A government study has revealed that even though the number of U.S. teenagers who smoke has come down, secondhand smoke remains a big problem for them, the Washington Post reports.

The study was published online Monday in the journal Pediatrics.

The report said that nearly half of nonsmoking students in middle school and high school were exposed to secondhand tobacco smoke in 2013.

The rate of exposure to second hand smoke was higher among smokers.

The exposure to secondhand smoke was defined as being around tobacco smoke at least once within the past week.

"These findings are concerning because the U.S. surgeon general has concluded that there is no safe level of secondhand smoke exposure," said Israel Agaku, lead author and a researcher at the Centers for Disease Control and Prevention.

The government study was based on a national survey of more than 17,000 middle school and high school students.

Nearly 1 in 4 nonsmokers who reported any secondhand smoke exposure said they were around tobacco smoke daily. Settings included home, school, cars and public places, Aljazeera America reports.

However, It was not clear how many had only brief exposure.

Secondhand smoke causes several illnesses in children, including breathing problems, ear infections, bronchitis and pneumonia. It can cause heart disease and lung cancer in adults.

Earlier studies on teens and secondhand smoke in specific places indicated that the problem of secondhand smoke has decreased in the past few years.

Medical express reports that the researchers said the study results show that efforts are needed to expand smoke-free zones.