Losing your sense of smell can seem like a minor annoyance—until you realize the nose is connected to some of the brain regions scientists watch most closely as we age.
Smell naturally becomes less sensitive for many people over time, and common problems such as respiratory infections, allergies, nasal disease and certain medications can interfere with it. But researchers have also found that impaired smell can appear early in neurodegenerative diseases including Parkinson’s and Alzheimer’s.
That does not make a poor sense of smell a dementia test. Far from it. But scientists increasingly view olfaction as a potentially useful window into brain health.
Smell is more neurologically complicated than it seems
When you smell coffee, perfume or smoke, airborne molecules interact with specialized sensory cells high in the nasal cavity. Those signals travel into brain networks involved in identifying odors and connecting them with memory and emotion.
This is one reason a familiar scent can instantly summon a person or place you have not thought about in decades.
The National Institute on Aging notes that odor identification depends on sensory, cognitive and semantic abilities, and that brain regions involved in olfactory processing overlap with regions affected early in Alzheimer’s and Parkinson’s disease.
Smell is not merely a function of the nose; successfully recognizing an odor requires several parts of the nervous system to work together.
That makes changes in olfaction scientifically interesting—but also means many different problems can interfere with it.
Smell loss has been associated with neurodegenerative disease
Researchers have known for years that reduced smell is common in Parkinson’s disease and can precede the movement symptoms for which the disease is best known. Olfactory changes have also been studied in Alzheimer’s disease and mild cognitive impairment.
In longitudinal NIA research, declining odor identification was associated with cognitive changes and with greater accumulation of Alzheimer’s-related amyloid and tau pathology in older adults.
A declining sense of smell has been associated with markers of neurodegeneration, but it cannot tell an individual person whether they have or will develop Alzheimer’s or Parkinson’s disease.
That distinction is crucial. Association at the population level is not the same thing as a diagnostic result for one person.
There are many ordinary reasons you may not smell as well
The neurological findings can sound frightening, particularly for someone who suddenly notices that food seems bland or flowers no longer smell as strong.
But smell can change for many reasons that have nothing to do with dementia.
Respiratory infections can temporarily or sometimes persistently alter smell. COVID-19 made that relationship dramatically familiar. Nasal polyps, sinus disease, allergies, smoking, head injuries and certain medications can also affect olfactory function.
A broad review of olfactory dysfunction also describes normal aging itself as an important contributor.
Smell loss is a symptom with many possible causes, not a diagnosis of neurological disease.
The pattern and timing matter. Sudden smell loss during a respiratory infection is very different from a gradual unexplained decline over years, and either situation may warrant medical discussion depending on the circumstances.
Losing smell can affect everyday health even without brain disease
Smell does more than make life pleasant. It helps us detect smoke, gas leaks and spoiled food. It also contributes heavily to what we experience as flavor.
When smell deteriorates, food can become less appealing. Some people may eat less; others may add more sugar or salt in an attempt to make food taste stronger.
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A reduced sense of smell can affect safety, nutrition and quality of life even when its cause has nothing to do with neurodegeneration.
This is particularly relevant for older adults, who may already face appetite changes, medication effects or nutritional challenges.
Practical adaptations can help. Working smoke and gas detectors become especially important, as does paying closer attention to food expiration dates rather than relying on smell alone.
Researchers hope smell testing could eventually help identify risk earlier
One appeal of olfactory testing is that it can be relatively inexpensive and noninvasive. Researchers can ask participants to identify standardized odors, discriminate between smells or determine the lowest concentration they can detect.
Scientists are investigating whether changes in these tests, combined with other information such as cognitive testing, imaging or biomarkers, could help identify people at increased risk of neurodegenerative disease.
Smell testing may eventually become one piece of a larger risk-assessment puzzle rather than a stand-alone screening test.
That would be important because disease-related brain changes can begin long before obvious symptoms appear. Earlier identification could become increasingly useful as treatments designed to slow disease progression improve.
For now, however, routine home smell tests cannot diagnose Alzheimer’s disease, and a disappointing result on an online odor quiz should not be interpreted as evidence of dementia.
Pay attention to change rather than trying to grade your nose
People vary enormously in how well they smell things. Some have always had sensitive noses; others have never been particularly good at identifying scents.
That makes change from your own baseline potentially more meaningful than comparing yourself with someone else.
A persistent, unexplained change in smell deserves attention, but it deserves evaluation rather than panic.
A clinician can consider common nasal and medical causes and decide whether further assessment makes sense based on age, symptoms and health history.
Final word
For most of us, smell sits far down the list of senses we worry about losing. Vision and hearing feel essential; smell can seem almost decorative.
Science suggests otherwise.
Olfaction is deeply connected with memory, behavior, nutrition and safety, and researchers have found intriguing associations between smell decline and neurodegenerative disease. Yet those findings need to be interpreted carefully because smell also declines with normal aging and can be disrupted by numerous common conditions.
Your nose cannot predict your neurological future, but a meaningful change in smell can be useful information about your health.
The larger lesson may be to stop treating smell as an expendable sense. Whether it leads to a diagnosis of sinus disease, a lingering viral effect or simply a better understanding of aging, noticing when it changes can be worth mentioning.
Has your sense of smell changed as you’ve gotten older?
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