Why Your Sense of Smell Is A Lot More Important Than You Think
Reviewed by Dr. Michael Leon, PhD, Professor Emeritus of Neurobiology and Behavior, University of California, Irvine.
If you asked most people which of their senses they could most easily live without, most would say smell.
When you go in for a regular check-up, you have your eyes and hearing tested. Yet almost nobody has ever had their sense of smell measured. So, many people are very surprised to learn that smell is the only sense directly connected to the brain’s memory and emotion centers.
What is Olfaction?
Olfaction is the scientific term for the sense of smell or the act of smelling. Olfaction is a chemical sense (chemosensation) that starts when airborne molecules enter the nasal cavity. Odor molecules bind to receptors on sensory neurons in a patch of tissue inside the nose. These neurons then send signals to the olfactory bulb in the brain.
History of Olfaction: The Oldest Sense
Smell is the oldest sense in evolutionary history. It originates directly from the basic survival mechanics of the earliest single-celled organisms. Long before animals developed noses, ancient single-celled life forms relied on chemical sensing—known as chemotaxis—to navigate their world. Then in early mammals, scent did the work of finding food, detecting predators, and navigating surroundings. CT scans of 190-million-year-old fossil skulls show that the olfactory regions were the first part of the brain to enlarge, ahead of the areas handling touch and coordination, and those early mammal brains ended up roughly 50% larger than their closest relatives.
The Nose-Brain Connection
Sight, sound, and touch go through the thalamus, a relay station in the brain that sorts and forwards signals before they reach the regions handling memory and emotion. Smell does not work that way. Odor molecules bind to receptors in the roof of the nasal cavity, and the signal travels through the olfactory bulb straight to the amygdala, which handles emotion, and to the entorhinal cortex, which then connects to the hippocampus, which forms memory. It is the only sensory system that reaches the cortex without an obligatory stop at the thalamus.
As Dr. Michael Leon, the creator of Memory Air, puts it, "The olfactory system has the only dedicated superhighway access to the memory centers and emotional centers of the brain. All other senses have to go through side streets."
One way you may have noticed this in your own life is when a smell returns you to a specific afternoon in your childhood more completely than a photograph did. In a 2019 review in Psychonomic Bulletin & Review, Hackländer and colleagues found that memories triggered by smell are reliably older than those triggered by words or images, and that they tend to feel more emotionally intense. A photograph of your grandmother's kitchen may prompt you to remember it, whereas the smell of that kitchen puts you back inside it.
Neurons in The Nose
When a neuron in your brain dies, it is generally gone for good. The ones in your nose are different. An olfactory receptor neuron lives about a month, then dies and is replaced by a new one grown from stem cells in the tissue at the roof of your nasal cavity. Your nose is one of the only places in the nervous system that rebuilds itself for life.
That is why people who lose their sense of smell to a virus can often get it back. Brandt and Huppert describe this regeneration as the basis "for long-term recovery of loss of olfaction due to respiratory infections, for example in pandemics like COVID-19," or after a head injury.
It also comes at a cost. These are the only neurons in your body sitting directly exposed to the outside world, with their endings in the air you breathe. Everything airborne reaches them first, and pollution and toxins kill them. Eventually, reducing your sense of smell (which affects your brain centers connected to memory).
Why Our Sense of Smell Has Changed
The human brain evolved in an environment saturated with smell. Body odors, other animals, cooking fires, unwashed everything, all day and every day.
"The brain evolved at a time when there was plenty of olfactory stimulation. Nobody took a shower, ever, so everybody was redolent, and everything around them was odorous."
— Dr. Michael Leon, UCI Magazine, Spring 2024, "The Nose Knows Where Memories Go"
Now take a deep breath where you are sitting. Most people, most of the time, smell essentially nothing.
Sealed buildings, climate control, refrigeration, deodorant, unscented products, and food engineered for shelf stability have removed almost all of it. On top of that, the olfactory system wears out over a lifetime from air pollution, smoking, toxins, chronic stress, chronic congestion, some medications, head injury, and viral infection.
A 2021 study in JAMA Network Open by researchers at Johns Hopkins examined 2,690 adults, 538 of whom were diagnosed with anosmia (the complete or partial loss of smell), and matched their records against long-term local air-quality data. Sustained exposure to fine particulate pollution was associated with roughly a 1.6 to 1.7-fold increase in the risk of losing the sense of smell, with older adults most affected.
The Bottom Line
Smell is easy to take for granted, but it's anything but minor. It's our oldest sense, and it's the only one with a direct line to the brain's memory and emotion centers. Its neurons are also unusual: they regenerate throughout life, yet they sit exposed to everything we breathe. That makes the sense of smell both remarkably resilient and vulnerable.
Modern life has left our brains with very little olfactory input. Sealed buildings, unscented products, and years of pollution, illness, and wear have stripped away much of the rich scent environment our brains evolved to expect.
For a system tied so closely to memory, that is concerning. The encouraging part is that the olfactory system responds to stimulation. Just as we exercise our bodies to keep them strong, we can give our sense of smell regular, varied input to keep it engaged. That idea is the foundation of Memory Air, developed by Dr. Michael Leon from decades of research on the nose-brain connection. Memory Air delivers a rotating series of scents while you sleep, giving your brain the olfactory enrichment modern life has taken away, with no effort required on your part.
Your sense of smell is a direct path to the parts of your brain that hold your memories. It's worth taking care of.