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 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.