The main causes of numbness of the left arm are related to peripheral nerve injury of the left upper extremity, radiculopathy, vascular lesions of the left upper extremity, or prolonged poor posture and exposure to cold. 1. Peripheral nerve injury of the left upper extremity: If there is peripheral nerve injury of the left upper extremity, it is easy to produce motor and sensory dysfunction of the left upper extremity, which leads to the symptom of numbness of the left arm. 2. Neurogenic cervical spondylosis: when there is neurogenic cervical spondylosis, changes in the structure of the cervical vertebrae can easily compress the nerve roots, so the symptoms of numbness of the left arm may appear. 3. Left upper extremity vascular lesions: left upper extremity vascular lesions, such as venous thrombosis, etc., which can easily affect the blood reflux, resulting in numbness of the left arm. 4. Prolonged poor posture and cold: usually if the poor posture leads to the left arm pressure or a long time in the cold environment, it is easy to affect the local blood circulation, resulting in numbness of the left arm symptoms. This situation can be relieved after adjusting posture and keeping warm. If the symptoms of numbness of the left arm last for a long time, it is necessary to identify the cause of the disease as soon as possible, and standardized treatment under the guidance of the doctor.