The air we breathe is composed of lots of different chemical components. Some of these components are pollutants that are harmful to human health. Air pollutants come from a variety of different sources, including power plants, factories, agriculture, and transportation. Transportation vehicles are powered by combustion of fossil fuels, such as gasoline or diesel, to move from one place to another, and combustion of these fuels results in the release of gases and very small particles. These substances are known as "pollution emissions." Different types of transportation can emit different amounts of air pollutants, which have an impact on local and regional air quality. Both trucks and trains are used to transport freight in the United States. Understanding how freight trucks and trains impact air quality is useful to learn ways to reduce pollution that is harmful to human health.

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