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Many fallout particles are especially hazardous biologically. Some of the principal radioactive elements are as follows:
Strontium 90 is very long-lived with a half-life of 28 years. It is chemically similar to calcium, causing it to accumulate in growing bones. This radiation can cause tumors, leukemia, and other blood abnormalities.
Iodine 131 has a half-life of 8.1 days. Ingestion of it concentrates in the thyroid gland. The radiation can destroy all or part of the thyroid. Taking potassium iodide can reduce the effects.
The amount of tritium released varies by bomb design. It has a half-life of 12.3 years and can be easily ingested, since it can replace a hydrogen in water. The beta radiation can cause lung cancer.
Cesium 137 has a half-life of 30 years. It does not present as large a biological threat as Strontium 90. It behaves similar to potassium, and will distribute fairly uniformly thoughout the body. This can contribute to gonadal irradiation and genetic damage.
When a plutonium weapon is exploded, not all of the plutonium is fissioned. Plutonium 239 has a half-life of 24,400 years. Ingestion of as little as 1 microgram of plutonium, a barely visible speck, is a serious health hazard causing the formation of bone and lung tumors.