COVID-19 has brought the importance of smell to the public's attention. In addition to the roles of olfaction in hygiene, pleasure, and nutrition, this underappreciated sense serves as an early warning system for environmental hazards such as spoiled food, fire, leaking natural gas, and air pollution. The olfactory system's receptor cells are uniquely exposed to the outside environment, making them, along with other epithelial cells crucial for their function, susceptible to damage from airborne viruses, bacteria, and nanoparticles. As first order neurons, olfactory receptor cells can transport xenobiotics from the environment directly to the brain. Smell loss has been associated with early mortality and can signal the first stages of Alzheimer's disease and Parkinson's disease.
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