The local factors of oral cavity that lead to bad breath: 1.Dental caries. There are many kinds of microorganisms in the cavity, and Streptococcus deformans and Lactobacillus are the leading pathogenic bacteria, which can give out a bad smell. 2.Gingivitis, periodontitis. The main pathogenic bacteria of periodontitis are Porphyromonas gingivalis, Clostridium perfringens and other anaerobic bacteria and micro aerobic bacteria. These bacteria themselves emit a special odor, most of the odor in the mouth is caused by this bacteria. The odor of Porphyromonas gingivalis is mainly caused by the necrosis of gingival tissues by Clostridium perfringens. Mucosal tissue necrosis of oral ulcers and gingival necrosis. Leukemic gingivitis gingival necrosis. 3, poor oral hygiene. Do not brush teeth, do not rinse, or brush teeth sloppy people, due to the long-term accumulation of food residues in the mouth, fermentation and decomposition under the action of bacteria, producing indole sulfur hydroxyl and amines and other substances, emitting a rotten stench. 4, some people who wear dentures do not pay attention to the cleaning of dentures, there will also be odor in the mouth, which is the most common cause of bad breath. Second, systemic diseases: Some halitosis is caused by diseases in other parts of the body, such as indigestion, purulent bronchitis, lung abscess, etc., will be discharged through the respiratory tract odor, manifested as halitosis. In addition, diseases of neighboring organs, such as nasopharyngeal and nasal cavity diseases, such as purulent maxillary sinusitis and atrophic rhinitis, can also lead to halitosis.