半衰期的長短與核素的危害程度的關係是?

The hazard of radioactive substances to the environment depends on two factors, half-life and activity.

Radioactive substances with high activity and long half-life causes the most hazard. The reason is apparent, as the substnace persist long in the environment.

Although radioactive substances with short half-life could decay in a short period of time, short half-life high activity substances still could bring much hazard. An example is iodine-131 (half-life 8 days), which is a substance that is released after a nuclear power plant accident. Most of the doses that incurred to the public after the nuclear accident at Chernobyle or Three-Mile Island did come from iodine-131.

Substances with long half-life but very low activity would bring minimal hazard. Examples are carbon-14 in the natural environment, or potasium-40 in our body.