An electronic cigarette in smoke. (iStock)

One in four middle school and high school students report that they have been exposed to secondhand smoke from e-cigarettes in the past 30 days, according to a new study by researchers at the Centers for Disease Control and Prevention.

The finding translates to about 6.5 million youths exposed. Some of the students had smoked e-cigarettes themselves during that time, but 4.4 million were not users. More than 1 million were exposed to secondhand smoke only from e-cigarettes, the study concluded.

The U.S. surgeon general reported last December that exposure to secondhand aerosol from electronic cigarettes is harmful because of the possible toxic substances, such as nicotine and heavy metals, they contain. Nicotine exposure can be particularly dangerous because it can affect adolescent brain development.

“We know that secondhand e-cigarette aerosol is not harmless, and it’s critical to protect our nation’s youth from this preventable health risk,” said Brian King, deputy director for research translation in CDC’s Office on Smoking and Health and a co-author of the new study. It was published in JAMA Pediatrics.

E-cigarette use among young people is a growing concern in the United States, with the surgeon general finding a 900 percent increase from 2011 to 2015. That jump makes e-cigarettes the most commonly used form of tobacco among children and adolescents in the United States, surpassing cigarettes, cigars, chewing tobacco and other conventional products.

Flavor is one of the most common reasons young people give for using e-cigarettes, according to the surgeon general's report. But some chemicals used for e-cigarette flavoring can be harmful to health, King said.

“One example is diacetyl, which is known to produce the buttery flavor in popcorn, and studies have linked inhalation of diacetyl to a severe respiratory illness,” King said. The ailment is commonly known as popcorn lung syndrome.

The CDC study drew on data from the 2015 National Youth Tobacco Survey. It found that secondhand exposure to e-cigarette aerosol was greater for girls (nearly 27 percent) than boys (22 percent). Fifteen percent of black students reported being exposed, compared to 27 percent of white students.

Youth who used e-cigarettes or conventional tobacco products are most likely to be exposed to secondhand smoke because of the company they often keep. “People who use multiple tobacco products are also in social environments with others who use e-cigarettes or other tobacco products,” King said.

To protect children from exposure to secondhand smoke from smoked tobacco products and aerosol from e-cigarettes, states and communities can consider modernizing smoke-free air policies to include e-cigarettes, he said.

“These policies can address the use of such products in indoor environments, and there has been considerable momentum across the country in the past few years,” he said. Eight states and 500 communities currently cover e-cigarette aerosol in their smoke-free policies, he said.

To decrease all forms of tobacco product use among young people, King recommends “access restrictions that prohibit youth who are less than a specified age from purchasing e-cigarettes, as well as educational campaigns that warn about the risks of e-cigarette use and secondhand aerosol exposure among youth.”

The latest report confirms certain trends, said Matthew Myers, president of the Campaign for Tobacco-Free Kids, and raises serious issues about how many youths are using e-cigarettes and being exposed to potentially harmful aerosol.