United States vehicle emission standards

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United States vehicle emission standards are set through a combination of legislative mandates enacted by Congress through Clean Air Act (CAA) amendments from 1970 onwards, and executive regulations managed nationally by the Environmental Protection Agency (EPA), and more recently along with the National Highway Traffic Safety Administration (NHTSA). These standards cover tailpipe pollution, including carbon monoxide, nitrogen oxides, and particulate emissions, and newer versions have incorporated fuel economy standards. However they lag behind European emission standards, which limit air pollution from brakes and tires.

In nearly all cases, these agencies set standards that are expected to be met on a fleet-wide basis from automobile and other vehicle manufacturers, with states delegated to enforce those standards but not allowed to set stricter requirements. California has generally been the exception, having been granted a waiver and given allowance to set stricter standards as it had established its own via the California Air Resources Board prior to the 1970 CAA amendments. Several other states have since also received waivers to follow California's standards, which have also become a de facto standard for vehicle manufacturers to follow.

Vehicle emission standards have generally been points of debate between the government, vehicle manufacturers, and environmental groups, and has become a point of political debate.