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Motor vehicle emissions have been determined to constitute a public health hazard, especially in the U.S. metropolitan areas. Under amendments to the Clean Air Act, the U.S. Congress has directed that national ambient air health standards must be met by specified dates. All reasonable control measures including vehicular emission inspection programs should be implemented in those areas not able to meet minimum public health ambient air levels. This potentially affects metropolitan areas in 40 states. This paper describes the implementation of an emission inspection program in the Portland, Oregon, metropolitan area, the effects of that program on the emissions from the area's motor vehicles, and the effect on ambient air quality.