Gray dirt in sputum may be caused by respiratory tract infections, smoking or inhaling dust particles, lung tumors, and so on. 1. Infections of the respiratory tract: Infectious diseases of the respiratory tract may cause coughing and coughing up thick sputum, which may look like grey dirt due to a change in the color of the sputum. 2. Smoking or inhalation of dust particles. In the above cases, the inhalation of dirty particles in the airway, which are mixed in the sputum after being expelled from the body, may be manifested as the phenomenon of gray dirt in the sputum. 3. Lung tumor disease: sometimes the necrotic material on the surface of the tumor in patients with lung tumor disease will fall off and mixed with sputum to be coughed out, which can also be manifested as the phenomenon of grey dirt in sputum. There may be other reasons for the gray dirt in sputum, such as tuberculosis, pulmonary fungal disease, the airway secretion will also contain similar necrotic material, if the symptoms fail to improve or even gradually aggravate, you need to consult the regular hospital in time, cooperate with the doctor to improve the examination and standardize the treatment.