People are exposed to benzene from tobacco smoking, automobile service stations, exhaust from motor vehicles, and industrial emissions. People living in cities or industrial areas are exposed to higher levels of benzene in air than those living in rural areas. Breathing high levels of benzene can cause drowsiness, dizziness, rapid heart rate, headaches, tremors, confusion, and unconsciousness. Long-term exposure causes harmful effects on the bone marrow, can lead to anemia, and can affect the immune system. Benzene is a known human carcinogen. Long-term exposure to high levels of benzene in the air can cause leukemia.