Leukemia doesn’t usually have one single cause. Most of the time, it develops from a mix of genetic changes in blood-forming cells plus certain environmental or lifestyle risk factors over time.

What leukemia is (quickly)

Leukemia is a cancer of the blood and bone marrow, where abnormal white blood cells grow uncontrollably and crowd out normal blood cells. There are several main types (like AML, ALL, CML, CLL), and the causes and risk patterns can differ between them.

Core idea: DNA damage in blood cells

At the heart of leukemia are mutations (damage or changes) in the DNA of cells in the bone marrow that make blood cells.

  • These mutations can switch on oncogenes (genes that drive growth) or switch off tumor suppressor genes (the “brakes” on cell growth).
  • As a result, immature or abnormal white blood cells multiply when they shouldn’t, don’t die when they should, and build up in the bone marrow and blood.
  • Different leukemias have different signature mutations, such as the Philadelphia chromosome t(9;22)t(9;22)t(9;22) in chronic myeloid leukemia (CML).

Often, doctors cannot pinpoint exactly when or why a specific person’s DNA changed; they just see the effects in the cells.

Major risk factors linked to leukemia

Doctors talk more about “risk factors” than direct causes, because many people with risk factors never get leukemia, and some people with leukemia have none of the known risks.

1. Radiation exposure

  • High-dose ionizing radiation (for example, atomic bomb exposure, certain nuclear accidents, or high doses of radiation therapy) is strongly associated with a higher risk of several leukemias, especially acute types.
  • Medical radiation used appropriately (like diagnostic X‑rays or CT scans) adds a much smaller risk, but doctors still try to use the lowest necessary dose, especially in children.

2. Certain chemicals

  • Long-term exposure to benzene (used in some industrial settings, petrochemicals, and previously in some products) clearly increases leukemia risk, especially acute myeloid leukemia (AML).
  • Some pesticides and industrial chemicals have been linked with a higher risk in some studies, though evidence strength varies.
  • Formaldehyde exposure has been associated with an increased risk for some myeloid leukemias.

3. Previous chemotherapy or radiotherapy

  • Some people develop “therapy-related” leukemias years after being treated for another cancer.
  • Certain chemotherapy drugs, especially alkylating agents and topoisomerase II inhibitors, and intense radiation therapy can damage bone marrow DNA and raise the chance of later leukemia.

4. Genetic and inherited conditions

Most leukemia is not directly inherited, but genetic makeup can influence risk.

  • Certain inherited syndromes significantly raise the risk of acute leukemias, for example:
    • Down syndrome
    • Fanconi anemia
    • Bloom syndrome
    • Li-Fraumeni syndrome
  • A family history of leukemia can slightly increase risk, especially for chronic lymphocytic leukemia (CLL) and in identical twins of children with ALL or AML.

5. Immune and blood disorders

  • Some chronic bone marrow or blood disorders (myeloproliferative neoplasms such as polycythemia vera, myelofibrosis, essential thrombocythemia) can evolve into AML over time.
  • Rare immune deficiencies or chronic immune activation can also alter risk patterns.

6. Viruses

A few viruses are clearly linked to specific leukemia types.

  • Human T‑cell leukemia virus type 1 (HTLV‑1) is associated with adult T‑cell leukemia/lymphoma.
  • Epstein–Barr virus (EBV) is associated with some lymphoid cancers and certain subtypes of acute lymphoblastic leukemia (ALL).

These viruses can alter the DNA and behavior of infected immune cells, making them more likely to become malignant.

7. Lifestyle and environmental factors

  • Smoking : Smoking is linked to a modestly higher risk of AML in adults, likely due to benzene and other carcinogens in tobacco smoke.
  • Air pollution and pesticides : Some studies suggest that air pollution and pesticide exposure (including parental exposures) may increase childhood leukemia risk, though results are not always consistent.
  • Diet : Diet seems to have at most a small effect; eating more vegetables may offer a slight protective benefit, but no specific food is a proven cause.

Why some people get leukemia and others don’t

Even with all these risk factors, in most individuals the exact trigger is still unknown.

  • Many people with strong risk factors (like radiation or benzene exposure) never develop leukemia.
  • Many people diagnosed with leukemia have no obvious risk exposures or family history.
  • This suggests a complex interplay of random DNA errors , inherited susceptibility, environment, age, and immune system factors.

A useful way to think of it: the bone marrow cells go through many divisions over a lifetime; with each division, there is a small chance of random DNA mistakes. Add extra stress (chemicals, radiation, viral infections), and the chance of harmful mistakes goes up.

Recent and ongoing research (up to 2026)

In recent years, researchers have been focusing on more precise causes at the molecular and environmental level.

  • Genomics and sequencing : Large studies using next-generation sequencing are cataloging the exact mutations that drive different leukemia subtypes, which helps explain why they behave differently and respond differently to treatment.
  • Early-life exposures : There is active research on how prenatal and early-childhood exposures (like maternal smoking, pollutants, pesticides, infections) might shape leukemia risk later in childhood.
  • Gene–environment interaction : Studies are trying to understand why people with similar exposures have different outcomes, suggesting some people’s genes make their bone marrow more or less vulnerable to damage.
  • Chronic leukemia drivers : For CML and some other chronic leukemias, specific chromosomal changes (like the Philadelphia chromosome) are well-characterized as the main driver events.

Because of this research, the World Health Organization and other bodies have updated leukemia classifications to be more genetics-focused, which also affects how doctors think about causes and prognosis.

Simple example to tie it together

Imagine a bone marrow cell like a factory with a detailed instruction manual (its DNA). Over time:

  • Some pages of the manual are damaged by random copying errors.
  • Others are stained or torn by chemicals like benzene or by high-dose radiation.
  • A virus might tape in an extra page with bad instructions.

If enough of the pages that control “growth,” “stop,” and “self-destruct” are altered in the wrong ways, that cell can start overproducing abnormal white blood cells—this is essentially leukemia.

Key takeaways about “what causes leukemia”

  • There is no single universal cause of leukemia; it arises when mutations build up in bone marrow cells that make blood cells.
  • Known contributors include high-dose radiation, benzene and some other chemicals, prior chemotherapy/radiation, certain viruses, inherited syndromes, some blood disorders, and smoking.
  • Most cases still appear “sporadic,” meaning no clear cause can be identified in that individual.
  • Ongoing research is making the picture more detailed, especially at the genetic and early-life exposure levels.

Information gathered from public forums or data available on the internet and portrayed here.