Left chest pain during inhalation is caused by tension, friction and irritation of the left chest muscles and fascia due to the thoracic movement and negative pressure changes in the chest cavity caused by inhalation. It is commonly seen in chest wall muscle neuropathy, pleural cavity disease, heart disease, lung disease, fatigue and mental and emotional changes. 1. Musculo-neuropathy of the left chest wall: chest wall bone fracture, muscle soft tissue contusion, costochondritis, intercostal neuritis and other diseases may cause left chest pain when pulling and stimulating during inhalation. 2. Pleural cavity diseases: pneumothorax, pleurisy, pleural mesothelioma and other diseases stimulate pleural friction and left chest pain during inspiration. 3. Heart diseases: pericarditis, viral myocarditis and other diseases cause chest pain by stimulating pericardial friction during inhalation. 4. Lung diseases: Inflammatory factors stimulate alveolar nerve endings during lung infection to cause chest pain, and respiratory movements or activities can induce or aggravate the pain. 5. Physical fatigue and mental and emotional changes, etc.: Muscle lactic acid accumulation during physical fatigue and mental and emotional changes can cause left chest pain during inhalation, but it can be relieved after rest. It is recommended to go to the hospital for further diagnosis and treatment for left chest pain during inhalation.