The Clean Air Act and its 1990 amendments appear to have contributed to a marked improvement in air quality nationwide. Of nearly 100 metropolitan areas not meeting air quality standards for ozone in 1990, more than two-thirds now do so. Even greater progress has been achieved with carbon monoxide: 36 of 42 areas not in attainment in 1990 now meet the standard. Nevertheless, EPA remains concerned about air pollution. In 1997, the Agency promulgated major revisions to its air quality standards for ozone and particulates, an action that would require most states and urban areas to establish additional controls on a wide range of pollution sources. The revised standards were challenged by numerous parties and the courts have remanded the standards to EPA. Implementation is currently in limbo, pending resolution of appeals by the Supreme Court.