A dangerous hemoglobin level is hemoglobin greater than 18.5 g/dL in men and 16.5 g/dL in women, Medscape states. This is known as polycythemia.
Someone suffering from polycythemia is at risk for blood clots, heart conditions, kidney dysfunction and spent marrow. When someone has a high hemoglobin count, their blood thickens, which makes it harder for the heart to pump it around the body. In severe cases, this can cause pulmonary embolisms, strokes and heart attacks, all of which may result in death.
In addition, a man with less than 13.5 g/dL and a woman with less than 12 g/dL hemoglobin is at risk of anemia, although this is rarely problematic, according to Mayo Clinic.