FIGURE D-1 Historical trend in emission standards for light-duty vehicles. Individual emission standards for HC, CO, and, NOx are combined for illustration only. SOURCE: Data from Ehlmann and Wolff (2005).

FIGURE D-2 Historical trend in emission standards for heavy-duty diesel engines.


The emission standards currently in effect in the United States are promulgated by the U.S. Environmental Protection Agency (EPA) for the 49 states and by the California Air Resources Board (CARB) for the state of California. California is the only state that has been granted authority, within the original Clean Air Act, to establish separate, stricter standards for vehicle emissions, independent of the federal government. All other states are required to comply with the federal vehicle emission standards or adopt the stricter California standards. Numerous states have adopted the latest CARB emission standards.

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