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. This section requires EPA to identify sources of the 188 hazardous air pollutants  (HAPs) listed in Sec. 112(b), including acid gases, asbestos, dioxin, benzene, chlorine, lead compounds, mercury, phosphorus, various metals and others. Facilities that emit 10 tons per year of a single HAP or 25 tons per year or more combined of several HAPs are defined as “major sources” under the Act.
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 boilers.
As with some other Clean Air Act programs, the Federal government establishes regulations and leaves it to state air quality programs to implement NESHAP programs.
Starting after 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. In 1998, EPA determined that industrial, commercial, and institutional boilers were sources of HAPs to be regulated because of their emissions of polycyclic organic matter and mercury. These regulations cover: boilers that burn fuels, including natural gas, fuel oil, coal, biomass, refinery gas, or other gas to produce steam that is then used to produce electricity or heat; and process heaters that heat raw or intermediate materials during an industrial process. This rule does not cover units that burn solid waste unless they are otherwise exempt from Clean Air Act standards for incinerators.
The new “Boiler MACT” rule regulates mercury, hydrogen chloride, particulate matter, carbon monoxide and dioxin/furan emissions from boilers and process heaters. It will affect about 13,840 major source boilers nationwide at industrial facilities and an additional 187,000 area boilers at a variety of smaller businesses and institutions. Major sources will be required to enact work practice improvements and tune ups for natural gas and refinery boilers, and to meet numeric emissions limits for HAPs for other types of boilers. For smaller, “area source” boilers (those that fall below the threshold for major sources), a scaled approach is taken under which boilers have different requirements to meet numeric emissions standards or receive tune ups given their size, age, and fuel type.
A Boiler MACT was finalized in September 2004. That regulation, however, was vacated and remanded by a 2007 court decision. EPA issued a revised rule was in March 2011, but then chose to immediately reconsider the rule. A new rule was released for public comment on December 2, 2011 and was finalized on December 20, 2012.
Read more from EPA  on the Boiler MACT.