POLLUTION LOCATOR|Definitions of Air Pollution Source Categories

Sources of air pollution like factories or cars are usually categorized by regulatory agencies into one of three groups: area, mobile, or point. Categorization of a specific source may vary depending on whether it is releasing "criteria" or "hazardous" air pollutants. Criteria pollutants refer to six chemicals which occur frequently in ambient air and can injure human health, harm the environment or cause property damage: carbon monoxide, lead, nitrogen dioxide, ozone, particulate matter, and sulfur dioxide. Hazardous air pollutants (HAPs) refer to other chemicals which can cause adverse effects to human health or the environment. Congress has identifed over 188 of these pollutants, including substances that cause cancer, neurological, respiratory, and reproductive effects.

Area sources include small pollution sources like dry cleaners, gas stations, and auto body paint shops. Area sources are defined as sources that emit less than 10 tons per year of a criteria or hazardous air pollutant or less than 25 tons per year of a combination of pollutants. The category also includes commercial buildings (heating and cooling units; surface coatings), residential buildings (fire places; surface coatings), fuel combustion in non-road machinery, boats, railroads, and even the family lawnmower or barbecue grill. Waste disposal in the form of open burning, landfills and wastewater treatment are significant area sources.

Though emissions from individual area sources are relatively small, collectively their emissions can be of concern - particularly where large numbers of sources are located in heavily populated areas. Area sources are responsible for over 50% of particulate matter emissions and more than point or mobile sources for volatile organic compound (VOC) emissions, which contribute significantly to the formation of ground-level ozone.

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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.

More on point sources.