(CNN) More middle school and high school students in the United States are being exposed to secondhand vapor from e-cigarettes in public places, new research finds.
About 1 in 3 students last year said they had breathed secondhand aerosol from e-cigarettes, which is an increase from the approximately 1 in 4 students who reported exposure in years past, according to the research published in the medical journal JAMA Network Open on Wednesday.
The research was based on self-reported data from the National Youth Tobacco Survey, which collected information on exposure to secondhand smoke from tobacco products and secondhand aerosol from e-cigarettes among middle school and high school students from 2015 to 2018.
Exposure in the survey was defined as breathing secondhand smoke or secondhand vapor in indoor or outdoor public places at least one day in the past 30 days. Those public places could include schools, stores, restaurants, sports arenas, parking lots, stadiums or parks, for instance.
The data showed that between 2015 and 2018, about half of students reported secondhand smoke exposure, but that prevalence has been following a downward trend.
Secondhand smoke exposure impacted 52.6% of students in 2015 compared with 48.7% last year, according to the data.
Meanwhile, the data showed that the prevalence of secondhand vapor has increased from exposure impacting 25.2% of students in 2015 to 33.2% last year.
The data that came from last year showed that young women, white individuals, those with a history of e-cigarette or tobacco use, and those living with someone who used e-cigarettes were more likely to be exposed to secondhand smoke and secondhand vapor.
The researchers noted that although about "16 states and more than 800 municipalities have introduced laws to restrict e-cigarette use in 100% smoke-free or other venues, including schools, over the past few years," an increasing proportion of youth in the United States have reported exposure to secondhand vapor in public places.
"This may be owing to the increase in youth using pod-based e-cigarettes and other devices, fewer vape-free policies than smoke-free policies, and fewer people who are willing to speak up against others vaping in public places," the researchers wrote.
In general, it's estimated that nearly 1 in 5 high school students and 1 in 20 middle school students use e-cigarettes, according to the US Centers for Disease Control and Prevention. The number of middle and high school students using e-cigarettes rose from 2.1 million in 2017 to 3.6 million last year -- a difference of about 1.5 million young people.
The new research came as no surprise since there has been an overall rise in e-cigarette use in the United States in recent years, and decline in cigarette smoking, said Dr. Theodore Wagener, director of the Center for Tobacco Research and co-leader of the cancer control program at the Ohio State University Comprehensive Cancer Center in Columbus, who was not involved in the new research.
Yet the potential health impacts of increasing exposure to secondhand e-cigarette vapor remains an enigma and has become an active area of study.
"We still don't know the long-term health effects and most people generally think that they're safer than smoking cigarettes, so they're not too worried about exposing others to secondhand vapor," Wagener said.
Wagener and his colleagues recently completed a study -- which is currently under review and has not yet published -- that compared levels of exposure to nicotine and certain carcinogens among children who lived with non-smokers, only e-cigarette users or only cigarette smokers.
When it came to children living with only e-cigarette users, "we definitely know that they're being exposed to many of these tobacco toxicants that we saw with cigarettes but it appears to be just at lower levels," Wagener said.
"What that means for downstream health, we still don't know. I wish we did," he said.