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Acetaldehyde is mainly used as an intermediate in the synthesis of other chemicals. Acetaldehyde is formed as a product of incomplete wood combustion in fireplaces and woodstoves, forest and wildfires, pulp and paper production, stationary internal combustion engines and turbines, vehicle exhaust fumes, and wastewater processing.

Acrolein is primarily used as an intermediate in the manufacture of acrylic acid. It can be formed from the breakdown of certain pollutants in outdoor air or from forest and wildfires, as well as vehicle exhaust.

Acrylonitrile is primarily used in the manufacture of acrylic and modacrylic fibers. It is also used as a raw material in the manufacture of plastics. Acrylonitrile may be released to the ambient air during its manufacture and use, from landfills, and through incineration of sewage sludge.

Arsenic, a naturally occurring element, is found throughout the environment; for most people, food is the major source of exposure. The air emissions are predominantly a result of the burning of coal or fuel oil, from metal smelters, iron foundries, and burning of wastes.

Benzene is found in the air from emissions from oil and natural gas production, petroleum refining, burning coal and oil, gasoline service stations, pulp and paper production, coke ovens, and motor vehicle exhaust. Benzene is used as a constituent in motor fuels; as a solvent for fats, waxes, resins, oils, inks, paints, plastics, and rubber; in the extraction of oils from seeds and nuts; and in photogravure printing. It is also used as a chemical intermediate. Benzene is also used in the manufacture of detergents, explosives, pharmaceuticals, and dyestuffs.

Beryllium emissions are predominantly a result of the burning of coal or fuel oil.

1,3-Butadiene is found in ambient air from motor vehicle exhaust as well as manufacturing and processing facilities, gasoline distribution, production of synthetic plastics and rubber, wastewater processing, forest and wildfires, or other combustion

Cadmium emissions are mainly from the burning of fossil fuels such as coal or oil, and the incineration of municipal waste. Cadmium may also be emitted into the air from zinc, lead, or copper smelters. For nonsmokers, food is generally the largest source of cadmium exposure. Cadmium levels in some foods can be increased by the application of phosphate fertilizers or sewage sludge to farm fields.

Carbon tetrachloride was produced in large quantities to make refrigerants and propellants for aerosol cans, as a solvent for oils, fats, lacquers, varnishes, rubber waxes, and resins, and as a grain fumigant and a dry cleaning agent. Consumer and fumigant uses have been discontinued and only industrial uses remain. Individuals may be exposed to carbon tetrachloride in the air from accidental releases from production and uses, its disposal in landfills, and wastewater processing.

Chloroform may be released to the air from a large number of sources related to its manufacture and use, as well as its formation in the chlorination of drinking water, wastewater, and swimming pools. Pulp and paper mills, hazardous waste sites, and sanitary landfills are also sources of air emissions. Chloroform was used in the past as an extraction solvent for fats, oils, greases, and other products; as a dry cleaning spot remover; in fire extinguishers; as a fumigant; and as an anesthetic. However, chloroform is no longer used in these products.

Chromium sources of emissions include the combustion of coal and oil, electroplating, vehicles, iron and steel plants, and metal smelters. The emissions reflected in this assessment are based on state and local agency reporting of chromium as "chromium and compounds," individual chromium compounds and chromium ions. Because of the inconsistent reporting, all of the chromium was lumped together and modeled as "chromium compounds." In assessing the risk, the Agency conservatively assumed that 34 percent of the chromium is hexavalent (which is the most toxic form).

Coke oven emissions are a mixture of coal tar, coal tar pitch, and creosote and contain chemicals such as benzo(a)pyrene, benzanthracene, chrysene, and phenanthrene. Coke oven emissions may occur from coke ovens and facilities associated with the manufacture of aluminum, steel, and graphite as well as electrical and construction industries.

1,3-Dichloropropene is used as a component in formulations for soil fumigants. Emissions are associated with its manufacture or application as a soil fumigant.

Diesel Particulate Matter (PM) is a mixture of particles and gases that is a component of diesel exhaust. Diesel exhaust is listed as a mobile source air toxic due to the cancer and noncancer health effects associated with exposure to whole diesel exhaust. EPA believes that exposure to whole diesel exhaust is best described, as many researchers have done over the years, by diesel particulate concentrations.

Dioxins/Furans are the common name for polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans. Examples of industries that emit dioxins/furans are hazardous and medical waste incinerators, cement production, and pulp/paper production. Because the states used different methodologies for reporting emissions information and the fact that various polychlorinated dioxin and furan compounds are all believed to cause adverse health effects by the same mechanism, EPA combined dioxins/furans for modeling purposes into "toxic equivalents" (TEQ) by normalizing each dioxin and furan species to an equivalent amount of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), then aggregating them. In this way, the dioxin/furan TEQ emissions, exposure, and risk estimates in this assessment include all polychlorinated dioxin and furan compounds for which the National Toxics Inventory contains emissions data.

Ethylene dichloride is primarily used in the production of vinyl chloride as well as other chemicals. It is used in solvents in closed systems for various extraction and cleaning purposes in organic synthesis.

Ethylene dibromide was used in the past as an additive to leaded gasoline and as a fumigant. Ethylene dibromide is currently used in the treatment of felled logs for bark beetles and termites, and control of wax moths in beehives. Ethylene dibromide is also used as an intermediate for dyes, resins, waxes, and gums.

Ethylene oxide is used mainly as a chemical intermediate in the manufacture of textiles, detergents, polyurethane foam, antifreeze, solvents, medicinals, adhesives, and other products. The major sources of emissions are commercial and hospital sterilizers.

Formaldehyde is used mainly to produce resins used in particleboard products and as an intermediate in the synthesis of other chemicals. The major sources of emissions to the air are forest and wildfires, stationary internal combustion engines and turbines, pulp and paper plants, petroleum refineries, power plants, manufacturing facilities, incinerators, and automobile exhaust emissions.

Hexachlorobenzene is formed as a byproduct during the manufacture of other chemicals (mainly solvents) and pesticides.  It was widely used as a pesticide until 1965.  There are currently no commercial uses of hexachlorobenzene in the United States.

Hydrazine is used in agricultural chemicals (pesticides), chemical blowing agents, pharmaceutical intermediates, photography chemicals, boiler water treatment for corrosion protection, textile dyes, and as fuel for rockets and spacecraft.

Lead is used in the manufacture of batteries. The largest source of lead in the atmosphere has been from leaded gasoline combustion, but with the phase down of lead in gasoline, air lead levels have decreased considerably. Other sources of air emissions include combustion of solid waste, coal, and oils, emissions from iron and steel production and lead smelters. Exposure to lead can also occur from food and soil. Children are at particular risk to lead exposure since they commonly put hands, toys, and other items in their mouths, which may come in contact with lead-containing dust and dirt. Lead-based paints were commonly used for many years and flaking paint, paint chips, and weathered paint powder may be a major source of lead exposure, particularly for children.

Manganese is a naturally occurring substance found in many types of rock and soil; it is ubiquitous in the environment and found in low levels in water air, soil, and food. Manganese can also be released into the air by combustion of coal and oil, residential combustion of wood, iron and steel production plants, and power plants.

Mercury is predominantly emitted to the air by the combustion of fossil fuels (mostly coal) and waste. Included in the "combustion" category are medical waste incinerators, which burn medical waste and municipal waste combustors which burn municipal waste. Once mercury enters waters, either directly or through air deposition, it can "bioaccumulate" in fish and animal tissue in its most toxic form, methylmercury. Bioaccumulation means that the concentration of mercury in predators at the top of the food web (for example, predatory fish and fish-eating birds and mammals) can be thousands or even millions of times greater than the concentrations of mercury found in the water.

Methylene chloride is predominantly used as a solvent in paint strippers and removers; as a process solvent in the manufacture of drugs, pharmaceuticals, and film coatings; as a metal cleaning and finishing solvent in electronics manufacturing; and as an agent in urethane foam blowing. Other sources of emissions are landfills and wastewater processing.

Nickel is found in the outside air as a result of releases from utility oil and coal combustion, residential heating, nickel metal refining, lead smelting, sewage sludge incineration, manufacturing facilities, mobile sources, and other sources.

Perchloroethylene is widely used for dry-cleaning fabrics and metal degreasing operations.

Polychlorinated biphenyls (PCBs) are a group of chemicals that contain 209 individual compounds. PCBs are no longer produced or used in the United States today; the major source of exposure to PCBs today is the redistribution of PCBs already present in soil and water. PCBs were used in capacitors, transformers, plasticizers, surface coatings, inks, adhesives, pesticide extenders, carbonless duplicating paper. Smaller amounts of PCBs may be released to the air from disposal sites and combustion. PCBs have been detected in food; they bioaccumulate through the food chain, with some of the highest concentrations found in fish.

Polycyclic organic matter (POM) defines a broad class of compounds that includes the polycyclic aromatic hydrocarbon compounds (PAHs). POM compounds are formed primarily from combustion and are present in the atmosphere in particulate form. Sources of air emissions are diverse and include, vehicle exhausts, forest and wildfires, asphalt roads, coal, coal tar, coke ovens, agricultural burning, residential wood burning, and hazardous waste sites. Because of limited emissions data, for this assessment Polycyclic Organic Matter (POM) data have been limited to either the group of 7 or group of 16 individual PAH species referred to as 7-PAH and 16-PAH, respectively. In this assessment POM refers to 16-PAH. The 16-PAH group includes the 7-PAH group.

Propylene dichloride is used as a chemical intermediate in the production of chlorinated organic chemicals, as an industrial solvent, in ion exchange manufacture, in toluene diisocyanate production, in photographic film manufacture, for paper coating, and for petroleum catalyst regeneration. Propylene dichloride is also emitted from landfills.

7-PAH (Polycyclic Aromatic Hydrocarbons) is a subset of the 16-PAH group of compounds. In this assessment the 16-PAH compounds are referred to as Polycyclic Organic Matter (POM). The species that make up 7-PAH are probable human carcinogens, and the 16-PAH are those species that are measured by an EPA test method (EPA Method 610).

Quinoline is used mainly as an intermediate in the manufacture of other products. Quinoline is also used as a catalyst, a corrosion inhibitor, in metallurgical processes, in the manufacture of dyes, as a preservative for anatomical specimens, in polymers and agricultural chemicals, and as a solvent for resins and terpenes. It is also used as an antimalarial medicine. A potential source of very low exposure to quinoline includes the inhalation of ambient air contaminated by emissions from petroleum refining, quenching and coking, and wastewater processing.

1,1,2,2-tetrachloroethane is no longer used much in the United States, current air emissions predominantly result from its use as a chemical intermediate during the manufacture of other chemicals.  One of the sources of emissions is landfills.

Trichloroethylene used in the United States is mainly associated with industrial degreasing operations and is also emitted from landfills. 

Vinyl chloride is used to make polyvinyl chloride (PVC) plastic and vinyl products. Sources of emissions include the discharge of exhaust gases from factories that manufacture or process vinyl chloride, landfills, or evaporation from areas where chemical wastes are stored.

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