Most of us live in places that barely smell like anything. Homes are sealed and climate-controlled, food is refrigerated, and almost everything we buy is made to be odorless. Our brains evolved in a world rich with smells, and today they're chronically deprived of them.
What does losing your sense of smell have to do with disease?
In 2024, Dr. Michael Leon, Emily Troscianko, and Cynthia Woo published a review in Frontiers in Molecular Neuroscience that searched the medical literature for conditions linked to a loss of smell. They found at least 139 of them.
The authors sorted those conditions into three groups:
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Neurological conditions, about 65 of them, including Alzheimer's disease, Parkinson's disease and multiple sclerosis
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Conditions affecting the rest of the body, about 60, covering everything from diabetes to kidney disease
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Inherited or congenital conditions, about 14, including Down syndrome, cystic fibrosis, and Fragile X syndrome
Some of these are conditions you would expect to see on that list. Many are not. That is a lot of separate conditions to have one thing in common.
Why does smell loss tend to show up first?
In many of these conditions, the olfactory loss does not arrive along with the disease. It arrives before it, sometimes by years. Doctors call an early sign like this a prodromal symptom, which means a warning sign that comes ahead of the illness.
Parkinson's disease is the clearest example, and olfactory loss has been measured carefully. Researchers followed 2,267 men aged 71 to 95, all of whom were free of Parkinson's and dementia when their sense of smell was tested. Over the next eight years, 35 of them were diagnosed with Parkinson's, on average about four years after the smell test.
When the men were divided into four groups by how well they identified odors, the rate of new Parkinson's cases dropped steadily from the group with the worst sense of smell to the group with the best. Men in the worst group had roughly five times the odds of a later diagnosis compared with the two best groups.
What do 139 different conditions have in common?
All 139 of these conditions are associated with chronic inflammation. Inflammation is the body's response to injury or illness, and it is helpful in the short term. However, when it becomes chronic and low-level, it damages tissue over time, including in the brain.
The olfactory system may be where that damage becomes visible first. The neurons that detect smell are the most exposed nerve cells in the body. They sit at the top of the nasal cavity with their endings directly in the air you breathe, which makes them sensitive and vulnerable.
They raise the possibility that, in some conditions, the smell loss is not only a signal. It may be part of what raises the risk. As Leon and colleagues wrote, "it is possible that for some conditions, it is the olfactory loss that raises the risk of expressing the symptoms of those conditions."
Dr. Leon goes on to say:
"Pleasant scents can decrease inflammation, potentially pointing to the mechanism by which such scents can improve brain health."
What has changed recently?
For about 25 to 30 years, the connection between smell loss and brain disease has been understood mainly as a warning sign. But it did not suggest anything to do about it.
What's changed in roughly the last five years is that researchers have begun testing whether the nose-to-brain connection could be used to help protect against these conditions before symptoms even appear.
The best-known result came from UC Irvine. In a six-month trial, 43 adults aged 60 to 85, all with normal memory at the start, slept next to a diffuser that released one of seven natural essential oils for two hours a night. The scent rotated, making it different each night.
This enriched group scored 226% better than the control group on a standard word list memory test. Brain imaging also showed improvement in the left uncinate fasciculus, which is a bundle of fibers connecting the memory regions to the front of the brain and one that normally declines with age.
Why do nonpharmacological approaches so often disappoint?
Some nonpharmacological approaches to Alzheimer's include regular exercise, cognitive training, and the MIND diet. Although these may reduce risk, they require someone to do something difficult every day, for years.
Olfactory enrichment can be done automatically. This is exactly why Memory Air was created. It is a bedside device that holds a belt of 40 small pouches of natural essential oil and is automatically released throughout the night. You press the button once. It starts an hour later, after you have fallen asleep, and does the work for you.
The Bottom Line
Smell loss is linked to at least 139 medical conditions. It often appears before other symptoms do, and every one of those conditions is also associated with inflammation.
For decades, that made smell loss useful mainly as an early warning sign. What's changed recently is that researchers have started testing what happens when you add smell back. In older adults, olfactory stimulation didn't just slow memory decline; it improved memory.