Answer
The biological half-life is shorter than the physical half-life because the biological half-life is how long it takes for the radioisotope to leave the body, not necessarily decay. Usually radioisotopes move through the body and are excreted in urine or feces fairly quickly. It takes much longer for the radioisotopes to physically decay.
Work Step by Step
The biological half-life of a radioisotope is the time it takes for half the amount to clear the body. This can happen either through radioactive decay or through elimination. Cesium-137 has a biological half-life of 17 days and is quickly excreted by the kidneys.
The physical half-life of a radioisotope is the time it takes for half the amount to radioactively decay to a more stable state. Cesium-137 has a physical half-life of 30 years.
