There are many reasons for decreased sense of smell, which are commonly caused by sinusitis, nasal polyps, trauma and so on. 1. Sinusitis: It is an inflammatory disease of the mucous membrane of the sinuses, mostly caused by bacterial, fungal and viral infections. It can lead to nasal congestion, purulent nasal discharge, head and face pain, and loss of smell and other symptoms. 2. Nasal polyps: a relatively common chronic disease of the nasal cavity and sinus mucosa, mainly in the extremely edematous nasal mucosa in the middle nasal tract to form a single or multiple polyps as a clinical feature, often occurring in bronchial asthma, aspirin intolerance, allergic fungal sinusitis patients. For the relatively large size of the nasal polyps, can lead to patients with a runny nose, nasal congestion, loss of smell and tinnitus and other uncomfortable symptoms. 3. Trauma: If the patient’s skull base is fractured and the sieve plate is damaged, it can lead to the avulsion of the olfactory nerve and the compression of the tumor at the bottom of the frontal lobe, which can lead to the loss of the sense of smell on one side or both sides of the patient. In addition, some Parkinson’s patients with early, nasal tumors, atrophic rhinitis, nasal septum deviation, olfactory nerve and olfactory center lesions, etc., can lead to olfactory loss, need to go to the hospital in a timely manner, a clear diagnosis, and comply with the doctor’s instructions for treatment.