From the process of biological evolution, smell is one of the most primitive sensory functions, with foraging, courtship and defense functions. In humans, olfaction, like vision and hearing, is an important basic sensory function of the human body. Although human olfactory function has degenerated, it still plays an important role in life, with functions such as identifying odors, participating in memory and regulating emotions. The human olfactory mucosa is exposed to the open environment and is highly vulnerable to external factors such as polluted air and viral infections. According to the literature, there are about 200 diseases and 40 kinds of drugs that can cause olfactory disorders. 1989, Frank reported that 1.2% of 1.5 million Americans have permanent olfactory loss and 62.4% have temporary loss. Although there is no exact statistics about olfactory disorders in China, the percentage of patients with olfactory disorders will not be very different from the literature reports. 1.Trauma: The number of patients with post-traumatic olfactory disorder occurred 35% of those with olfactory dysfunction, accounting for 7.5% of trauma patients. Most of them occur in young and middle-aged men, which may be related to the fact that these people often go out. The site of trauma is mostly frontal and occipital area, which may be caused by the following three mechanisms: 1) When the head is hit, the olfactory nerve fibers are cut at the sieve plate due to the relative movement of brain and skull, which is mostly accompanied by blunt contusion of brain; 2) After trauma, local bone fracture and misalignment occurs, and after a period of recovery, the bone heals deformely, causing mechanical obstruction of nasal cavity; 3) Trauma causes central nervous system widespread injury. 2.Virus: Virus-induced olfactory disorder is called post-viral infection of upper respiratory tract (PVOD), which mostly occurs in middle-aged and elderly people. The main viruses that cause upper respiratory tract infection are influenza virus A, B, C, adenovirus, rhinovirus, parainfluenza virus, respiratory syncytial virus and so on. However, after research, it was found that rhinovirus and coronavirus only cause cold symptoms and do not damage nasal epithelium, while influenza virus and adenovirus damage nasal epithelium. a study by Temmel et al. on 278 patients with olfactory disorders showed that upper respiratory tract infections accounted for 39% of the cases, higher than those caused by head trauma and nasal and sinus diseases. 3. Inflammatory: Inflammatory olfactory disorders (rhinitis and sinusitis) are prevalent, some patients have hyposmia, while others have olfactory loss. Chronic sinusitis is a clinical syndrome caused by long-term inflammatory reaction of the mucous membrane of the nasal cavity and sinuses, often accompanied by allergic or non-allergic rhinitis, nasal polyps, etc. Pathological histology shows chronic inflammatory cell invasion and fibrosis and thickening of the mucosal substrate. The disease is due to nasal obstruction and edema of the respiratory mucosa, which reduces the airflow reaching the olfactory fissure. According to the classification method of Snow et al, it belongs to conductive olfactory disorder. 4.Drug: There are many drugs that cause olfactory disturbance, including anticancer drugs (cisplatin, furofluazone, etc.), drugs for angina pectoris, hyperthyroidism, antidepressants, antibiotics (doxycycline, streptomycin, kanamycin sulfate, etc.), antihypertensive drugs, anxiety drugs, anesthetics, etc., but the exact mechanism is not clear. Yoshio Yamagishi (1993) reported that furofluridine olfactory disorder is due to a disorder of transformation of olfactory epithelium, especially olfactory cells leading to peripheral neurogenic disorder, which cannot cause differentiation to olfactory cells. 5, Congenital: congenital factors account for 3%, common ones are Kallmann’s syndrome, deafness-retinitis pigmentosa syndrome, acromegaly, congenital nasopharyngeal atresia, etc. There are also families with congenital loss of smell, which is currently considered to be an autosomal dominant mode of inheritance. 6. Other: including endocrine diseases (e.g. hypothyroidism, adrenal hyperplasia, etc.), tumors (frontal lobe tumors, etc.), psychological (phantom sniffing), medical (nasal examination importing bacteria or virus, inappropriate medication, etc.), obesity, neurodegeneration, radiotherapy, surgery, toxic exposure (multiple chemical materials, pesticides, etc.), age-related factors, etc. In particular, it is important to suggest that some factors that we may not have noticed may play a role in the development of olfactory disorders (e.g., hot steam, etc.).