Slight pain at the edge of the sternum when breathing may be caused by physiological factors such as cold and mental tension, or it may be caused by pathologic factors such as costochondritis, trauma, pleural or pulmonary lesions. 1. Physiological factors: cold or mental tension may induce pain at the edge of the sternum when breathing. The pain is mild and can be relieved through rest, relaxation and other lifestyle modifications. 2. Costochondritis: overwork, prolonged incorrect posture and other factors may lead to aseptic inflammation of the costochondral bone, which may cause slight pain when breathing, being stimulated or pulled. 3. Trauma: when the muscles around the sternum are injured, due to the local soft tissue oozing, edema and other pathological changes, there will also be pain when breathing. 4. Pleural or lung lesions: when there are pleurisy, pneumonia and other diseases, due to the presence of inflammation itself, there will also be pain when breathing, partly accompanied by coughing, coughing up sputum and other symptoms. Slight pain in the sternum during breathing requires prompt medical attention to identify the cause of the problem and follow the doctor’s instructions.