Preliminary data from two long-standing nationwide surveys reveals a sustained surge in e-cigarette use—aka vaping—by teens over the last two years.

About 25% of 12th graders in 2019 reported using nicotine-containing vaping products within 30 days of taking one of the surveys. That figure was about 21% in the 2018 edition of the same survey and 11% in 2017.

Nearly 12% of 12th graders in 2019 reported using nicotine-containing e-cigarettes on a daily basis (defined as use during 20 or more days in the last 30.)

That data is all from the Monitoring the Future survey (MTF), a survey run every year since 1975 by researchers at the University of Michigan. The survey probes the opinions and behaviors of a nationally representative sample of students in grades 8, 10, and 12. Researchers aim to get data from around 50,000 students each year and typically release results in December. This year, the researchers got answers from around 42,500 students and released some preliminary results this week in The New England Journal of Medicine.

Overall, the MTF has revealed a stunning decline in cigarette smoking in its time. High school seniors reporting smoking in the 30 days prior to the survey plummeted from nearly 40% in 1976 to 7.6% in 2018. That number fell to 5.7% in the preliminary 2019 data. Likewise, daily cigarette smoking in seniors had dropped from nearly 29% in 1976 to 3.6% in 2018.

But—while the MTF hints at a troubling surge in vaping since 2017—it only offers a glimpse of trend data for e-cigarettes. The MTF only started asking questions about vaping in 2017 and only asked about daily use this year.

That’s where a second massive survey comes in: the National Youth Tobacco Survey (NYTS). This survey is run by the Centers for Disease Control and Prevention and gauges middle and high school students’ use of tobacco products. (The CDC generally considers e-cigarettes tobacco products, noting in survey questions that e-cigarettes “are battery powered devices that usually contain a nicotine-based liquid that is vaporized and inhaled”). In annual surveys from 2011, the CDC has gotten responses from between around 17,000 and 25,000 students.

This data offers a wider picture of teen vaping. After e-cigarettes hit the market in 2007, the NYTS revealed a slow uptake by high schoolers from 2011 to 2013. High school students reporting use in the past 30 days rose only from 1.5% to around 5% in that time frame. But use jumped to over 15% by 2015 before dipping in 2016 and 2017.

At that point, the CDC data echoes that from the MTF—both show an abrupt surge. High school students reporting use in the past 30 days jumped from 11.7% in 2017 to 20.8% in 2018. Preliminary 2019 surveys show recent use jumped to more than 25%. An “overwhelming” majority of students who reported vaping said they used fruity, minty, or menthol-flavored products.

So what happened between 2017 and 2019? Public health officials have blamed flavored products targeted to youth, particularly those by the trendy e-cigarette maker Juul, which has a lion’s share of the market. Between June 2017 and June 2018, Juul’s dollar sales rose a whopping 783%, reaching $942.6 million, according to a Wells Fargo analysis of Nielsen data, which was reported by CNBC at the time.

A Congressional hearing this summer highlighted that Juul has marketed its products directly to students in schools and pitched its products as harmless. That’s despite the fact that they contain a variety of untested additives as well as nicotine, which is addictive and may harm health, particularly that of teens.

In turn, the Trump administration has moved to ban flavored e-cigarettes, and the Food and Drug Administration has issued warnings and launched investigations into Juul.