This is the time taken for half of the atoms in a radioactive substance to decay. The more unstable the atom the quicker it will decay and the shorter its half-life will be. Half-lives can range from less than a millionth of a second to millions of years depending on the element concerned. After one half-life the level of radioactivity of a substance is halved, after two half-lives it is reduced to one quarter, after three half-lives to one-eighth and so on. After ten half-lives the radioactivity is reduced to one-thousandth of the original.