Radioactive isotopes were isotopes found in atmospheres of planets bombarded with nuclear energy and were radioactive.
Shortly after the USS Enterprise-E emerged from a temporal vortex near Earth in the year 2063 from 2373, Lieutenant Commander Data reported to Captain Jean-Luc Picard that he believed the ship had time traveled back to the mid-21st century. According to Data, due to the number of radioactive isotopes found in Earth's atmosphere, the Enterprise had arrived around ten years after the end of World War III. (Star Trek: First Contact)
In 2373, a Changeling impersonating Julian Bashir told Odo that he had purged the radioactive isotopes from a infant Changeling's morphogenic matrix, but it was still slightly unstable. (DS9: "The Begotten")