Radioactivity-results from the decay of radiogenic isotopes to produce other isotopes; energy is released in three forms:
The rate of decay varies from a fraction of a second to millions of years, depending on the isotope. The rate is usually expressed as the 'half-life:' the time required for ½ of the radioisotopes in a sample to decay.
If the decay occurs with a lot of energy release, the alpha and beta particles can cause molecular damage, but if the exposure is low or the decay is low energy, most living tissues in most organisms can repair the damage. See DOE summary.