A new study released on Thursday blamed the dirtiest U.S. power plants for pumping 2 billion tons of pollution into the air, causing smog, global warming and scores of health and environmental problems. The report, from U.S. Public Interest Research Group (PIRG) and Clear the Air, says it documented reports from 594 of the nation's most polluting power plants, ranking the dirtiest facilities, states and companies for four pollutants.