Breathing is very smelly may have the following causes: 1, respiratory system diseases, if there is a respiratory system infection, such as bronchitis, bronchitis or other infectious diseases of the respiratory system, bacteria may decompose some substances, producing smelly gas, through the respiratory tract to the mouth or nasal cavity. 2, there may be a bad smell of the mouth, if There is gingivitis or periodontitis, the bacteria in the mouth will use the food residue and soft scale attached to the neck of the teeth to decompose and produce smelly gas. The odor can be smelled when you open your mouth, and can also enter the nasal cavity, making it smelly when you breathe. 3, it may be a digestive system disease, digestive system disease, producing smelly gas, through the esophagus, pharynx into the nasal cavity, and make it smelly when you breathe.