The Clean Air Act requires the Environmental Protection Agency (EPA) to regulate hazardous air pollutants , through the National Emissions Standards for Hazardous Air Pollutants program established in Sec. 112  of the Act. EPA must identify sources of the 188 hazardous air pollutants  (HAPs) listed in section 112(b), including acid gases, asbestos, dioxin, benzene, chlorine, lead compounds, mercury, phosphorus, various metals and others. Major sources of these pollutants are those that emit 10 tons per year of a single HAP or 25 tons per year or more combined of several HAPs.
EPA promulgates technology-based standards for reducing HAP emissions using maximum achievable control technology  (MACT) for both new and existing sources. Determination of the MACT considers a number of factors, including cost, energy requirements, and non-air quality health and environmental impacts. The Act established certain stringency requirements for MACT for new sources, known as "floor" requirements. Existing sources are also subject to MACT, which may be less stringent than the MACT for new sources, but must not be less stringent than the best-performing 12 percent of existing sources in the same category. EPA has recently established new MACT requirements for power plants, which leads some to refer to this rule as the "Utility MACT rule."
As with some other Act programs, the Federal government establishes and state air quality programs implement NESHAP programs.
Starting with the mandate of the Clean Air Act Amendments of 1990 and then revised every eight years, EPA determines a list of categories and subcategories of sources of HAPs. After Congressionally-mandated EPA studies indicated that other rules were not substantially reducing HAP emissions from coal- and oil-fueled power plants, these plants were determined to be source categories of HAPs in 2000. Mercury is the most significant HAP emitted from coal- and oil-fueled power plants. These power plants are also significant emitters of other carcinogenic HAP metals, such as arsenic, nickel, cadmium, and chromium; HAP metals with potentially serious non-cancer health effect such as lead and selenium; and other toxic air pollutants such as the acid gases hydrogen chloride and hydrogen fluoride. There are 1,325 units at 525 power plants around the United States that will need to comply with the recently announced rule. Some of those power plants are more than fifty years old, and complying with the new regulations will be significantly more challenging and expensive than for newer facilities. Some of these older plants may be retired rather than incur the costs of installing new pollution control equipment.
After coal- and oil-fueled power plants were determined to be a source category in 2000 and therefore subject to regulation, EPA undertook to regulate mercury from these sources. This Bush Administration rule was known as the Clean Air Mercury Rule (CAMR) and would have instituted a cap-and-trade program for mercury emissions. A court ruled that EPA must regulate HAPs under Sec. 112 and not under another section as proposed in CAMR. It vacated the 2005 rulemaking. EPA's new Mercury Rule, also known as the Mercury and Air Toxic Standards (MATS), was proposed in March 2011 and scheduled to be finalized in November 2011, although it was delayed until December 12, 2011.
In September 2011, the House of Representatives passed H.R. 2401 , the Transparency in Regulatory Analysis of Impacts on the Nation (TRAIN) Act, that would, among other regulations, delay implementation of the Mercury Rule pending completion of additional economic studies (other than those EPA and the Office of Management and Budget have already conducted). This bill is unlikely to advance in the Senate.
On February 16, 2012, Senator Jim Inhofe (R-OK) introduced a Congressional Review Act (CRA) joint resolution to revoke the Mercury Rule. Under CRA procedure, the CRA bill was required to pass both chambers of Congress and be signed by the President to go into effect. The bill was voted down 46 to 53 in the Senate on June 20, 2012.
Read more from EPA  on the Mercury Rule.