Deep breathing feels left back pain, the reasons are mainly the following: first, it may be that the thorax has obvious expansion when deep breathing, which produces extrusion stimulation to the surrounding intercostal muscles. If the intercostal muscles are strained and cooled by the formation of inflammatory foci, it will produce obvious pain in the left back area. Secondly, there may be damage to the location of the ribs and scapula, and deep inhalation will also stimulate the location of the damage, resulting in pain in the left back, and thirdly, there may be inflammation of the pleura. If there is an inflammatory lesion in the pleura of the left back position, when taking a deep breath, due to the expansion of the lungs, there will be obvious extrusion stimulation, which will also produce the feeling of pain in the left back. Fourth, there may be intercostal neuritis. If the nerves between the ribs, such as viral infection or cold, cause an inflammatory lesion, it will produce stimulation and pain when taking a deep breath.