
The odor information is processed in these brain regions before it reaches the higher cognitive centers responsible for consciously identifying the smell. This explains why a scent can evoke a powerful emotional reaction or memory long before a person can verbally name what they are smelling. While the physical odor molecules may rapidly dissipate in the air, the sensation and the meaning they create are consolidated and stored deep within the brain’s emotional and memory circuits.