The reduction of NOx and particulate matter (PM) has been central to the U.S. Environmental Protection Agency's (EPA) strategy for cleaner air. Since the passing of the Clean Air Act in the 1970s, the EPA has made steps to lower emissions of NOx, PM and other pollutants from road vehicles, electric utilities and off-road equipment, and periodically reviews national air quality standards.
The EPA has a specific responsibility under the Clean Air Act to manage and enforce emissions limits at a federal level. It sets the limits for emissions standards for diesel engines and takes actions against manufacturers who do not meet the standards. It also takes an active interest in the technology choices that are made by the manufacturers (although it cannot express a preference) and assesses their current progress.