Sources And Types Of Air Pollution

The sources of air pollution may be manmade, such as the internal combustion engine, or natural, such as plants (pollens). The pollutants may be in the form of particulates, aerosols, and gases or microorganisms. Included are pesticides, odors, and radioactive particles carried in the air.

Particulates range from less than 0.01 to 1000 ^m* in size; generally they are smaller than 50 ^m. Smoke is generally less than 0.1 ^m size soot or carbon particles. Those below 10 ^m can penetrate the lower respiratory tract; particles less than 3 ^m reach the tissues in the deep parts of the lung. Particles over 10 ^m are removed by the hairs at front of nose. Included are dust and inorganic, organic, fibrous, and nonfibrous particles. Aerosols are usually particles 50 ^m to less than 0.01 ^m in size; although generally they are less than 1 ^m in diameter. Gases include organic gases such as hydrocarbons, aldehydes, and ketones and inorganic gases (oxides of nitrogen and sulfur, carbon monoxide, hydrogen sulfide, ammonia, and chlorine).

Manmade Sources

Air pollution in the United States is the result of industrialization and mechanization. The major sources and pollutants are shown in Table 4.3. It can be seen that carbon monoxide is the principal pollutant by weight and that the motor vehicle is the major contributor, followed by industrial processes and stationary fuel combustion. However, in terms of hazard, it is not the tons of pollutant that is important but the toxicity or harm that can be done by the particular pollutant released. Lead has shown the most dramatic reduction, due to the use of nonleaded gasoline.

Agricultural spraying of pesticides, orchard-heating devices, exhaust from various commercial processes, rubber from tires, mists from spray-type cooling towers, and the use of cleaning solvents and household chemicals add to the pollution load. Toxic pollutant emissions and their fate in the environment need further study.

Particulates, gases, and vapors that find their way into the air without being vented through a stack are referred to as fugitive emissions. They include uncontrolled releases from industrial processes, street dust, and dust from construction and farm cultivation. These need to be controlled at the source on an individual basis.

Wood stoves contribute significantly to air pollution. This type of pollution is a potential health threat to children with asthma and elderly people with chronic lung problems. Wood stove use may have to be limited. Stoves are being redesigned to keep the air pollution at acceptable levels.

Natural Sources

Discussions of air pollution frequently overlook the natural sources. These include dust, plant and tree pollens, arboreal emissions, bacteria and spores, gases and dusts from forest and grass fires, ocean sprays and fog, esters and terpenes from

* A micron (^m) is 1/1000 of a millimeter, or 1/25,000 of an inch. Particles of 10 ^m and larger in size can be seen with the naked eye.

TABLE 4.3 Air Pollution According to Source and Type of Pollutant: United States, Selected Years 1970-1998


All Sources

On-Road Transportation

Nonroad Engines and Vehicles


Fuel Combustion

Industrial Process

Waste Disposal and Recycling


Carbon Monoxide (Millions of Short Tons)

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