CONTROL OF Toxic Air Pollutants
The Clean Air Act Amendments of 1990 require the US EPA to regulate emissions of toxic air pollutants (hazardous air pollutants) using technology-based standards. These standards are known as the National Emissions Standards for Hazardous Air Pollutants (NESHAP) or the MACT (Maximum Achievable Control Technology) standards. EPA identified and compiled a list of the major sources of air toxics and established a timeframe in which MACT standards for each source category will be promulgated. When all of the 90+ MACT rules are finalized, around 170 source types will be affected. This site contains information to assist air pollution control
agencies in conducting inspections and writing permits under these MACT Rules.
Introduction to MACT or NESHAP Rules
Source: US EPA - Office of Air and Radiation
- What are the Maximum Achievable Control Technology (MACT)
Rules? (See below)
- What Is the Purpose of the NESHAP?
1. What are the Maximum Achievable Control Technology (MACT)
Rules?
 US EPA's MACT standards require affected sources to meet specific emissions limits that are based on the emissions levels already
achieved by the best-performing similar facilities. This straightforward,
performance-based approach yields standards that are both reasonable
and effective in reducing toxic emissions. This approach also provides
a level economic playing field by ensuring that facilities with
good controls are not disadvantaged relative to competitors with
poorer controls.
When developing a MACT standard for a particular source category,
US EPA looks at the level of emissions currently being achieved
by the best-performing similar sources through use of HAP compliant
materials, clean processes, control devices, work practices, or
other methods. These emissions levels set a baseline (often referred
to as the "MACT floor") for the new standard. At a minimum,
a MACT standard must achieve, throughout the industry, a level
of emissions control that is at least equivalent to the MACT floor.
EPA can establish a more stringent standard when this makes economic,
environmental, and public health sense.
The MACT floor is established differently for existing sources
and new sources:
For existing sources, the MACT floor must equal the average emissions
limitations currently achieved by the best-performing 12 percent
of sources in that source category, if there are 30 or more existing
sources. If there are fewer than 30 existing sources, then the
MACT floor must equal the average emissions limitation achieved
by the best-performing five sources in the category.
For new sources, the MACT floor must equal the level of emissions
control currently achieved by the best-controlled similar source.
Wherever feasible, US EPA writes the final MACT standard as an
emissions limit
(i.e., as a percent reduction in emissions or a concentration limit
that regulated sources must achieve). Emissions limits provide
flexibility for industry to determine the most effective way to
comply with the standard.
You can link to Federal Register publications (5-15-03) [PDF] (5-30-03) [PDF]
or go to a
US EPA site for more information on source categories.
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2. What is the purpose of the NESHAP?
 The purpose of the final National Emission Standards for Hazardous
Air Pollutants (NESHAP) is to protect the public health by
reducing discharges of air toxics, or hazardous air pollutants (HAPs), from air emission sources. Section
112 of the Clean Air Act Amendments (CAAA) of 1990 requires EPA to promulgate
standards for the control of hazardous air pollutants from both new and existing major
sources. A major source of HAP is defined as any stationary source
or group of stationary sources within a contiguous area and under
common control that emits or has the potential to emit [PDF], considering
physical and operational design, in the aggregate, 10 tons per year or more of any single
HAP or 25 tons per year or more of multiple HAPs. Sources that
emit levels less than these amounts are known as area sources.
Under the Urban Air Toxic Strategy, some types of area sources will have also have air toxic standards.
The CAAA requires the standards to reflect the maximum degree of
reduction in HAP emissions that is achievable taking into consideration
the cost of achieving the emissions reductions, any non-air-quality
health and environmental impacts, and energy requirements. This
level of control is commonly referred to as the MACT. The MACT
floor is the minimum control level allowed for NESHAP. In essence,
the MACT floor ensures that all major HAP emission sources achieve
the level of control already achieved by the better-controlled
and lower-emitting sources in each category.
For new sources, the MACT floor cannot be less stringent than
the emission control that is achieved in practice by the best-controlled
similar source. The standards for existing sources can be less
stringent than standards for new sources, but they cannot be less
stringent than the average emission limitation achieved by the
best-performing 12 percent of existing sources (or the best-performing
5 sources for categories or subcategories with fewer than 30 sources).
In developing MACT, control options are considered that are more
stringent than the floor. Standards may be established more stringent
than the floor based on the consideration of cost, non-air-quality
health and environmental impacts, and energy requirements.
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Updated 8-30-2007
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