Diffusion function is a measurement in gas exchange function. It is used to evaluate the efficiency of gas exchange by alveolar capillary membranes. It is important for early detection of lung and airway lesions, assessment of disease severity and prognosis, evaluation of the efficacy of drugs or other treatments, identification of the cause of dyspnea, diagnosis of the site of the lesion, assessment of lung function for surgical tolerance or labor intensity tolerance and monitoring of critically ill patients. Lung diffusion refers to the process of gas exchange between oxygen and carbon dioxide through the walls of alveoli and pulmonary capillaries within the lungs. The pathway of diffusion includes the alveolar gas, the alveolar capillary walls, and the plasma, red blood cells, and hemoglobin within the pulmonary capillaries. Gases are exchanged along this pathway depending on which end has the higher concentration, so the process can be bidirectional. Oxygen diffuses much more slowly than carbon dioxide because oxygen is not easily dissolved in body fluids. Therefore, when a patient has an abnormality in diffusion function, oxygen exchange is more likely to be affected than carbon dioxide, and clinically impaired pulmonary diffusion can significantly affect arterial oxygen levels. How is decreased diffusion function diagnosed? The diagnosis can be made by clinical examination of spirometry. It is (28.84±4.84) ml/(mmHg?min) in men and (22.13±3.09) ml/(mmHg?min) in women. Below this value, there is a decrease in gas diffusion function. It is important to go to the respiratory department of a regular hospital for examination and to pay attention to the standardization of the examination. Reduced diffusion function can be seen in: ① reduced diffusion area: such as emphysema, lobectomy, lung infection, pulmonary edema, pulmonary hemorrhage, pneumothorax, scoliosis, etc.; ② thickening of alveolar capillary membranes: such as interstitial fibrosis, tuberculosis, asbestosis, scleroderma, etc.; ③ decreased capacity of haemoglobin oxygen carrying capacity: such as anemia, carbonic haemoglobinopathy. It is also important to pay attention to the possibility of secondary formation of infectious diseases.