Background:
Increased knowledge of the harmful effects of SHS is an evidence-based key indicator for eliminating nonsmokers’ exposure to SHS. This study assessed the prevalence and predictors of perceptions about the harm of secondhand smoke (SHS) exposure among U.S. middle and high school students.

Findings:
Data were obtained from the 2012 National Youth Tobacco Survey, a nationally representative school-based survey of U.S. students in grades 6–12. Respondents who reported that they thought breathing smoke from other people’s cigarettes or other tobacco products causes “some” or “a lot” of harm were considered to have the perception that SHS is harmful. Multivariate logistic regression was used to identify predictors of the perception that SHS is harmful. Predictors included sex, race/ethnicity, school grade level, current tobacco use, and whether the respondent currently lived with a tobacco user. Overall, 75.4% of students perceived SHS exposure as harmful. The adjusted odds of perceiving SHS exposure as harmful were higher among non-Hispanic Asians than among non-Hispanic whites, and among students in 10th-12th grades than among students in 8th grade. Adjusted odds were lower among boys than among girls, among non-Hispanic blacks than among non-Hispanic whites, among students living with a tobacco user than among those not, and among those who use combustible tobacco only or both combustible and non-combustible tobacco than among those who use no tobacco.

Conclusions:
Most middle and high school students perceive SHS exposure as harmful, but efforts are needed to increase the prevalence of this perception in certain subpopulations, particularly tobacco users.