Nearly 2 million U.S. adults who have never consistently smoked traditional cigarettes use e-cigarettes, according to results from a national survey. Of these sole e-cig users, about 60 percent are young adults, aged 18 to 24, researchers report [...] E-cigarette companies have marketed the devices — which heat and vaporize liquids that typically contain nicotine — as a way to help adults quit smoking. But some public health officials worry that e-cigarettes could become a means to nicotine addiction, rather than an end.