Smell loss affects up to 22% of the population and has been observed in millions of COVID‑19 cases, prompting renewed research into the sense of smell. The condition, known as anosmia or hyposmia, can be permanent or temporary and is associated with a range of neurological and psychiatric disorders.

The sense of smell relies on olfactory neurons in the nasal cavity that transmit signals to the olfactory bulbs, which directly connect to brain regions involved in emotion and memory. These bulbs are among the few brain areas that generate new neurons in adulthood and are vulnerable to viral infection, trauma, inflammation, and neurodegeneration.

Chrissi Kelly lost her sense of smell after a viral infection during a trip to the Czech Republic in 2012. After six months of complete anosmia, she described the loss as “catastrophic” and began searching for information and community support. She later co‑founded patient groups and co‑authored more than 30 academic papers.

Research has linked prolonged smell loss to increased risk of neurodegenerative diseases. A wine enthusiast who requested anonymity reported that loss of smell preceded his diagnosis of Parkinson’s disease after 20 years, while clinicians noted that olfactory dysfunction can appear early in Alzheimer’s and Lewy body dementia. A 2023 survey found that 60% of COVID‑19 patients experienced smell loss, most temporarily but some for extended periods.

Scientists estimate that humans can distinguish millions of odors, yet the neural code translating odor molecules into perception remains unclear. In the early 1990s, researchers identified roughly 1,000 olfactory receptor genes, a discovery that earned a Nobel Prize in 2004 and launched modern olfactory research.

Current investigations focus on olfactory training, a regimen in which patients repeatedly smell familiar scents such as lemon, rose, clove, and eucalyptus. Controlled trials have shown that about 30% of participants with long‑standing smell loss improve after several months of training, with some studies reporting improvements of 10% or more on standardized smell tests. A 2024 meta‑analysis of 36 trials found a statistically significant positive effect across all domains of olfactory function, with reported gains ranging from 11% to 68%.

While olfactory training appears beneficial for some, it is not universally effective, and more research is needed to determine its long‑term impact on cognition, mood, and disease prevention. The ability to smell also influences safety, as loss of odor detection can prevent recognition of smoke, gas leaks, or spoiled food.

Overall, recent findings underscore the importance of the olfactory system for quality of life and brain health, and they suggest that smell loss may serve as an early marker for several medical conditions.