Sources of Toxic Air Pollution
Inventory: As part of the Cumulative Exposure
the U.S. EPA prepared a comprehensive list of air toxics emissions
for the entire country in 1990. This is the first time that such
a list (known as an emissions inventory) had ever been prepared
on a national scale. Although there are bound to be some errors
in the details of a massive undertaking such as this, a summary
of the emissions inventory can give us some indication of what may
be the most important sources of air toxic emissions in Vermont.
Sources: On and off road vehicles and aircraft.
Sources: Burn barrels, gasoline filling stations, woodstoves,
paint stripper, surface coatings, drycleaners, industrial boilers,
etc. (small stationary sources)
Manufacturing operations (large stationary sources).
In the late 1990s, the U.S. EPA undertook
the National-scale Air Toxics Assessment (NATA). This assessment,
based on the 1996 national emissions inventory, yielded results useful
in understanding the amounts and possible health effects of 33 toxic air
pollutants plus diesel particulate matter. (For more information regarding
this list of federal hazardous air pollutants (HAPs), please click here).
The analysis consisted of four primary steps. These include: (1) an emissions
inventory which estimates the release of the 33 HAPs nationwide from mobile,
area, and point (or major) sources (2) modeling estimates of the concentrations
of these pollutants in the air (3) estimates of the exposure of populations
to the HAPs and (4) the resultant risk of both cancer and noncancer health
effects due to exposure to these toxic pollutants.
Similar to the 1990 CEP inventory results,
the 1996 NATA inventory indicates that approximately 53% of air toxics
still come from mobile sources while 47% of toxic air emissions come from
area sources. The CEP categorized sources as point, area and mobile. Due
to the 1990 Clean Air Act Amendments (CAAA), the definition of the source
categories was revised to include major, area and mobile sources. Vermont
has very few large stationary sources that are classified as "major"
under the CAAA. Therefore, our small industrial sources are classified
as "area" sources and hence the difference between the 1990
CEP pie chart above and the 1996 NATA pie chart below.
Eight of the top 10 air toxic pollutants
in Vermont, excluding methyl chloride (chloromethane) and styrene, are
considered in the National Air Toxics Assessment. NATA divides emission
sources into four categories. They are
major sources (large point sources),
area and other (small point sources), onroad mobile sources and nonroad
mobile sources. For simplicity's sake, onroad mobile and nonroad mobile
were combined in the following pie charts. These pie charts depict the
estimated statewide Vermont emissions in 1996 by source category.
of 1996 Air Toxic Pollutant Emissions by Source Category
Unlike many larger states, a great majority
of the toxic air pollutants emitted in Vermont come from mobile sources
and smaller area sources, as the pie charts indicate. In fact, according
to the federal definition of
major source, Vermont has very few major sources of HAPs. Based
on the 1996 NATA results, the operation of mobile sources is responsible
for a majority of the benzene (85%) and 1,3-Butadiene (67%) emissions
in the state as well as approximately 49% of the primary formaldehyde
emissions. The remainder of these three pollutants is emitted by area
and other sources. In 1996, sources within the area and other category
emitted 100% of the carbon tetrachloride, chloroform, methylene chloride
and tetrachloroethylene and nearly 90% of the mercury compounds.
for the top 10 air toxic pollutants in Vermont are listed below:
||Benzene in the atmosphere comes
from numerous sources. Motor vehicles are considered a significant
source of benzene. Benzene is present in both evaporative emissions
and in exhaust emissions from motor vehicles. Refueling of motor
vehicles is another source of benzene. Benzene is also released
by industries in the state and is a component of fossil fuel
emissions, including wood stoves. Benzene in Vermont appears
to be locally generated as the highest concentrations are observed
in urban areas and the concentrations decrease as the sites
become more rural. Benzene has an atmospheric half life of 10-12
||The primary source of 1,3-butadiene
is motor vehicles. 1,3-Butadiene is formed in vehicle exhaust
due to incomplete combustion of fuel. Other sources of 1,3-butadiene
are waste incinerators and wood fires. The 1,3-butadiene concentrations
observed appear to be locally generated as the concentrations
are highest in the urban areas of Burlington and Rutland with
levels decreasing as the sites become more rural. 1,3-Butadiene
has a short atmospheric half-life of 4-6 hours, which also indicates
that observed levels are locally generated, as opposed to transported
from outside the state.
|3. Carbon Tetrachloride
||Carbon tetrachloride in the
atmosphere in Vermont is probably due to transported pollution.
The compound was used extensively in degreasers, cleaning fluids
and fire extinguishers until it was withdrawn from the market
in the 1960s. Carbon tetrachloride is still used as a refrigerant
and an aerosol propellant but, these uses are being phased out.
Carbon tetrachloride appears to be a regional or transported
pollutant as the concentrations do not vary significantly from
urban sites to rural sites. The atmospheric half life of carbon
tetrachloride is 50-100 years, indicating that it will take
a long time for current concentrations to decrease.
||The amount of chloroform normally
expected to be present in air ranges from 0.02 to 0.05 ppb (0.1
to 0.2 µg/m 3 ). Chloroform has been found in the air from all
areas of the United States. Sources of chloroform in the atmosphere
are pulp and paper mills and water and wastewater plants that
use chlorine as a disinfectant. Chloroform appears to be a transported
pollutant. The concentrations do not vary significantly from
urban to rural sites and the compound has an atmospheric half
life of 2-3 months.
||Formaldehyde has numerous
atmospheric sources. The compound is a byproduct of combustion,
which covers a wide range of sources from internal combustion
engines to wood stoves. Formaldehyde is also generated by wood
processing plants and glues. Formaldehyde is produced by the
atmospheric reactions of other pollutants, including 1,3-butadiene.
Formaldehyde in Vermont follows the locally generated pattern.
Concentrations are highest in urban sites and decrease at rural
locations (except for Winooski 1995). The atmospheric half life
of formaldehyde is short, 4-10 hours.
|6. Methyl Chloride
||Outside air contains less
than 0.001 ppm (2 µg/m 3 ) and city air contains up to 0.001
ppm methyl chloride. These levels are much lower than the levels
shown to have toxic effects . Methyl chloride is naturally released
into the atmosphere from oceans and biomass, producing low ambient
concentrations. Other sources of methyl chloride are wood burning
and chlorinated swimming pools. The methyl chloride in the outdoor
environment, however, is almost totally from natural sources.
Methyl chloride appears to be a transported pollutant. The variation
in concentrations between urban and rural sites is not significant
and the atmospheric half life is fairly long: 1-2 years.
|7. Methylene Chloride
||Methylene chloride comes from
both transported and locally generated sources. Local sources
are regulated industrial sources and area sources. Area sources
include businesses where methylene chloride is used as a general
solvent such as garages and the compound is also found in spray
cans and furniture strippers. Methylene chloride has a fairly
long atmospheric half life, 3-4 months, indicating the fairly
long persistence typical of transported pollutants.
||Sources of tetrachloroethylene
are local area and point sources. Tetrachloroethylene is used
in many industries as a general solvent. The compound is used
extensively in dry cleaning and is also produced by waste incinerators.
Tetrachloroethylene concentrations are highest at urban sites
and decrease at rural sites, indicating that the compound is
locally generated. The atmospheric half life is 70-100 days.
||Sources of mercury include
both transported and local emissions. Waste incineration is
a local source of atmospheric mercury.
||Styrene sources include local
industrial point sources and area sources such as vehicle exhaust
and auto body shops. Styrene has a fairly short atmospheric
half life, 6-7 hours, indicating that it is probably a locally
generated, rather than a transported pollutant.
to Air Toxics Program Page
Last Updated: 1/22/03