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Why can radioactive fallout cause illness long after a nuclear explosion?

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U.S. hydrogen bomb test in the Marshall Islands — nuclear weapon A test of a U.S. thermonuclear weapon (hydrogen bomb) at Enewetak atoll in the Marshall Islands, November 1, 1952.

Radioactive fallout causes illness long after a nuclear explosion primarily because it contains long-lived radioisotopes that remain hazardous for years. These materials can enter the human body through the food chain, such as when plants absorb isotopes like strontium-90 from contaminated soil or water.312

Once ingested or inhaled, these radionuclides can accumulate in specific "critical organs" where they deliver continuous, protracted radiation. For example:2
* Strontium-90 is chemically similar to calcium and can be retained in the skeleton and bone marrow for years.23
* Cesium-137 also persists in the environment and can be incorporated into the bodies of animals and plants.31
* Iodine-131, while having a shorter half-life, concentrates in the thyroid gland, increasing the risk of metabolic disturbances and cancer.21

Because these isotopes continue to emit ionizing radiation while inside the body, they cause ongoing tissue damage, which is linked to long-term health risks such as leukemia and various solid cancers.24