The number of adults in the United States who smoke has hit an all-time low, according to a new report from the Centers for Disease Control and Prevention. While 13.9 percent of the adult population — roughly 30 million people — smoke, that’s well below the 41.9 percent of adults who smoked in 1965, when tracking began. Once common in offices, restaurants, ballparks and elsewhere, smoking has been banned from most public places and become taboo inside many private homes, too. Many believe that broader awareness of the proven links between smoking and an array of health problems — cancer, lung disease, heart attacks, thickened blood, nicotine addiction and a shorter life span among them — has prompted people to quit, as have antismoking advertising campaigns and higher taxes on cigarettes. The CDC found that those who still smoke are more likely to live in smaller towns or rural settings than in metropolitan areas with a million or more residents. Today, 21.5 percent of non-metro residents smoke, versus 11.4 percent of urban dwellers. Each year, the deaths of more than 450,000 people nationwide — about one in five deaths — are linked to smoking, which the CDC describes as the leading cause of preventable disease and death in the United States.