Mercury regulation in the United States limit the maximum concentrations of mercury (Hg) that is permitted in air, water, soil, food and drugs. The regulations are promulgated by agencies such as the Environmental Protection Agency (EPA) and Food and Drug Administration (FDA), as well as a variety of state and local authorities. EPA published the Mercury and Air Toxics Standards (MATS) regulation in 2012; the first federal standards requiring power plants to limit emissions of mercury and other toxic gases.
Background
Forms of mercury
Mercury occurs naturally in the environment and exists in many forms. In pure form, it is known as "elemental" or "metallic" mercury. Elemental mercury is a shiny, silver-white metal that is liquid at room temperature. It is not found in this form in nature, but in compounds and inorganic salts. If not sealed off, mercury slowly evaporates into the air, forming a vapor. The quantity of vapor formed increases as temperatures rise. Elemental mercury is traditionally used in thermometers and some electrical switches.
Inorganic mercury compounds or mercury salts, more commonly found in nature, include mercuric sulphide (HgS), mercuric oxide (HgO) and mercuric chloride (HgCl2). Most of these are white powders or crystals, except for mercuric sulphide which is red and turns black after exposure to light.
Organic mercury is formed when mercury combines with carbon and other elements. Examples of organic mercury compounds are dimethylmercury, phenylmercuric acetate, and methylmercuric chloride. The form most commonly found in the environment is methylmercury.
How mercury exists in the environment
Elemental mercury in the atmosphere can undergo transformation into inorganic mercury forms, providing a significant pathway for deposition of emitted elemental mercury.
Some micro-organisms can produce organic mercury, particularly methylmercury, from other mercury forms. Methylmercury can accumulate in living organisms and reach high levels in fish and marine mammals via a process called biomagnification (i.e. concentrations increase in the food chain).
Being an element, mercury cannot be broken down or degraded into harmless substances. Mercury may change between different states and species in its cycle, but its simplest form is elemental mercury, which itself is harmful to humans and the environment. Once mercury has been liberated from either ores or from fossil fuel and mineral deposits hidden in the earth's crust and released into the biosphere, it can be highly mobile, cycling between the earth's surface and the atmosphere. The earth's surface soils, water bodies and bottom sediments are thought to be the primary biospheric sinks for mercury.
Mercury speciation
The different forms mercury exists in (such as elemental mercury vapour, methylmercury, or mercuric chloride) are commonly designated "species". As mentioned above, the main groups of mercury species are elemental mercury, inorganic and organic mercury forms. Speciation is the term commonly used to represent the distribution of a quantity of mercury among various species.
Speciation influences the transport of mercury within and between environmental compartments including the atmosphere and oceans, among others. For example, the speciation is a determining factor for how far from the source mercury emitted to air is transported. Mercury adsorbed on particles and ionic (e.g., divalent) mercury compounds will fall on land and water mainly in the vicinity of the sources (local to regional distances), while elemental mercury vapour is transported on a hemispherical/global scale making mercury emissions a global concern. Another example is the so-called "polar sunrise mercury depletion incidence", where the transformation of elemental mercury to divalent mercury is influenced by increased solar activity and the presence of ice crystals, resulting in a substantial increase in mercury deposition during a three-month period (approximately March to June).
Moreover, speciation determines how to control mercury emissions to air. For example, emissions of inorganic mercuric compounds (such as mercuric chloride) are captured reasonably well by some control devices (such as wet-scrubbers), while capture of elemental mercury tends to be low for most emission control devices.