Lung nodules with glass density of 0.4 cm are usually associated with lung inflammation, tuberculosis and lung cancer. 1. Lung inflammation: for example, pneumonia, which mainly includes bacterial infection, fungal infection and viral infection, etc. When inflammation repeatedly stimulates the lung tissues and interalveolar spaces, it will easily cause inflammatory exudate fluid, which will usually be found as ground-glass nodule shadow through examination. 2. Tuberculosis: it is mainly related to the infection of tuberculosis bacillus, when tuberculosis bacillus stimulates the lung tissue repeatedly, it is easy to cause erosive infection, so it will be detected in the early stage of the existence of ground-glass nodule shadow. 3. Lung cancer: for example, lung carcinoma in situ, micro-invasive carcinoma, etc., all of them may cause lung ground-glass nodule shadow, which can be found through imaging examination. If the density of ground-glass nodule is 0.4 cm, it is recommended to carry out regular testing, and if it increases significantly, it can be diagnosed and analyzed by pathological biopsy, and then targeted treatment can be carried out according to the diagnosis results.