The causes of bronchitis may be microbial infection, physical and chemical factors, allergic reactions and so on. 1. Microbial infection: when the patient’s ability to resist disease is poor, the bronchial area is easily infected by pathogens, which can cause bronchitis. In addition to close contact with bronchitis patients, it is also easy to cause the spread of pathogens, inducing the disease. 2. Physical and chemical factors: seasonal changes, the patient’s bronchial area is susceptible to cold air stimulation, which can lead to damage to the bronchial mucosa, easily caused by bronchitis. In addition, inhalation of dust, irritating gases can also cause acute damage to the bronchial mucosa, leading to bronchitis. 3. Allergic reaction: Inhalation of allergens such as pollen, organic dust, etc., can also easily irritate the bronchial mucosa, inducing allergic reaction and leading to bronchitis. In addition, when the bronchial tube is repeatedly infected many times may lead to the destruction of the bronchial tube mucosa and structure, thus leading to chronic bronchitis. There are many causes of bronchitis, and patients with symptoms are advised to go to the hospital in time for treatment.