That Emergency Exit Sign Is Radioactive (But That's Okay)Esther Inglis-Arkell3/18/15 10:30am427EditPromoteGo to permalinkOne of the major requirements of an illuminated exit sign is it has to keep glowing no matter what. That means that, if power is cut to the building, or if the sign itself gets knocked around, power has to keep flowing. What's the power source? Radiation. But there's no need to panic.