The radioactivity of an element depends on what isotope is present. Polonium-210 for example is regarded as an extremely volatile, dangerous, and radioactive isotope. By mass, polonium-210 is nearly 250,000 times more toxic than hydrogen cyanide. To put that into perspective, hydrogen cyanide was used as a chemical weapon in World War I. It interferes with the normal function of oxygen in nearly every organ of the body, causing rapid death in under a minute.
In the following infographic, Alan’s Factory Outlet lists 38 radioactive elements and what they are used for.