But risk factors are rarely absolute. Having a risk factor, or even several risk factors, does not mean that you will get the disease. And many people who get the disease may not have had any known risk factors.

Genetic Risk Factors

Genetic risk factors are those that are part of our DNA. They are most often inherited from our parents. While some genetic factors increase the risk of childhood leukemia, most cases of leukemia are not linked to any known genetic causes.

Inherited Syndromes

There are several inherited disorders that increase a child's risk of developing leukemia:

• Li-Fraumeni syndrome: This is a rare condition caused by change in the p53 tumor suppressor gene. People with this change have an increased risk of developing several kids of caner, especially leukemia, bone or soft tissue sarcomas, breast cancer, and brain tumors. 
• Down syndrome (trisomy 21): Children with Down syndrome have an extra (third) copy of chromosome 21. In ways that are not completely understood, this extra chromosome 21 causes mental retardation and a characteristic facial appearance. Children with Down syndrome are 10 to 20 times more likely to develop either ALL or AML than are other children. Down syndrome has also been linked with transient leukemia -- a leukemia-like condition within the first month of life, which often resolves on its own without the use of chemotherapy. 
• Klinefelter syndrome: This is a genetic condition in which males have an extra "X" chromosome. This causes infertility, prevents normal development of male features (such as body hair, deep voice, etc.) and is linked to an increased risk of developing leukemia.

Several other genetic disorders (such as neurofibromatosis, ataxia telangiectasia, Wiscott-Aldrich syndrome, and Fanconi anemia) also carry an increased risk of leukemia, although these disorders more commonly lead to non-Hodgkin lymphoma and other types of cancers.

Inherited Immune Deficiencies

Certain inherited diseases cause children to be born with immune system problems. Along with being at increased risk of getting serious infections due to reduced immune defenses, these children may also have an increased risk of leukemia.

Having a Brother or Sister With Leukemia

Siblings (brothers and sisters) of children with leukemia have a slightly increased chance (2 to 4 times normal) of getting leukemia, although the overall risk is still low. The risk is much higher among identical twins. If an identical twin develops childhood leukemia, the other twin has about a 20% chance of getting leukemia as well. This risk is even higher if the leukemia develops in the first year of life.

Having a parent who develops leukemia as an adult does not seem to raise a person's risk of leukemia.

Lifestyle-related Risk Factors

Lifestyle-related risk factors for some cancers include an unhealthy diet, harmful habits such as smoking, drinking excessive amounts of alcohol, and too much sun exposure. While lifestyle-related factors are important in many adult cancers, they play much less of a role in childhood cancer risk.

Some studies have suggested that a mother drinking a lot of alcohol during pregnancy may increase the risk of leukemia in her child, but not all studies have found such a link.

Environmental Risk Factors

Environmental risk factors are influences in our surroundings, such as radiation and certain chemicals, which increase the risk of getting diseases such as leukemias.

Radiation Exposure

Exposure to high levels of radiation is a risk factor for childhood leukemia. Japanese atomic bomb survivors had a 20-fold increased risk of developing AML, usually within 6 to 8 years after exposure. Similar risks have occurred after exposure to radiation from nuclear reactor accidents. Exposure of a fetus to radiation within the first months of development may also carry an increased risk of childhood leukemia, although the extent of the risk is not clear.

The possible risks from fetal or childhood exposure to lower levels of radiation, such as from x-rays or CT scans, is not well-defined. Some studies have found a slight increase in risk, while others have found no increased risk. Any risk increase is likely to be small, but to be safe, most doctors do not order these tests for fetuses or children unless they are absolutely necessary.

Radiation Therapy and Chemotherapy

Children and adults treated for other cancers with radiation therapy and chemotherapy have a higher risk of getting a second cancer, usually AML, later in life. Chemotherapy drugs such as alkylating agents (a class that includes cyclophosphamide and chlorambucil) and epipodophyllotoxins (such as etoposide and teniposide) have been linked to a higher risk of leukemia. These leukemias usually develop within 5 to 10 years of treatment and tend to be hard to treat.

Immune System Suppression

Patients getting intensive treatment to suppress their immune function (mainly organ transplant patients) have an increased risk of certain cancers, such as lymphoma and ALL.

Certain Chemicals

Exposure to chemicals such as benzene may cause AML in adults and, rarely, in children. ALL has not been linked to any cancer-causing chemicals.

Uncertain, Unproven, or Controversial Risk Factors

Other factors that have been studied for a possible link to childhood leukemia include:

•  exposure to insecticides 
• exposure to electromagnetic fields (such as living near power lines) 
• mother's age when child was born 
• parent's smoking history 
• fetal exposure to hormones (such as diethylstilbestrol (DES) or birth control pills)
•  father's workplace exposure to chemicals and solvents 
•  chemical contamination of ground water

So far, none of these factors has been linked conclusively to childhood leukemia.