In a recent review published in Frontiers in Molecular Neuroscience, researchers from UC Irvine and the University of Oxford have highlighted a profound connection between olfactory dysfunction—loss or impairment of the sense of smell—and inflammation across 139 distinct medical conditions. The research suggests that the sense of smell plays an underestimated but vital role in physical and mental health. Olfactory dysfunction may not merely be a symptom but could actively contribute to the onset and progression of diseases.
The researchers were motivated by the observation that olfactory dysfunction frequently accompanies a wide range of medical conditions, yet the reasons behind these associations remained unclear. The study’s lead author, UC Irvine’s Professor Emeritus Michael Leon, noted that he has long “been interested in the role that scents play in keeping the brain healthy.” Leon and his colleagues aimed to investigate whether olfactory loss could be more than a symptom, potentially serving as an early warning sign or even a contributing factor to the development of diseases.
To investigate, the team conducted an extensive review of existing literature, analyzing studies that documented olfactory dysfunction in conditions across three broad categories: neurological, somatic, and congenital or hereditary disorders.
“The loss of the ability to smell things is associated with at least 139 medical conditions, including heart disease, cancer, lung disease, liver disease and kidney disease – the diseases most likely to kill you,” Leon told PsyPost.
The review highlighted inflammation as a potential mechanism behind these associations. The olfactory system, due to its direct exposure to environmental pollutants and volatile substances, is particularly vulnerable to damage and inflammation. This damage can spread to other parts of the brain or body, exacerbating or triggering symptoms of various conditions.
For instance, inflammation in the olfactory bulb—the brain region responsible for processing smells—has been linked to cognitive decline and memory loss. Furthermore, the study showed that inflammation associated with olfactory loss could be aggravated by environmental factors such as air pollution, smoking, or stress.
“First, I was surprised by the sheer number of medical disorders associated with olfactory loss. Then, I was struck by the fact that all of them were also linked to chronic inflammation,” Leon said.
One of the most striking findings was the predictive power of olfactory loss. Studies cited in the research revealed that impaired smell ability could forecast cognitive decline, mortality, and the likelihood of developing specific diseases more accurately than traditional risk factors like cardiovascular health. For example, older adults with reduced olfactory function were shown to have a significantly higher risk of mortality over a 17-year period compared to those with normal smell ability.
The review also explored the potential for reversing or mitigating these effects through olfactory enrichment—exposure to pleasant scents that stimulate the olfactory system. Previous research has shown that olfactory enrichment can improve cognitive performance, reduce inflammation, and enhance memory in both healthy adults and individuals with dementia.
For instance, older adults exposed to daily scents such as lavender or eucalyptus experienced improvements in verbal fluency, attention, and memory recall. These findings suggest that olfactory stimulation could serve as a therapeutic intervention for preventing or slowing the progression of diseases linked to olfactory dysfunction.
Despite its groundbreaking contributions, the research is not without limitations. Much of the evidence comes from correlational studies, making it difficult to establish a direct causal link between olfactory dysfunction and specific medical conditions. Additionally, while inflammation appears to be a key factor, other mechanisms—such as direct neuroanatomical connections between the olfactory system and memory-related brain regions—may also play a role.
Future research should focus on longitudinal studies that track individuals over time to better understand these relationships. Experimental studies that manipulate inflammation levels or olfactory stimulation could further clarify causality.
“The memory centers of the brain need a great deal of olfactory stimulation to maintain their health,” Leon said. “We have developed a device called Memory Air that sits on your nightstand and delivers 40 scents twice a night as you are sleeping. This therapy has been shown to improve the memory of older adults, even those with dementia (Alzheimer’s disease). Nothing else comes close. Everyone probably needs more olfactory stimulation throughout their lives.”
The paper, “Inflammation and olfactory loss are associated with at least 139 medical conditions,” was authored by Michael Leon, Emily T. Troscianko, and Cynthia C. Woo.
