The negative health impacts of secondhand smoke (SHS) exposure are well documented, and there is no risk-free level of SHS exposure.¹ In 2017, an estimated 13.2 million middle- and high-school students were exposed to SHS in indoor or outdoor public places.² SHS-associated health conditions differ by age, with exposed children at risk of respiratory infections and more frequent and severe asthma attacks, whereas adults are at a higher risk of coronary heart disease and lung cancer than those who are unexposed.¹ Comprehensive smoke-free air laws, which prohibit smoking in indoor public places, including workplaces, bars, and restaurants, have been passed in states and communities to protect the public from SHS exposure. To date, 27 U.S. states, the District of Columbia, several U.S. territories, and >1,000 municipalities have implemented such laws.^3,4 However, as of January 2021, it is estimated that approximately 40% of Americans are not covered by smoke-free indoor air policies in bars, restaurants, and work sites and may be exposed to SHS.⁵ Thus, it remains important to characterize the levels of SHS exposure, particularly among youth and other population groups that may lack coverage.

The recent increased use of E-cigarettes introduces secondhand aerosol (SHA) exposure as an additional public health concern. Although the long-term health impacts of SHA exposure are unknown, the aerosol from E-cigarettes is not harmless.⁶ SHA can contain harmful and potentially harmful chemicals, including volatile organic compounds such as benzene and heavy metals such as nickel, tin, and lead.⁶ Data from the National Youth Tobacco Survey (NYTS) show that although SHS exposure decreased during 2015–2018, there was a significant increase in SHA exposure.⁷

There are few studies that report SHS and SHA exposure among youth^2,7−10; earlier studies show that exposure to SHA was associated with higher susceptibility to E-cigarette use,^7,8 highlighting the importance of reducing SHA exposure among youth.

No studies have reported on SHS and SHA exposure in indoor and outdoor settings, separately, among U.S. youth. Given that policies aiming to prevent SHS and SHA exposures in these settings differ, and that, indoor policies are more common than outdoor policies,¹¹ this study can increase the awareness of exposure and provide important evidence to help inform SHS and SHA prevention strategies. Currently, 15 states, the District of Columbia, and Puerto Rico include E-cigarettes in their statewide smoke-free laws³; this study provides information for states and communities about youth exposure to SHS and SHA. Building on previous studies that provide estimates of SHS and SHA exposure,^2,7 this study aims to present estimates for prevalence and correlates of SHS and SHA exposures, overall and separately, for indoor and outdoor public settings among a nationally representative sample of U.S. middle- and high-school students.

Exposure to any SHS or SHA, either indoor or outdoor, was reported by 67.7% (95% CI=65.6, 69.8; 18.1 million) of students (Table 1). Indoor or outdoor SHS exposure was reported by 60.6% (95% CI=58.7, 62.4; 16.1 million) of students. Indoor or outdoor SHA exposure was reported by 44.5% (95% CI=42.1, 46.9; 12.0 million) of students. Indoor exposure to either SHS or SHA was reported by 51.1% (95% CI=48.9, 53.2; 13.7 million) of students. Prevalence of outdoor exposure to either SHS or SHA was slightly greater, at 60.1% (95% CI=58.0, 62.2; 15.9 million students).

Indoor SHS exposure was reported by 37.6% (95% CI=36.0, 39.2; 10.0 million) of students, and outdoor SHS exposure was reported by 53.3% (95% CI=51.4, 55.2; 14.2 million) (Table 2). Indoor SHA exposure was reported by 34.9% (95% CI=32.9, 37.4; 9.4 million) of students, and outdoor SHA exposure was reported by 36.8% (95% CI=34.6, 38.9; 9.7 million). Among students who reported exposure to any of the queried indoor or outdoor outcomes, most reported exposure on 1–2 days in the past 30 days (Figure 1).

In multivariable-adjusted logistic regression models examining the correlates for indoor SHS, outdoor SHS, indoor SHA, and outdoor SHA among all students, female students were more likely to report all outcomes than male students (indoor SHS: APR=1.16, 95% CI=1.11, 1.20; outdoor SHS: APR=1.30, 95% CI=1.24, 1.36; indoor SHA: APR=1.15, 95% CI=1.08, 1.22; outdoor SHA: APR=1.25, 95% CI=1.19, 1.32) (Table 2). Middle-school students were less likely to report exposure to indoor SHA (APR=0.63, 95% CI=0.56, 0.70) and outdoor SHA (APR=0.80, 95% CI=0.73, 0.87) than high-school students but were more likely to report exposure to indoor SHS (APR=1.07, 95% CI=1.00, 1.13). Non-Hispanic Black students were significantly less likely to report exposure to outdoor SHS, indoor SHA, and outdoor SHA than non-Hispanic White students (range, APRs: 0.74 [outdoor SHA] to 0.80 [outdoor SHS]). Students identifying as gay, lesbian, or bisexual were more likely to report exposure to indoor SHS (APR=1.10, 95% CI=1.02, 1.19) and outdoor SHS (APR=1.11, 95% CI=1.05, 1.17) than heterosexual students. Students who currently used combustible tobacco products (versus not) were more likely to report both indoor and outdoor exposure to both SHS and SHA (range, APRs: 1.15 [outdoor SHA] to 1.36 [indoor SHS]); those who currently used E-cigarettes were more likely to report exposure to SHS in indoor settings (APR=1.19, 95% CI=1.09, 1.29) and SHA in indoor (APR=1.61, 95% CI=1.46, 1.77) and outdoor (APR=1.50, 95% CI=1.38, 1.62) settings than those who did not currently use E-cigarettes. Finally, all categories of household tobacco use were associated with increased prevalence of all outcomes, with the exception of smokeless tobacco and exclusive E-cigarette use on outdoor SHS (Table 3).

Correlate patterns were similar for the 5 studied composite measures (Appendix Table 2, available online). For example, female students were more likely to report all-composite outcomes than male students (range, APRs: 1.13 [indoor SHS or indoor SHA] to 1.23 [outdoor SHS or outdoor SHA]). An additional analysis examining the prevalences of indoor SHS, outdoor SHS, indoor SHA, and outdoor SHA among those not currently using any tobacco products also yielded similar results as those reported among the entire study population in Table 2 (Appendix Table 1, available online).

This study is the first to report separate estimates of exposure to public indoor and outdoor SHS and SHA for U.S. youth. Approximately 1 in 3 U.S. middle- and high-school students reported exposure to indoor SHA, indoor SHS, and outdoor SHA in the past 30 days. In addition, more than half of U.S. students reported outdoor SHS exposure in the past 30 days. Separate estimates for indoor and outdoor exposure provide greater detail on where students report being exposed to these constituents and therefore could be used to develop targeted strategies to reduce such exposures. Observed differences in exposure by demographic characteristics can be used to aid public health practitioners in the development of prevention programs. Estimated SHS exposure in indoor and outdoor public places combined was higher in this study at 60.6% than at 48.7% of 2018 NYTS respondents.⁷ This result is surprising because SHS exposure has decreased over the past few years⁷; it is possible that the combination of the indoor and outdoor measures to determine this prevalence provides a better representation of the exposure of students. By contrast, it is also possible that combining 2 questions led to an overestimation of the true prevalence. Nonetheless, given its known health effects,¹ updated estimates of SHS exposure are important to track the progress of smoke-free policies and to inform states and communities about the extent and burden of exposure among youth.

Concurrent with the increase of E-cigarette use among both youth⁶ and adults,^13,14 reported SHA exposure among youth has increased.⁷ Both the combined (44.5%) and separate (34.9%) public indoor and outdoor (36.8%) SHA exposures presented in this report are higher than the combined indoor/outdoor youth SHA exposure estimate reported in 2018 (33.2%).⁷ Notably, E-cigarettes have been the most commonly used tobacco product among youth since 2014.¹⁵ One important source of SHA for youth may be school grounds. In the 2019 NYTS, 33% of youth reported observing E-cigarette use inside a school bathroom or locker room; 33% also reported observing E-cigarette use outside of the school-on-school grounds.¹⁶ Because SHA is not harmless,⁶ it remains important to monitor youths’ SHA exposures, given that youth E-cigarette use is an epidemic and that the E-cigarette marketplace is evolving.¹⁷

Prohibitions on indoor tobacco product use are more common than those on outdoor use, with 22,710 U.S. municipalities covered by smoke-free provisions in nonhospitality workplaces, restaurants, and bars compared with 3,861 states, commonwealths, territories, cities, and counties with outdoor regulations as of January 1, 2021.¹¹ This may be reflected in the finding that reported that outdoor SHS exposure was higher than indoor SHS exposure overall and among all studied subgroups. In addition, more students reported outdoor SHS/SHA exposure than those who reported indoor SHS/SHA exposure, underlying the importance of addressing outdoor exposure. Thus, understanding both indoor and outdoor youth secondhand tobacco product exposure patterns can provide support for future policies aimed at reducing SHS and SHA exposure, particularly in public outdoor locations.

Demographic differences observed in this study for public indoor and outdoor exposures were similar to those reported previously for overall SHS/SHA exposures.^2,8 Furthermore, youth identifying as gay, lesbian, or bisexual reported a higher prevalence of SHS and SHA exposure than heterosexual students; although this has been observed among sexual minority adults in California¹⁸ and lesbian and bisexual adult women nationally,¹⁹ not as much is known about SHS and SHA exposures by sexual orientation among youth. However, youth identifying as gay, lesbian, or bisexual have higher tobacco product use overall.²⁰ Together, these findings suggest that further work needs to be done to ascertain where these students are being exposed to SHS and SHA publicly to inform future programs to reduce SHS and SHA exposures.

Results of this study show that millions of youth report being exposed to SHS and SHA in indoor and outdoor public places. These results emphasize the continued exposure that exists in both indoor and outdoor locations even with the comprehensive smoke-free policies in some states. Among states with comprehensive smoke-free laws that address SHS exposure, only more than half (15 states, the District of Columbia, and Puerto Rico) currently include E-cigarettes in their jurisdiction-wide policies.³ Opportunities exist for the improvement of current comprehensive smoke-free laws through the integration of E-cigarettes and other novel tobacco products into state, territorial, and local laws. In addition, because there are some subgroups (e.g., girls) who consistently reported elevated SHS and SHA exposure, these may be groups that may benefit from enhanced public policies or messaging to reduce their exposure to SHS and SHA. These findings can be used by communities who are exploring opportunities to protect individuals from SHS/SHA.