Heath Alseike/flickr

Teens are using fewer traditional tobacco products, but they're smoking more pot than ever, according to a new CDC report.

Between 1997 and 2013, the share of students in grades 9-12 saying they exclusively smoked cigars or cigarettes fell to 7.4 percent from 20.5 percent, the report found. Meanwhile, exclusive marijuana use more than doubled—from 4.2 percent in 1997 to 10.2 percent in 2013. Smoking in any combination, the most prevalent practice (in other words, if you smoke one, you're likely to smoke the other), fell to 29.9 from 46.1 percent.

Fusion, data via CDC

The CDC study's authors say an increasing perception of pot as harmless has likely contributed to increased usage.

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"Specifically, decriminalization and legalization of recreational marijuana use in some states with minimal concomitant public health messaging to address potential detrimental health effects of marijuana use might be contributing to this perception," they write.

"Further, legalization of medical marijuana use in 24 states, the District of Columbia, and Guam might increase perceptions of benefits of use, including that it is not harmful."

The study did not measure use of e-cigarettes. In a separate report published earlier this year, the CDC found e-cigarette use had tripled in just 12 months among middle and high school students, to 13.4 percent, or approximately 2 million students in 2014.

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In 2010, the proportion of U.S. 12th grade students who said they used marijuana in the past 30 days surpassed the proportion saying they'd smoked cigarettes. That figure now stands at 21.4 percent, versus 19.2 percent for cigarettes.

Usage rates of any product have converged across ethnic groups to about 30 percent. Since 1997, white high school students have seen the greatest declines, while rates have been increasing among blacks and Hispanics since 2007 (though they're still down from '90s highs). Chart:

Fusion, data via CDC

Rob covers business, economics and the environment for Fusion. He previously worked at Business Insider. He grew up in Chicago.