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 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.”

E-cigarettes have been sold in the U.S. since 2007. Most devices heat a liquid nicotine solution into vapor and have been promoted to smokers as a less dangerous alternative since they don’t have all the chemicals, tar or smoke of regular cigarettes. E-cigarettes and similar vaping devices have grown into a $4 billion-dollar U.S. industry with thousands of varieties of flavors and customizable products available in specialty shops and online.

There are few long-term studies on their health impacts and no consensus on whether they are effective in helping smokers quit, according to the report requested by the Food and Drug Administration.

The FDA gained authority to regulate the devices in 2016 after years of pushback from the industry. But last year the agency said it would delay the deadline for manufacturers to submit their devices for review until 2022. The decision was blasted by anti-smoking advocates who say some e-cigarette manufacturers target kids with candy and fruit flavors.

The FDA has signaled its intention to begin pushing U.S. consumers away from traditional cigarettes toward alternative products, including e-cigarettes. The regulatory delay was intended, in part, to give companies more time to research their products.

After reviewing more than 800 studies, the panel reached a series of conclusions largely in line with prior assessments by other researchers. For instance, the panel found “conclusive evidence” that most e-cigarettes contain numerous chemicals that can be toxic.