Reuters: Dramatic improvements in U.S. air quality over the last two decades have added 21 weeks to the life of the average American, researchers reported on Wednesday. Reducing fine particles given off by automobiles, diesel engines, steel mills and coal-fired power plants have added as much as 15 percent of the 2.72 years of extra longevity seen in the United States since the early 1980s, they wrote in the New England Journal of Medicine. Changes in smoking habits are the biggest …