Once mobilized in the environment, mercury can cycle through land, air, and water, undergoing a number of complex chemical and physical transformations. An understanding of the global mercury cycle is necessary to understand the causes of mercury accumulation and to evaluate the role played by human activities and by different sources of mercury emissions.

Typically, mercury is emitted to the atmosphere as a gas or as particulate matter; once released it may return to the earth's surface by either dry (e.g., gravitational settling) or wet (e.g., with precipitation) deposition. Erosion, rainfall and leaching transport mercury from land surfaces to streams, lakes and oceans.

Mercury circulates through the environment in different chemical forms and different physical states. In inorganic form, it exists in three oxidation states as elemental mercury (Hg⁰), monovalent mercury (Hg⁺¹), or divalent mercury (Hg⁺²). Elemental mercury, in liquid form, is the type of mercury found in many consumer products (e.g., household fever thermometers with the silver bulb). When open to the atmosphere, elemental mercury vaporizes from its liquid state into the atmosphere. Mercury may also exist in organic forms (i.e. in combination with carbon-containing compounds) such as methylmercury (CH₃Hg⁺). Methylmercury is the chemical formed when bacteria in soil or water convert deposited mercury through ingestion and absorption.

While it circulates in the environment and changes its form, mercury is persistent and is not biodegradable. It tends to accumulate in sediments - in rivers, streams, lakes and the ocean. Once present in a biological system, mercury can be passed up the food chain, "bioaccumulating" (increasing its concentration) accordingly. Mercury can even accumulate in sewer pipes, which can lead to long-term releases of mercury to municipal wastewater that may continue even after the original source has been eliminated. Mercury, once released, is thus hard to control.

A combination of local, regional, and distant anthropogenic sources, natural sources, and re-emitted mercury (of either human or natural origin from existing reservoirs of previously mobilized mercury) may contribute to mercury deposition at a given location. The cycling nature of mercury presents many uncertainties in attempting to model mercury deposition and concentrations.

Sources: NESCAUM (Northeast States for Coordinated Air Use Management) et. al., "Northeast States and Eastern Canadian Provinces: Mercury Study: A Framework for Action," February 1998; Massachusetts Department of Environmental Protection, "Mercury in Massachusetts: An Evaluation of Sources, Impacts, Emissions and Controls," June 1996.