The trefoil radiation symbol — used to mark genuinely radioactive material, but online it's just as often deployed as hyperbole for something "toxic," dangerously spicy food, or a scandal that's about to blow up. It long predates emoji: the trefoil was designed in 1946 at UC Berkeley, and its three blades are said to evoke a fan blowing danger outward from a central point, though the exact origin story is debated even among physicists.
Keywords: radioactive, sign