People in some parts of the world, notably Asia, may still be picking up smoking, and puffing away at home; Americans, not so much. Almost twice as many U.S. households are smoke-free compared with two decades ago, a new federal study shows. The study suggests the decline reflects increased awareness of the dangers of secondhand smoke, although it notes that still too many people remain exposed. The study, released by the Centers for Disease Control and Prevention, shows that 83 percent of American homes were smoke-free in 2010-2011, versus just 43 percent in the early 1990s. “It’s a shift in social norms,” said Brian King, lead author of the study, adding that people no longer think it is acceptable to smoke around non-smokers. The CDC says some 18 percent of Americans were smokers in 2012, down from 42 percent in 1965.