1. An air pollutant identified as potentially harmful, deserving monitoring and careful study. 2. An air pollutant that is controlled by government regulation. In the United States, there are two primary classes of designated pollutants regulated by the Environmental Protection Agency: criteria pollutants and toxic pollutants. For the 188 “toxic” air pollutants that are known or suspected to cause cancer or other serious health effects (such as birth or developmental defects), the regulations control emissions from sources such as industrial factories. See also air toxins.