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10 Lawrence K. Wang et al.

the US EPA has submitted annual reports to Congress. It has been noted in these reports
that sulfur dioxide emissions have been cut significantly, but the reduction in automo-
tive emissions has been offset by increasing motor vehicle use and fuel consumption,
so that total nationwide reductions are not as high as they are per vehicle mile. The 2003
US EPA air emission standards can be found on the agency’s website (www.epa.gov).
4. SOURCES

As previously noted, there is often much difficulty and little agreement in how to
accurately classify the various emissions. The US EPA, in an extensive attempt, classi-
fied the estimated emissions in the United States in a 433-page document (7). In
response to public demand, the US EPA summarized air pollutant emissions in the
United States in 1998. These emissions are listed in seven categories in Table 2, which
also includes data on natural and miscellaneous sources: forest fires, agricultural
burning, structural fires, and coal refuse fires. The values in parentheses represent the
percentage of total pollutants emitted.
Much of the data in Table 2 comes from such sources as State Emission Inventories.
However, it is sometimes necessary to estimate emissions by using “emission factors,”
which are published values of expected emissions from a particular source and are
usually expressed as quantity of pollutant per unit weight of raw material consumed or
product produced. The most complete listing of emission factors is found in the US EPA
publication (8), which is periodically updated. The 2003 update can be found on the US
EPA website.
As shown in Table 2, highway and off-highway transportation account for most of the
total pollutants emitted. Fuel combustion emissions from electrical utilities, industries,
and other categories are other major sources of air pollution emissions.
Fossil fuels, especially coal, contain sulfur. When burned, most of the sulfur is con-
verted to SO . Most of the SO pollution (77%) comes from fuel combustion sources.
2 2
Eastern coal has a high sulfur content, compared to coal from the West, with values as
high as 6%. The weighted average is in the 2.5–3.5% sulfur range. The content of west-
ern coal is lower in sulfur, with a weighted average of about 0.5–1.0%. However, the
heating value of this coal is lower, and so a direct comparison should not be made
between the two types of coal based only on sulfur content. It is estimated that 87% of the
coal is used from the eastern reserves. To reduce sulfur emission, a greater percentage
should come from the western coal reserve. As recycling and conservation increase, pol-
lution from the waste disposal and recycling category should also decrease. Much of
this material could be used to produce energy and thus reduce the use of high-pollution
fuels.
There are over 20,000 major stationary sources of air pollution in the United States.
They include mainly power plants, industries, and incinerators. Over 80% of these sta-
tionary sources have been either in compliance with US EPA standards or are meeting
an abatement schedule.

5. EFFECTS
One of the requirements of Document PL 91-601 was that the US EPA publish crite-
ria documents related to the effects of various air pollutants. A number of these docu-