It takes only a single breath. Freshly mowed grass or the delicate aroma from the pages of an aging book can instantly elicit nostalgia. Smell is our most emotionally potent sense, capable of evoking memories and stirring long-forgotten sentiments. But before scent becomes memory, it is first chemistry.
The building blocks of scents
The aromas that people encounter begin with volatile organic compounds, lightweight molecules that easily evaporate into the air until they reach the tip of the nose. Their unique chemical structures provide the familiar fragrance that people recognize, from the earthy scent that permeates after rainfall to the sweetness of vanilla in a morning brew.
Our sense of smell then starts with the olfactory receptors—specialized proteins in the nasal cavity that detect airborne molecules. When scent molecules bind to these receptors, they activate a nerve pathway that directly sends signals to the olfactory bulb, a structure in front of the brain that organizes these signals into a distinct “scent map” that allows us to distinguish between various scents.
Concentration also fuels how odors are perceived. Chemist and NAST academician Fabian Antonio Dayrit explains, “Some compounds at low concentrations have a different reaction, and at high concentration, [they have] an [entirely] different reaction.” Rather than producing a fixed aroma, compounds behave differently depending on how concentrated they are. In small amounts, they may add subtle or pleasant notes, while higher amounts can make them overpowering or unappealing. Hence, there is no universal formula for “pleasantness.” Dayrit cited ammonia as an example; while it is generally considered unpleasant on its own, tiny amounts can enhance the overall character of a fragrance. Still, certain groups of compounds appear more frequently in popular fragrances. Dayrit identified esters as the most common, referring to a class of compounds responsible for fruity and floral aromas. Meanwhile, terpenes are compounds that provide flowers, herbs, and forests their fresh, woody, and, sometimes, citrus-like notes.
These compounds give familiar scents their signature fragrance, but they alone cannot explain why a single and short whiff can stir an emotion or memory in a person.
Neuroscience of smell
“It’s not just how it smells, [it’s] also the association.” As Dayrit emphasized, fragrances may evoke nostalgia not merely because of the molecules they contain, but also because the brain associates those scents to places, people, or memories. Additionally, memories associated with smell carry more emotion than visual memories because of how our brain processes smell. Unlike other senses, the neural pathway for smell bypasses the thalamus and travels straight to the limbic system—particularly the amygdala and the hippocampus—which governs emotions and memory. A medical article details how these two parts work together to send nerve signals to the frontal lobes of our brain. These pathways explain why scents trigger such vivid memories and evoke nostalgia.
That said, scents do not have a fixed quality of “pleasantness.” Scents are a dynamic interplay of biology, culture, and personal experience. Just like how beauty is subjective, “pleasantness” differs from one person to another. Preferences vary widely across cultures as well. For instance, the sharp smell of gasoline may be oddly comforting to some due to early life experiences like riding on a family member’s motorcycle during childhood. Others, however, may find it pungent and unpleasant if they were not as exposed compared to others.
The direct relationship between smell and memory does not only tell us about what’s good, but also alert us about what’s bad. A study reported that scents could trigger people with post-traumatic stress disorder or PTSD when certain scents associated with negative and intense memories are smelled. The study documented cases where exposure to specific smells—such as burning diesel fuel in a combat veteran or the foul odor of vomit in a former paramedic—instantly triggered vivid flashbacks, nausea, and emotional distress linked to their original traumas.
Furthermore, genetics shape these preferences, too, by influencing how our olfactory receptors respond to different molecules, making certain individuals more sensitive to specific odors. Overall, these individual differences are what gives scents their remarkable ability to linger in our lives, long after the molecules have faded.
The lasting power of scents
Scents begin as tiny molecules drifting through the air, but no two people will perceive it in exactly the same way. That is why some may find comfort in the fragrances of flowers, while others are drawn to the aroma of coffee, yellowing pages of books, gasoline, or freshly opened markers. What seems unusual or weird to some may feel strangely captivating to others, as every scent carries a different set of memories and associations.
There is no single scent that everyone likes, as our preferences are shaped by a mix of chemistry, biology, and personal experiences. Every familiar aroma becomes attached to a place, person, or memory.
Perhaps that is the lasting power of scent: invisible to the naked eye, but deeply woven into our minds, much like storytellers of our emotions. They leave traces not just in the air, but also in our memories, continuing to linger even after the scents fade.