Electronic cigarettes may be less risky than the regular kind, but that still doesn’t mean they’re safe, according to the most exhaustive review of the research yet.

The booming, $10-billion vaping industry is expected to grow to $34 billion by 2021, but there’s still a lot scientists don’t know about how e-cigarettes affect health. So Congress asked a panel of experts to wade through more than 800 scientific studies. The result, published today by the National Academies of Sciences, Engineering, and Medicine, reports that swapping e-cigarettes for the regular kind reduces some of the health risks associated with smoking. But overall, the report calls for more research into the health effects of vaping, especially since e-cigarettes are the favored tobacco product of middle and high school-aged students.

The researchers grouped their conclusions by the amount of evidence supporting them. In the best-supported category, the team reports conclusive evidence that completely switching to e-cigarettes can cut down on a smoker’s exposure to the toxic and cancer-causing chemicals found in regular cigarettes. But e-cigarettes also pose new and unusual risks, the review says: “e-cigarette devices can explode and cause burns and projectile injuries” and “intentionally or unintentionally drinking or injecting e-liquids can be fatal.”

In the second-best supported category, the team reports substantial evidence that the puff of vapor from an e-cig can contain metals and chemicals known to damage DNA, like formaldehyde. The nicotine in e-cigs can cause a short-term boost in heart rate, and smoking them can lead to dependence, the report says. Teens who vape are also more likely to try smoking regular cigarettes at least once and there’s moderate evidence that this could set teens up for heavier cigarette use later on.

When it came to kicking the smoking habit, there weren’t enough randomized controlled trials to say how e-cigarettes stacked up against, say, nicotine gum or just going cold turkey. But there were hints that people who used e-cigarettes more often were also more likely to quit smoking the regular kind of cigarettes.

There’s still a lot scientists don’t know about e-cigarettes. For instance, while the FDA generally considers the flavorings in e-liquids safe to eat, we don’t actually know if most of them are safe to inhale. It’s also not clear whether smoking e-cigarettes causes cancer, or how vaping during pregnancy affects the fetus. To figure out if e-cigarettes are a boon or a danger to public health, more high-quality research needs to be conducted without the tobacco industry’s influence. Until then, we won’t know which of the vast range of poorly-regulated e-cigarette products are the safest — or whom vaping is most likely to benefit, or harm.

“When it got down to answering the questions about what the impacts on health are, there is still a lot to be learned,” David Eaton at the University of Washington, who led the committee, told The New York Times. As it stands right now, he says, “E-cigarettes cannot be simply categorized as either beneficial or harmful.”