Atmospheric release, mercury levels

Mercury is a toxic substance that, through human and natural activities, cycles through the atmosphere, hydrosphere, and ecosphere affecting the health of both humans and wildlife. It enters the environment naturally through erosion, fire, and volcanic processes, as well as a result of human industrial practices. The human activities such as combustion, smelting, and mining have elevated global mercury levels to approximately three times those found before industrialization. Once released, mercury persists in the environment where it circulates between air, water, sediments, and biota in various forms. Mercury is present everywhere in the environment. The level of Hg in air varies from 0.5 ng/m3 to 10 ig/m3. 

There are virtually no sources of drinking water on Earth that are not contaminated with xenobiotics. Rain water cleanses the atmosphere as it forms and falls. As a result, it contains dissolved acids, organic compounds, and heavy metals such as mercury and selenium in many areas. Surface collection basins from which potable water is drawn—rivers, streams, and lakes—accumulate ground level pollutants in addition to those carried in rain water. Underground water, which is somewhat filtered and generally contains lesser quantities of pollutants than surface water, may itself be contaminated by ground releases of toxicants and by contaminants produced by chemical reactions in the soil and water. 

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