Mobile sources include both onroad vehicles (such as cars, trucks and buses) and offroad equipment (such as ships, airplanes, agricultural and construction equipment).
Mobile sources contribute significantly to air pollution. Driving a car is probably a person's single most polluting daily activity. Nationwide, mobile sources are responsible for about 75% of carbon monoxide pollution, and more oxides of nitrogen emissions than area or point sources. In urban areas, the motor vehicle contribution to carbon monoxide pollution can exceed 90 percent. In a typical urban area, at least half of the hydrocarbon and nitrogen oxide pollutants come from mobile sources. Motor vehicles are also substantial sources of hazardous air pollutants, such as the recognized carcinogens benzene, formaldehyde, acetaldehyde, 1,3-butadiene and diesel particulate matter.
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Point sources include major industrial facilities like chemical plants, steel mills, oil refineries, power plants, and hazardous waste incinerators. Point sources are defined as those that emit 10 tons per year of any of the criteria pollutants or hazardous air pollutants or 25 tons per year of a mixture of air toxics.
Nationwide, point sources like power plants, petroleum refineries, fertilizer manufacturers, industrial paper mills, copper smelters and iron and steel mills contribute the majority of sulfur dioxide emissions, accounting for nearly 90% of this criteria air pollutant. Point sources (predominantly electrical utilities and industrial boilers) are also major emitters of nitrogen oxides (NOx) - accounting for about 40% of total releases. Point sources are less important sources of VOCs - releasing less than 15% of total volatile organic compounds.
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