0.3 cm solid micronodules are mostly tuberculosis, pneumonia and lung cancer. 1. Tuberculosis: usually caused by Mycobacterium tuberculosis infection, if not treated in time, may lead to caseous necrosis of the lungs, thus causing the symptoms of 0.3 cm solid microscopic nodules. 2. Pneumonia: generally due to bacterial and viral infections and other factors, resulting in inflammatory reactions in the lungs. When the mucosa of the lungs is repeatedly stimulated by inflammation, congestion and edema will occur, which will lead to the protrusion of granulation tissue and the emergence of 0.3 cm solid micronodules. 3. Lung cancer: long-term smoking and exposure to harmful gases may lead to lung cancer, and if it is a malignant tumor, it may cause 0.3 cm solid microscopic nodules. The appearance of 0.3 cm solid micronodules may also be due to other reasons, so you should go to the hospital as soon as possible to avoid delaying your condition.