The Clean Air Act requires the establishment of monitoring networks to collect data to help identify and track movement of air pollutants into Great Waters ecosystems and determine overall pollution loadings from the air. EPA must report the findings of the investigations in their biennial reports to Congress. There is widespread evidence in the United States, Canada, and Europe of high concentrations of mercury, a toxic metal, in fish tissue that exceed local, national, or international public health guidelines. This contamination represents a serious human health concern as well as a significant economic threat to both the commercial and sportfishing industries. Many of these advisories, particularly in Minnesota, Wisconsin, and Florida, have been issued for relatively pristine waterbodies where atmospheric deposition is thought to be the main source of the mercury contamination.
Mercury comes from natural and man-made sources.
Natural sources of atmospheric mercury include degassing of the earth's crust and forest fires.