A little more than 3% of American adults regularly use e-cigarettes, and 15% say they have tried them, according to 2016 data from the Centers for Disease Control and Prevention (CDC). That means millions of Americans vape, either consistently or sporadically — and there’s evidence that the number is rising.

As they become more prevalent, e-cigarettes — which are typically used as alternatives to traditional cigarettes, or as smoking cessation aids — are coming under increasing scrutiny from doctors and regulatory groups. Youth recreational use, in particular, has drawn the attention of lawmakers and public health experts, as trends like “Juuling” and “dripping” spread throughout schools across the country.

Meanwhile, research parsing the health effects of e-cigarettes continues to accumulate. Here’s what some of the latest studies say about the habit, and what’s still unknown.

The chemicals in e-cigs likely come with health risks

While e-cigarette aerosol “generally contains fewer harmful chemicals than smoke from burned tobacco products,” according to the CDC, it still may pose some risks. Formulas vary, but many e-cigarettes expose users to particles that can get into the lungs, potentially cancer-causing chemicals, heavy metals and other toxins, according to the CDC. A new study published in Scientific Reports also found that e-cigarette aerosol may contain more formaldehyde than previously estimated. Formaldehyde can be carcinogenic at high levels of exposure, according to the Environmental Protection Agency (EPA).

E-cigs come in a wide variety of flavors, from strawberry to creme brûlée, and the chemicals used to flavor them have come under scrutiny. A 2016 study linked diacetyl, a chemical often used in e-cig flavoring agents, to a serious respiratory condition nicknamed popcorn lung. The American Thoracic Society also recently released data from human cell research that connects another common flavoring agent called cinnamaldehyde, which is used in cinnamon flavors, to lung damage.

E-cigarettes may contribute to teen smoking

E-cigarettes cannot legally be sold to anyone under age 18, but that hasn’t stopped teenage use from skyrocketing over the years. A 2015 study found that teenagers who smoke e-cigarettes may be more likely than their peers to also use other tobacco products, perhaps because the habit can contribute to nicotine addiction.

Given that risk, the Food and Drug Administration (FDA) has made curtailing teenage e-cigarette use a priority, and has requested information from Juul and other e-cig manufacturers as part of that campaign. “There is no acceptable number of children using tobacco products,” said FDA Commissioner Scott Gottlieb in an April statement. “These products should never be marketed to, sold to, or used by kids — and we need to make every effort to prevent kids from getting hooked on nicotine.”

It’s not clear whether e-cigarettes help you quit smoking

Lots of people turn to e-cigs as a bridge between conventional cigarettes and total smoking cessation, since they typically deliver nicotine and mimic the ritual of smoking. But more research is needed to determine whether or not that’s actually an effective strategy.

Some studies, including one published in The BMJ in 2017, have found that e-cigarettes may improve a smoker’s chances of quitting successfully, but others have found the opposite. For example, a study published in March in the Annals of Internal Medicine found that among smokers trying to quit, those who used e-cigarettes were actually more likely to use tobacco after six months, compared to non-users. Another study, published in the New England Journal of Medicine in May, found that cash incentives were more effective at encouraging smokers to quit than free e-cigarettes.

Different smoking cessation strategies may work for different people, but currently, the data around vaping’s efficacy as a stop-smoking tool is murky.

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Write to Jamie Ducharme at jamie.ducharme@time.com.