In outpatient clinics and at work in general, I am often asked to look at a CT film or report of the chest and say that there is a nodule on the lung. Immediately afterwards, they ask what is a pulmonary nodule? How were they created? Is it a serious problem? Then the very worrying question is asked: Is it cancer? In the dictionary, a nodule is explained as a protrusion, or a lump. Then the word lung nodule is a descriptive language in chest x-ray imaging, which usually refers to a single, round, opaque, well-defined nodule less than 75px in diameter in the lung. Those larger than 75px are called lung masses and the vast majority are lung cancers. However, not all lung nodules are lung cancer; some may be inflammatory lesions and some may be tuberculomas. And early lesions of lung cancer, such as carcinoma in situ and adenomatous hyperplasia, can develop into lung cancer, while some are lung cancer when they are found. Then there are some nodules that can slowly fade away after a period of time. Therefore, no generalization can be made about lung nodules. Chest CT can perform thin layer scans to find smaller nodules, those smaller than 5.0mm are called microscopic nodules, and those 5.0mm – 1.0mm are called small nodules. Of course, the size of a lung nodule is closely related to whether it is a malignant nodule, and the larger the nodule the greater the likelihood of lung cancer. The mechanism of lung nodule generation is not very clear, but some nodules can be explained, such as having tuberculosis, some small tuberculosis foci of bacteria are eliminated, calcium salt deposits form high-density nodules, which are benign at first glance; there are also sclerosing angiomas, which are themselves lesions of small blood vessels and capillaries in the lungs; there are also inflammatory lesions, which form inflammatory nodules after mechanization; all these can be roughly seen. However, most of the nodules seen on CT films are difficult to make a clear interpretation, which is why pulmonary nodules are a difficult and hot spot in thoracic surgical diseases. However, there are many triggers for the formation of pulmonary nodules that we can list, which are the factors that trigger the formation of nodules, and this is the basis for the prevention of pulmonary nodules or lung cancer in the future. Because the lungs are organs directly connected to the outside world through the mouth, nose, throat, and trachea, polluted air and smoke from smoking are very harmful to the lungs. This mixture of dust, greasy particles, and smoke in the atmosphere, when inhaled into the lungs, part of it can be coughed out. However, when the PM value of inhaled substances is less than or equal to 2.5 microns, the lung’s own cleaning function cannot eliminate them, and these small particles will reside in the mucous membrane of fine bronchial tubes, and long-term stimulation will cause the mucous membrane epithelial cells to mutate and grow abnormally, and nodules will be formed when multiple cells are involved, which may develop into lung cancer if the situation continues. Also, intravenous infusion, hanging water and drip. There are many tiny impurities in saline and glucose water, and there are more than 100,000 particles in a 500ml bottle of medicine, and the diameter of these impurities is mostly 2-16 microns. The smallest capillary of the lung allows only one red blood cell to pass through, with a diameter of only about 8 microns. Particles exceeding 8 microns are filtered and remain in the capillary, which are engulfed by cells and form granulomas locally in the lung, which may also be the cause of the formation of some lung nodules. Therefore, the exact mechanism for the occurrence of pulmonary nodules is not well understood. Generally speaking, benign nodules account for most of the nodules, including pulmonary tuberculomas, sclerosing hemangiomas, inflammatory granulomas, local inflammation, and pulmonary arteriovenous fistulas. We will discuss in the next article how to determine the benignity and malignancy of a pulmonary nodule found.