Although bismuth-209 is now known to be radioactive, it has classically been considered to be a stable isotope because it has a half-life of approximately 2.01×1019 years, which is more than a billion times the age of the universe.
Of naturally occurring radioisotopes, the most stable is radiogenic 210Bi with a half-life of 5.012 days.
210mBi is unusual for being a nuclear isomer with a half-life multiple orders of magnitude longer than that of the ground state.
Commercially, bismuth-213 can be produced by bombarding radium with bremsstrahlung photons from a linear particle accelerator, which populates its progenitor actinium-225.
This isotope has also been tried in targeted alpha therapy (TAT) program to treat a variety of cancers.