Hemoglobin carries oxygen to your body’s organs. When levels drop too low, your heart and lungs have to work overtime to keep you alive. Symptoms of dangerously low hemoglobin may include: -Severe fatigue -Shortness of breath -Dizziness or fainting -Rapid heart rate -Chest pain -Pale skin Common causes: -Gastrointestinal bleeding -Heavy menstrual bleeding -Bleeding ulcers -Certain cancers -Severe iron deficiency A hemoglobin of 4–5 g/dL can be life-threatening and often requires urgent evaluatio