Generally speaking, if there is inflammation in the airways, phlegm will appear in most cases, and phlegm is actually the body’s response to inflammation inhibition or resistance. Sputum is also divided into white sputum and yellow sputum. Generally speaking, white sputum is related to chronic inflammation of the airways, such as smoking, and it is easy to cough up white sputum after exposure to dust. If one coughs up yellow sputum, most of the time it is due to bacterial infection and the neutrophils in the human body flow out from the body to the airway and then swallow the bacteria to form pus, and the pus is coughed out with the airway and becomes yellow sputum. Therefore, white sputum is mostly caused by chronic inflammation, such as smoking, etc.; while yellow sputum is caused by bacterial infection, so pay attention to the difference.