When you're told that you have cancer, it's natural to wonder what may have caused the disease. No one knows the exact causes of leukemia. Doctors seldom know why one person gets leukemia and another doesn't. However, research shows that certain risk factors increase the chance that a person will get this disease.

The risk factors may be different for the different types of leukemia:

• Radiation: People exposed to very high levels of radiation are much more likely than others to get acute myeloid leukemia, chronic myeloid leukemia, or acute lymphocytic leukemia.
  • Atomic bomb explosions: Very high levels of radiation have been caused by atomic bomb explosions (such as those in Japan during World War II). People, especially children, who survive atomic bomb explosions are at increased risk of leukemia.
  • Radiation therapy: Another source of exposure to high levels of radiation is medical treatment for cancer and other conditions. Radiation therapy can increase the risk of leukemia.
  • Diagnostic x-rays: Dental x-rays and other diagnostic x-rays (such as CT scans) expose people to much lower levels of radiation. It's not known yet whether this low level of radiation to children or adults is linked to leukemia. Researchers are studying whether having many x-rays may increase the risk of leukemia. They are also studying whether CT scans during childhood are linked with increased risk of developing leukemia.
  
  
• Smoking: Smoking cigarettes increases the risk of acute myeloid leukemia.
  
  
• Benzene: Exposure to benzene in the workplace can cause acute myeloid leukemia. It may also cause chronic myeloid leukemia or acute lymphocytic leukemia. Benzene is used widely in the chemical industry. It's also found in cigarette smoke and gasoline.
  
  
• Chemotherapy: Cancer patients treated with certain types of cancer-fighting drugs sometimes later get acute myeloid leukemia or acute lymphocytic leukemia. For example, being treated with drugs known as alkylating agents or topoisomerase inhibitors is linked with a small chance of later developing acute leukemia.
  
  
• Down syndrome and certain other inherited diseases: Down syndrome and certain other inherited diseases increase the risk of developing acute leukemia.
  
  
• Myelodysplastic syndrome and certain other blood disorders: People with certain blood disorders are at increased risk of acute myeloid leukemia.
  
  
• Human T-cell leukemia virus type I (HTLV-I): People with HTLV-I infection are at increased risk of a rare type of leukemia known as adult T-cell leukemia. Although the HTLV-I virus may cause this rare disease, adult T-cell leukemia and other types of leukemia are not contagious.
  
  
• Family history of leukemia: It's rare for more than one person in a family to have leukemia. When it does happen, it's most likely to involve chronic lymphocytic leukemia. However, only a few people with chronic lymphocytic leukemia have a father, mother, brother, sister, or child who also has the disease.

Having one or more risk factors does not mean that a person will get leukemia. Most people who have risk factors never develop the disease.

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