Uranium in the environment is a global health concern, and comes from both natural and man-made sources. Mining, phosphates in agriculture, weapons manufacturing, and nuclear power are sources of uranium in the environment.
In the natural environment, radioactivity of uranium is generally low, but uranium is a toxic metal that can disrupt normal functioning of the kidney, brain, liver, heart, and numerous other systems. Chemical toxicity can cause public health issues when uranium is present in groundwater, especially if concentrations in food and water are increased by mining activity. 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. 
Uranium's radioactivity can present health and environmental issues in the case of nuclear waste produced by nuclear power plants or weapons manufacturing.
Uranium is weakly radioactive and remains so because of its long physical half-life (4.468 billion years for uranium-238). The use of depleted uranium (DU) in munitions is controversial because of questions about potential long-term health effects.