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. On the other hand, experts found only "limited evidence" that cigarettes are effective tools to help adult smokers quit.

The committee's review of more than 800 studies yielded many findings that were 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. However, there was equally strong evidence that e-cigarettes contain fewer toxicants and at lower levels than regular cigarettes.

E-cigarettes have been sold in the U.S. since at least 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.

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, such as e-cigarettes. The regulatory delay was intended, in part, to give companies more time to research their products.

FDA Commissioner Scott Gottlieb called the link between e-cigarette use and trying smoking in young people "troubling."

"We need to put novel products like e-cigarettes through an appropriate series of regulatory gates to fully evaluate their risks and maximize their potential benefits," he said in a statement.

Some other key takeaways and questions from the report:

— Chemicals in e-cigarette vapor, such as formaldehyde, are capable of damaging DNA in humans. However, it's unclear if the chemicals exist at levels high enough to cause cancer.

— Switching completely from traditional cigarettes to e-cigarettes significantly reduces exposure to numerous cancer-causing chemicals.

— E-cigarettes can sometimes explode causing burns and injuries. The risk of such accidents is higher with devices that are stored improperly or contain low-quality batteries.

— There is substantial evidence that e-cigarette vapor contains traces of metal, possibly due to the metallic coils used to heat liquid that the devices vaporize.