Breathing hard left chest pain is considered to be related to trauma, left pleural disease, lung infection, cardiac disease, etc., and needs to be integrated with other clinical symptoms as well as medical history to make a comprehensive judgment. 1. Trauma: If the left chest muscle is contused, the left lung is contused, or the left rib is fractured due to violent impact, car accident, collision and other external injuries. At this time, the pain will be aggravated because the sternum will be pulled when breathing hard, which will damage the nerves. 2. Left pleural diseases: such as pleurisy, pleural tumor. After inflammation or tumor occurs in the lung, the surface of pleura becomes rough, which further causes the contact friction between the wall pleura and the dirty pleura, thus producing a kind of stimulation reaction, causing chest pain when taking a deep breath. 3. Lung infection: for example, pneumonia, resulting in localized infiltration of lesions and even pleural adhesions. Patients will experience chest pain, especially when deep breathing or coughing. 4. Heart disease: such as coronary heart disease, will show the left anterior chest pain, usually compression pain, accompanied by dyspnea, patients may be consciously for the left side of the chest pain when breathing hard. The causes of left chest pain are more, it is recommended to consult a doctor in a timely manner, with the help of a doctor to clarify the cause of the disease, and targeted treatment or therapy.