Uranium is a naturally radioactive heavy metal represented by the symbol U. Its most commonly occurring (and most commonly mined) isotope is U-238. U-238 decays along a defined path, changing to the next element in the series and releasing fixed amounts of radiation. The decay series for Uranium-238 contains some 18 radioactive elements before reaching a stable lead form. The amount of radiation released is measured in Becquerels, but the radioactivity of an element is usually described in half-lives, which is the time it takes for half the amount of the substance to break down into the next element in the decay series. Here are some examples from the Uranium-238 series:
Uranium-238 4.5 billion years
Thorium-234 24 days
Thorium-230 77,000 years
Radium-226 1,600 years
Radon-222 (a gas) 3.8 days
Polonium-218 3.1 minutes
Radon-222 is the only gas in the series, and it decays into Polonium-218, a solid, which further decays to other solid radioactive elements.
Generally speaking, elements with higher radioactivity give off more radiation in a shorter period of time so it can be more damaging to living cells. Although they will disappear faster from the environment, they leave behind their decay products. Elements with longer half-lives will decay more slowly and be a “constant” source of low levels of radiation and will continue to produce the element’s decay products.
Uranium-238 and its decay products are also considered heavy metals that are chemically toxic to living cells.