The Chernobyl (also spelled Chornobyl) nuclear accident was by far the most devastating in the history of nuclear power. It released into the Earth's atmosphere about 400 times more radioactive material than the atomic bomb dropped on Hiroshima, but less than 1 percent of the amount from atmospheric weapons tests conducted in the 1950s and 1960s. This keyword site summarizes what is known about the nuclear accident and its aftermath—what happened, how it happened, how many people were seriously exposed to radiation, the immediate health effects, and the anticipated long-term health and environmental effects of this environmental disaster.

Among 400 people, about 200 will develop at least one tumor of the colon (large bowel), 20 of these cases will progress to malignancy, and 1 will be associated with an inherited genetic abnormality. Among the inherited forms of colon cancer, two important types are familial adenomatous polyposis (FAP) and hereditary nonpolyposis colorectal cancer (HNPCC). They demonstrate the difference between tumor initiation and tumor progression. Patients with FAC develop thousands of colon polyps, only a few of which progress to cancer, whereas patients with HNPCC have only a few polyps, but each has a high probability of progressing to cancer. Consult this keyword site to learn more about these and other types of colon cancer.

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