Using a nationwide network of monitoring sites, EPA has developed ambient air quality trends for particle pollution, also called Particulate Matter (PM). PM_2.5 describes fine inhalable particles, with diameters that are generally 2.5 micrometers and smaller.  Under the Clean Air Act, EPA sets and reviews national air quality standards for PM. Air quality monitors measure concentrations of PM throughout the country. EPA, state, tribal and local agencies use that data to ensure that PM in the air is at levels that protect public health and the environment. Nationally, average PM_2.5 concentrations have decreased over the years. For information on PM standards, sources, health effects, and programs to reduce PM, please see our Particulate Matter Pollution page.