Sources of Toxic Air Pollution

Emissions Inventory: As part of the Cumulative Exposure Project (CEP), 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.

Mobile Sources: On and off road vehicles and aircraft.

Area Sources: Burn barrels, gasoline filling stations, woodstoves, paint stripper, surface coatings, drycleaners, industrial boilers, etc. (small stationary sources)

Point 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.

Pie Charts of 1996 Air Toxic Pollutant Emissions by Source Category
(as estimated by NATA)

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.

Specific sources for the top 10 air toxic pollutants in Vermont are listed below: