The olfactory region is the only part of the brain that is
constantly exposed to the external environment.
It is critical to protect this sensitive area from airborne pollutants and pathogens <2.5 microns (PM 2.5).
Due to its size, PM 2.5 is able to "drift" all the way to the olfactory region of the nasal cavity, where it can damage the cells of the olfactory epithelium, leading to anosmia.
PM 2.5 can also use the olfactory nerves as a path to the rest of the brain, bypassing the blood brain barrier and causing neuroinflammation and neurodegeneration.
Anosmia is one of the first and most common clinical signs of Parkinson's Disease and Alzheimer's Disease, appearing decades before cognitive decline.
"Persistent olfactory impairment with or without perceptual distortions (ie, parosmias or phantosmias) after SARS-CoV-2 infection could, therefore, serve as a marker to identify people with an increased long-term risk of neurological disease.”