WASHINGTON (AP) — Electronic cigarettes could be a boon to public health or a major liability, depending on whether they help Americans quit smoking or encourage more young people to try traditional cigarettes, a new report concludes.The report issued Tuesday wrestles with the potential benefits and harms of the vapor-emitting devices which have been sold in the U.S. for more than a decade. But those effects may not be known for decades, in part, because of how slowly illnesses caused by smoking emerge."In some circumstances, such as their use by non-smoking adolescents and young adults, their adverse effects clearly warrant concern," said David Eaton, of the University of Washington, who headed the National Academies of Sciences, Engineering and Medicine committee that studied the issue. "In other cases, such as when adult smokers use them to quit smoking, they offer an opportunity to reduce smoking-related illness."There are no long-term studies on the health consequences of e-cigarettes and little consensus on whether they are effective in helping smokers quit, according to the report requested by the Food and Drug Administration.The experts found "substantial" evidence that young people who use e-cigarettes are more likely to try cigarettes.