How Smell Works and Why It Matters

How does the sense of smell work?

Odor molecules in the air dissolve in the nasal mucus and bind to olfactory receptor neurons at the top of the nose. These neurons send signals to the olfactory bulb at the brain’s base, which then relays information to multiple brain regions, including those involved in memory and emotion. This network explains why smells can trigger strong memories and feelings.

How are smell and taste related?

Flavor perception is largely driven by smell—about 80–90% of what we perceive as flavor depends on olfaction. Basic tastes (sweet, sour, salty, bitter, umami) come from the tongue, but the nuanced flavors of food come from smell, including through retronasal pathways when aromas travel from the mouth to the nose during eating.

Why might someone still detect a few smells despite overall smell loss?

Not all olfactory neurons fail at once. Different smells depend on combinations of receptor types, so partial neuron loss can leave some smells intact or distorted. Day-to-day nasal inflammation and air quality can also affect detection; on a less inflamed day with better airflow and mucus balance, residual function may temporarily improve.

Smell Loss: Causes, Patterns, and Testing

What causes smell loss and where can the problem occur?

Smell loss can stem from issues anywhere along the olfactory pathway. Peripheral causes include nasal inflammation or nerve damage from viruses (such as COVID-19) and toxins/chemicals. Central causes involve changes in the olfactory bulb and related brain regions, seen in neurodegenerative diseases. Multiple sites can be involved in a single person.

Is smell loss usually gradual or sudden?

It varies. Many people lose smell gradually and may not notice until a marked change occurs; studies suggest up to a quarter of people with smell loss are unaware of it. Others experience a relatively sudden decline, for example after a viral illness or head trauma.

How is smell currently tested, and what are the limitations?

Common tests include the University of Pennsylvania Smell Identification Test (UPSIT) and Sniffin’ Sticks, which assess identification, discrimination, and thresholds. These are subjective and influenced by familiarity with odors, language, and literacy. Researchers are developing objective tools, such as devices that directly record olfactory neuron activity, to reduce bias and improve precision.

What are phantom smells and smell distortions?

Phantosmia refers to smelling odors that aren’t present, while parosmia is distorted smell perception. Causes can be peripheral (e.g., nasal inflammation) or central (e.g., migraine aura, epilepsy). Treatments may start with reducing nasal inflammation; further evaluation may be needed if symptoms suggest central causes.

Smell Loss and Parkinson’s Disease

How is smell loss linked to Parkinson’s disease?

Smell loss is common in Parkinson’s and can appear years before motor symptoms. Abnormal alpha-synuclein protein can accumulate in the olfactory bulb early, contributing to olfactory dysfunction. However, smell loss also occurs in other brain diseases and from non-neurologic causes, so smell loss alone does not diagnose Parkinson’s.

Do all people with Parkinson’s lose their sense of smell?

No. While most people with Parkinson’s have reduced smell, a subset retains near-normal smell. Research using spinal fluid alpha-synuclein assays suggests those with confirmed alpha-synuclein pathology are more likely to have smell loss, but some still smell normally. Scientists are studying what preserved smell means for disease biology and progression.

Can smell testing predict who will develop Parkinson’s?

Smell testing can identify reduced olfaction, a risk marker associated with Parkinson’s, but it cannot specify future diagnosis on its own. It is most useful as a first step to flag potential risk and guide further evaluation with additional biomarkers and assessments.

Do Parkinson’s treatments change sense of smell?

Standard Parkinson’s medications like levodopa are not known to worsen smell and generally have not shown consistent improvements in olfaction. Rare anecdotal reports suggest deep brain stimulation might affect aspects of smell perception in some individuals, but this is not a reliable or primary reason to pursue DBS.

Participating in Research: The PPMI Study and Smell Screening

What is PPMI and why does it focus on smell?

The Parkinson’s Progression Markers Initiative (PPMI) is a large, international study aimed at finding and validating biomarkers to diagnose Parkinson’s earlier and track its progression. Because smell loss can precede diagnosis, PPMI uses smell testing to identify people who may be at risk or have distinct biological profiles, enabling deeper biomarker study and better matching to future therapies.

Who can take the PPMI smell test and what happens next?

Adults 40 and older without Parkinson’s can request a smell test to screen for reduced olfaction. Individuals with Parkinson’s diagnosed within the past seven years may also be screened, with a focus on identifying those who retain smell. Based on results, participants may be invited to complete online surveys or attend clinical sites for advanced tests and imaging.

Will I receive my smell test results from PPMI?

PPMI currently does not return UPSIT smell test scores because interpretation depends on age- and sex-adjusted percentiles, and single-time results can vary with factors like nasal inflammation. The study is evaluating how to share results responsibly and consistently at scale. Other selected study results may be returned to participants who choose to receive them.

What is the value of participating in PPMI?

Participation helps researchers link symptoms like smell loss with underlying biology (e.g., alpha-synuclein, dopamine imaging), accelerating the development of targeted diagnostics and treatments. Many participants also receive structured follow-up and may opt to access certain test results, contributing to both personal insight and scientific progress.

Managing and Treating Smell Loss

What evidence-based treatments can help smell loss?

Options with supportive randomized trial evidence include: (1) Olfactory training—structured, twice-daily smelling of distinct odors (e.g., rose, lemon, eucalyptus, clove) for at least six months, focusing attention on the expected smell to retrain neural pathways; (2) High-volume steroid nasal irrigations to reduce inflammation around olfactory neurons; (3) High-dose omega-3 supplementation for nerve support in appropriate patients (avoid if you have bleeding risks); (4) Platelet-rich plasma (PRP) injections into the olfactory region, studied primarily in post-viral cases but showing promise across causes. Discuss suitability and safety with a clinician.

Is there a time window when treatments are most effective?

Earlier intervention is generally better, but improvement can still occur years later in select cases. While regeneration and plasticity decline with age and duration of loss, some patients have shown meaningful gains with treatments like PRP even decades after onset. Because many interventions are low-risk, a trial under medical guidance is often reasonable.

What practical steps can support smell recovery or coping?

Consider daily olfactory training for at least six months; optimize nasal health and reduce inflammation; discuss omega-3s or PRP with a specialist if appropriate; and enhance food enjoyment with texture, temperature, and seasoning. For safety, use detectors for smoke and gas, label perishable foods by date, and seek social support or counseling if smell loss affects quality of life.