Nasal blood may be caused by environmental factors, local lesions, systemic diseases and other reasons. 1. Environmental factors: in too dry conditions, the mucous membrane of the nasal cavity will become congested and eroded due to excessive dryness, thus leading to nasal discharge. 2. Local lesions: local lesions in the nasal cavity itself is one of the common causes of nasal blood, such as local trauma, nasal polyps, nasal septum deviation, sinusitis, nasal foreign body, nasal tumors and so on. 3. Systemic diseases: Nasal blood may also be caused by systemic diseases, such as thrombocytopenic purpura, leukemia, aplastic anemia, systemic lupus erythematosus and so on. To summarize, there are many factors that can cause nasal blood, as soon as possible to seek medical treatment.