Uranium: A metallic element that is used as nuclear fuel and is highly toxic and radioactive. Exposure to radiation from uranium can occur in various ways. The breakdown of uranium products creates radon daughters. These can attach to dust particles and, if workers inhale the dust, the particles lodge in their lungs, where they release high doses of radiation. This is believed to be why Navajo uranium miners face a risk of lung cancer that is 28 times normal.