Mercury is a naturally occurring element in minerals, rocks, soil, water, air, plants, and animals. The predominant forms in the atmosphere, water, and aerobic soils and sediments are elemental and mercuric mercury; while cinnabar is commonly found in mineralized ore deposits and anaerobic soils and sediments. Mercury is present throughout the atmosphere because of its relatively high vapor pressure. It vaporizes from the earth's surface and is transported in a global cycle, sometimes for hundreds of kilometers, before being deposited again with particulates, rain, or snow. The background concentrations in rocks and soils typically range between 20 and 100 μg Hg/kg with a worldwide average of about 50 μg Hg/kg. Natural background concentrations in the uncontaminated atmosphere are in the order of between 1 and 10 ng/m³ increasing to between 50 and 1,000,000 ng/m³ or more over mineralized areas. Mercury is transported to aquatic ecosystems via surface runoff and atmospheric deposition...