Breath odor is an unpleasant gas that appears when breathing, and its causes mainly include the following: intraoral factors, respiratory diseases, digestive diseases and mental factors. 1. Intraoral factors: bad breath may be related to intraoral factors, such as periodontal disease, dental caries, necrotizing stomatitis and gangrenous stomatitis, oral candidiasis, oral dryness. The main source of bad breath is bacteria in periodontal tissue and tongue. 2. Respiratory diseases: patients suffering from respiratory diseases can also cause bad breath, such as lung abscess, bronchial dilatation with infection, acute suppurative tonsillitis and so on. 3. Digestive system diseases: digestive system diseases can also cause bad breath. The common ones are chronic gastritis, pyloric obstruction, peptic ulcer, advanced gastric cancer. 4. Psychological factors: also known as delusional halitosis, refers to some patients consciously breathing odor, but the doctor did not see any odor breathing, clinical examination can not be found in any cause of halitosis. Breath odor may also be seen in other diseases, such as sinusitis, diabetic ketoacidosis, organophosphorus poisoning, it is recommended that the patient consult a doctor in a timely manner, a clear diagnosis, under the guidance of the doctor to receive treatment.