The common causes of poor ventilation in the left nostril are: deviated nasal septum, nasal foreign bodies, nasal tumors and so on. 1. Deviated nasal septum: It can be caused by congenital factors or trauma. If the nasal septum is deviated to the left side, blocking the left side of the airway, it will lead to poor ventilation in the left nostril and symptoms of nasal congestion. 2. Nasal foreign body: Mostly occur in younger children, if accidentally stuffed small toys into the left side of the nasal cavity, it will lead to nasal blockage, poor ventilation. If it is not removed in time, it can lead to infection. 3. Nasal tumors: including benign and malignant tumors. Such as nasal involutional papilloma, nasal hemangioma, etc., with the gradual increase of the tumor, it will block one side of the nasal cavity, which will lead to poor ventilation, and it is recommended to remove it in time, and if necessary, cooperate with radiotherapy and chemotherapy. As we can see, there are many reasons that cause the left nostril to be poorly ventilated, it is recommended to go to the regular hospital in time to get a clear diagnosis and symptomatic treatment.