According to statistics, 80% ~ 90% of bad breath comes from the mouth. Untreated dental caries, residual roots, residual crowns, bad restorations, abnormal anatomical structures, gingivitis, periodontitis and oral mucosal diseases can all cause bad breath. Dental caries and periodontal diseases are the most common related diseases. Food residue and plaque often remain in the cavity of deep caries and under the overhang of bad restorations, and bacteria are fermented and decomposed to produce odor. Dental pulp necrosis or suppurative pulpitis, without treatment, can also give off a bad smell; Periodontal patients are often accompanied by a large number of tartar and plaque, and bacteria in periodontal pockets ferment to produce hydrogen sulfide, indole and ammonia, thus producing odor. In addition, periodontal abscess and periodontal pocket empyema, mostly staphylococcus aureus combined with periodontal pathogen infection, will also give off a bad smell. The quality and quantity of saliva also play an important role. The decrease of saliva and the increase of organic components such as protein reduce the scouring and buffering effects of saliva, which leads to the proliferation of bacteria, the decomposition of organic components in saliva, gingival crevicular fluid and food residues, and the generation of a large number of volatile sulfides, indole and other substances, resulting in bad breath.