Cosmic rays were a form of high-energy radiation originating in space. In high concentration cosmic rays could be dangerous for both lifeforms and technology, though the atmosphere of a planet usually protected its inhabitants.
In 2268, the star of the Minarian star system became unstable. It produced a huge solar flare with high levels of cosmic rays accompanying, with the concentration reaching as much as 3.51 on the Ritter scale. The USS Enterprise therefore had to leave the area, while an away team stayed on Minara. (TOS: "The Empath")