One in five high school students — and more than 6% of American adults overall — now say they vape. The industry has ballooned to more than $6 billion, led by Juul.

Some researchers worry that a major crackdown on Juul and other manufacturers would simply send teenagers reaching for traditional cigarettes instead.

Virtually all e-cigarette products contain nicotine, some in high levels — including Juul's 5% pods, each of which contains as much nicotine as an entire pack of cigarettes.