What nuclear accidents taught us about radiation and cancer

Most of what we reliably know about radiation released from a nuclear power plant comes from studying what happened after April 1986 at Chernobyl. NCI's fact sheet on nuclear power plant accidents is built largely on that research, and this page follows the same source rather than going beyond it.

The radioactive forms of elements (isotopes) released included I-131, Cs-134, Cs-137, and Sr-90. These are different radioactive substances, and they behave differently in the body. That is part of why the health effects were not all the same.

Studies of children and teens exposed to I-131 from the accident found a higher risk of thyroid cancer.

The reason comes down to ordinary body function. The thyroid gland uses iodine to make its hormones, and it cannot tell the difference between ordinary iodine and the radioactive kind. Radioactive iodine that enters the body collects there, delivering its dose to one small organ.

An NCI-led study measured this link. The higher a person's dose of I-131, the more likely they were to develop thyroid cancer. Each gray (a unit of radiation dose) of exposure was linked to a doubling of risk.

I-131 has a half-life of about eight days. A half-life is the time it takes for half of a radioactive substance to break down. Within a few months, almost none of it is left. You might expect the risk to fade quickly too.

It did not. The study found that the higher thyroid cancer risk stayed high for at least 30 years.

The radiation left quickly. The harm it caused did not. This one fact changed how experts think about long-term follow-up after radiation events.

A second group studied here is the people who worked on cleanup in the years after the accident. Studies of this group found a higher risk of leukemia — a cancer of the blood-forming tissue. That is a different pattern from the thyroid cancers seen in exposed children.

Different exposures, different ages, different cancers. Radiation does not cause just one kind of harm.

Fear that radiation could damage future generations is common and long-standing. Some animal studies have shown effects passed down to offspring, so this fear was not baseless.

But a genomic study of children born to people exposed at Chernobyl found no increase in new genetic changes in those children.

That is a clear, reassuring answer to a question that mattered enormously to many families.

More recent work looked at the tumors themselves. A 2021 study found that thyroid tumors in children exposed to the accident's fallout had more of a certain kind of DNA damage than tumors in people who were not exposed. This damage involved breaks in both strands of the DNA.

This finding makes the mechanism visible. Radiation physically damages DNA, and here, that damage could still be read in tumors decades later.

Potassium iodide is the tool that comes out of all this research. Authorities may tell people at risk to take it after a radiation release, to stop I-131 from building up in the thyroid. It works by filling the gland with ordinary iodine, leaving no room for the radioactive kind. NCI also notes it should not pose a danger to someone who has already had radiation therapy or chemotherapy.

The bigger lesson is about who needs protecting most, and how fast. Children's thyroids took the brunt of the harm. The window to prevent uptake is short, but the effects can last for decades. That combination is why emergency plans near nuclear facilities are built the way they are.

Research on this population has now run for four decades, and it is still producing new findings — the genomic work above is recent. That is one of the lessons on its own.

Health effects that take thirty years to fully show up cannot be measured by a study that stops after ten. Long-running studies are expensive and slow, but they are the only way questions like these ever get answered.