Uranium in the environment refers to the science of the sources, environmental behaviour, and effects of uranium on humans and other animals. Uranium is weakly radioactive and remains so because of its long physical half-life (4.468 billion years for uranium-238). The biological half-life (the average time it takes for the human body to eliminate half the amount in the body) for uranium is about 15 days. Normal functioning of the kidney, brain, liver, heart, and numerous other systems can be affected by uranium exposure, because uranium is a toxic metal. The use of depleted uranium (DU) in munitions is controversial because of questions about potential long-term health effects.