Kids who vape and use other forms of e-cigarettes are likely to try more harmful tobacco products like regular cigarettes, but e-cigarettes do hold some promise for helping adults quit.

That's according to the National Academies of Science, Engineering and Medicine, which published a comprehensive public health review of more than 800 studies on e-cigarettes on Tuesday.

"There is conclusive evidence that most products emit a variety of potentially toxic substances. However the number and intensity is highly variable," says David Eaton, who heads the committee that wrote the report. He is also the dean and vice provost of the graduate school of the University of Washington, Seattle.

"In some circumstances, such as their use by nonsmoking adolescents and young adults, their adverse effects clearly warrant concern. In other cases, such as when adult smokers use them to quit smoking, they offer an opportunity to reduce smoking-related illness."

In fact, 15 of the studies NAS reviewed found that when teens and young adults use e-cigarettes, they are more likely to try regular tobacco within a year.

"We found that kids who tried e-cigarettes, hookah, or smokeless tobacco or cigars — any non-cigarette tobacco product — were all twice as likely to try cigarettes a year later, compared to kids who hadn't used any of those other tobacco products," says Shannon Lea Watkins, a public policy researcher at University of California, San Francisco. Watkins and her colleagues also found that the effects of using non-cigarette products compound: "Kids using two or more non-cigarette products were four times as likely to report using cigarettes a year later."

However, it is not yet known how much tobacco they use on a regular basis and whether it became a habit. More long-term studies are needed.

The NAS report also indicates that there is some evidence that e-cigarettes may help adults already smoking regular cigarettes quit, but only if they switch exclusively to e-cigarettes.

Eaton emphasized that there is insufficient evidence on whether e-cigarettes work as well as either no treatment or FDA-approved smoking-cessation treatments, such as nicotine patches or gum, to get smokers to quit.

E-cigarettes are still fairly new and are largely unregulated. The were introduced only in 2003 as an alternative to tobacco-containing cigarettes, which is why the government requested that the analysis be conducted.

The report found that most e-cigarettes contain nicotine, even if they are marketed as flavored like bubble gum and potpourri.

The report also found that emissions from e-cigarettes do contain some harmful byproducts like metal, but far less than conventional cigarettes.

"Conventional cigarettes burn, and in the process of burning tobacco, a large number of fairly toxic chemicals are generated in the tobacco smoke," Eaton says. So there are more adverse health effects related to these combustible products.

However, a recent national survey suggests adults do not believe that e-cigarette vapor is toxic.

Food and Drug Administration Commissioner Scott Gottlieb says the report "helps identify areas that need further study to better understand the net public health impact of e-cigarettes ... . 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."

FDA commissioned the report at the direction of Congress to evaluate evidence of health effects of e-cigarettes after it delayed proposed regulations last year.

Matthew Myers, president of the Campaign for Tobacco-Free Kids, issued a statement expressing disappointment that the FDA has not acted quickly enough to regulate these products. "This report shows what happens when a new product is introduced without meaningful government oversight. It demonstrates why the FDA should fully and aggressively implement the overdue e-cigarette regulations that took effect in August 2016," it says.

The national academies report calls for more studies.

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