(covers information from several alternate timelines)
Radioisotopes are atoms that emit radiation as they decay.
One type of radioisotope are "delta-series radioisotopes", which can be produced in plasma conduits, and which can interact with temporal energy. Prolonged exposure to these isotopes can prove fatal for humanoids.
In 2371, Miles O'Brien became exposed to a large amount of delta-series radioisotopes when a plasma conduit he was working on exploded. The radioisotopes in his body interacted with temporal energy being emitted by the artificial quantum singularity of a cloaked Romulan Warbird orbiting Deep Space 9, causing him to shift forward in time by approximately five hours every time the singularity emitted a particular temporal frequency. Later, O'Brien and Doctor Bashir were able to calibrate the decay rate of the isotopes to match the temporal frequency, thereby controlling O'Brien's time shifts. This allowed him to travel three hours into the future, and uncover a Romulan plot to destroy the station. Unfortunately, due to the toxic effects of the radioisotopes, O'Brien died, necessitating his counterpart from the future making the return trip to warn the crew of the imminent attack. (DS9: "Visionary")