Source: BTS.

Estimated Air Pollutants Emitted by Transportation in the United States, 1970-2002

The most important emissions are related to carbon monoxide, Volatile Organic Compounds (VOC) and nitrogen oxides, while transportation plays a marginal role for particulates and sulfur oxides emissions. Contributions patterns are fairly consistent among different developed countries, but differences exist. They may be attributed to modal choice, the industrial structure, commercial energy sources and general climatic conditions.