The Pollutants
Hazardous air pollutants, also known as toxic air pollutants or
air toxics, are those pollutants that cause or may cause cancer
or other serious health effects, such as reproductive effects or
birth defects, or adverse environmental and ecological effects.
EPA is required to control 188
hazardous air pollutants. Examples of toxic air pollutants include
benzene, which is found in gasoline; perchlorethlyene, which is
emitted from some dry cleaning facilities; and methylene chloride,
which is used as a solvent and paint stripper by a number of industries.
Through appropriate rulemaking, the Clean Air Act list can be modified.
A current list of modifications
is available. Some clarification on certain pollutant
aggregation is also available.
The Sources
Most air toxics originate from human-made sources,
including mobile
sources (e.g., cars, trucks, buses) and stationary sources (e.g.,
factories, refineries, power plants), as well as indoor
sources (e.g., building materials and activities such as cleaning).
There are two types of stationary sources that generate routine
emissions of air toxics:
- "Major" sources are defined as sources that emit
10 tons per year of any of the listed toxic air pollutants,
or 25 tons per year of a mixture of air toxics. These sources
may release air toxics from equipment leaks, when materials
are transferred from one location to another, or during discharge
through emission stacks or vents
- "Area" sources consist of smaller-size facilities
that release lesser quantities of toxic pollutants into the
air. Area sources are defined as sources that emit less than
10 tons per year of a single air toxic, or less than 25 tons
per year of a combination of air toxics. Though emissions from
individual area sources are often relatively small, collectively
their emissions can be of concern - particularly where large
numbers of sources are located in heavily populated areas.
EPA published the initial list of "source categories"
in 1992 (57FR31576 , July 16, 1992) and since that time has issued
several revisions and
updates to the list and promulgation schedule. For each listed
source category, EPA indicates whether the sources are considered
to be "major" sources or "area" sources. The
1990 Clean Air Act Amendments direct EPA to set standards for
all major sources of air toxics (and some area sources that are
of particular concern).
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