If the nodule is caused by inflammation, it is not serious; if it is caused by tumor, it is more serious. 1. Inflammation: If the infected lesion is caused by inflammation, it is benign, and there may be exudate foci around the infected lesion nodule, such as pneumonia with fuzzy boundaries, and the nodule can be reduced or disappeared after using antibiotic treatment. There may be bleeding around the nodules, which may be caused by fungal infection invading blood vessels. After bleeding, small nodules with unclear boundaries can be seen on the image, which will shrink or disappear after antifungal treatment. 2. Tumor: if the nodule is a tumor, it can also cause unclear boundary by invading the surrounding blood vessels. 3mm nodule in the lower lobe of the left lung is more serious if it is a tumor, especially malignant tumor, which needs surgery and radiotherapy if necessary. To summarize, it is recommended that patients with 3mm nodules in the lower lobe of the left lung should consult a doctor in time to get a clear diagnosis and symptomatic treatment, so as to avoid delaying the condition.