A nosebleed, also known as epistaxis, is an instance of bleeding from the nose. Blood can flow down into the stomach, and cause nausea and vomiting. In more severe cases, blood may come out of both nostrils. Rarely, bleeding may be so significant that low blood pressure occurs. Allergens like pollen, pet dander, and dust can aggravate the nasal membranes, causing irritation and subsequently bleeding. Blood group O appears over-represented in Caucasian patients admitted with epistaxis, compared with the control population, raising the possibility that blood group O is a risk factor for epistaxis.