The United States has made progress in reducing dangerous air pollution since 1990 but work remains to reduce risks for the country's most overburdened urban areas, the U.S. Environmental Protection Agency's top official said. The EPA released to Congress its second report on integrated air toxics, citing "substantial progress" toward reducing levels of contaminants such as arsenic, mercury and lead since it launched an Integrated Urban Air Toxics Strategy in 1999. Air toxics, also known as toxic air pollutants or hazardous air pollutants, are pollutants that may increase the risk of cancer or other serious health effects, such as birth defects.