Leukemia is a cancer of the blood and the blood-forming organs. The word “leukemia” technically means cancer of the leukocytes (white blood cells), although it is generally used to refer to all types of blood cancers.

Every year, nearly 27,000 adults and more than 2,000 children in the United States learn that they have leukemia.

There are four major types of leukemia: Acute Myelogenous Leukemia (AML), Acute Lymphocytic Leukemia (ALL), Chronic Myeloid Leukemia (CML), and Chronic Lymphocytic Leukemia (CLL). All four types of leukemia can be caused by exposure to benzene and other leukemogens, although the scientific evidence is strongest for AML.

Several chemicals are known to cause leukemia. These include the industrial chemicals benzene, 1,3-butadiene and formaldehyde, and likely ethylene oxide and hydroquinone as well; various pesticides (especially herbicides), various chemotherapy and immunosuppressive drugs (especially alkylating agents and Topoisomerase II inhibitors), and, of course, ionizing radiation. Cigarette smoking has also been recognized as a cause of myeloid leukemias.

In 2009, the International Agency for Research on Cancer (IARC) concluded that there is “sufficient evidence” that benzene causes AML and “limited evidence” that benzene causes ALL, and CLL. According to IARC, the designation “limited evidence” means that “a positive association has been observed between exposure to the agent and cancer for which a causal interpretation is considered . . . to be credible, but chance, bias, or confounding could not be ruled out with reasonable confidence.”