A new report by the National Academy of Sciences estimates that burning fossil fuels costs the United States about $120 billion a year in hidden costs. The study estimated that nearly 20,000 people die prematurely each year from air pollutants emitted by power plants and vehicles. The study found that coal burning was the biggest single source of such external costs. Environmental groups said the actual hidden cost of burning fossil fuels is even higher, because the study did not include expenses related to global warming. The National Mining Association criticized the report for ignoring what it described as the hidden benefits of coal-based generation.