Preplanned Studies: Prevalence and Determinants of Secondhand Smoke Exposure Among Adolescent Girls — China, 2019

Secondhand smoke (SHS) exposure is harmful to health: there is no risk-free level, and it negatively influences smoking behaviors as well (1-2). Girls are particularly vulnerable to the adverse health outcomes associated with SHS; a recent study indicated exposure to SHS increases the risk of developing breast cancer, especially when the exposure occurs in puberty when breast cell proliferation is most rapid (3). The prevalence of SHS exposure in China (69.9%) in 2013–2014 was much higher than that in 68 other low-income and middle-income countries (54.1%) in 2006–2013 for girls aged 12–15 (4). However, the prevalence of SHS exposure at the present is unknown, and the research on its determinants among adolescent girls is limited in China. This study aims to describe the status quo of SHS exposure and explore its determinants among adolescent girls.

The data for this study were extracted from the 2019 China National Youth Tobacco Survey (NYTS), in which a multi-stage, stratified, cluster random sampling design was used to acquire a nationally representative sample. First, 347 districts or counties were selected with a probability proportional to the population size sampling (PPS) method from 31 provincial-level administrative divisions (PLADs) in China. Second, 3 junior high schools and 3 senior high schools (2 academic and 1 vocational) were selected using PPS in each sampled district or county. Third, a class was selected randomly in every grade of each sampled school. All students of the selected class were invited to complete paper-based questionnaires distributed by well-trained investigators independently without the presence of teachers. A total of 154,287 junior high and 149,764 senior high school students were included, with the overall response rates being 95.5% and 94.1%, respectively. The average median age of students surveyed was 15 years old.

In the 2019 China NYTS, all the participants were asked: 1) In the past 7 days, how many days did someone smoke in your home in your presence? 2) In the past 7 days, how many days did someone smoke in any indoor public places in your presence, such as teaching buildings, indoor venues, gymnasiums, internet cafes, stores, restaurants, shopping malls, or cinemas? 3) During the past 7 days, how many days did someone smoke in any outdoor public places in your presence, such as playgrounds, sidewalks, stations, building entrances, or parks? and 4) How many days did someone smoke in your presence in public transportation such as trains, buses, or taxis over the past 7 days? Being exposed to SHS at any place (i.e., at home, in indoor public places, in outdoor public places, or on public transportation) on ≥1 days in the past 7 days was defined as SHS exposure.

All parameter estimations were weighted based on a complex sampling design. Multilevel modeling of complex survey data was used to explore determinants of SHS exposure, with individuals being level-1 units and schools being level-2 clusters. Point values with 95% confidence intervals (CI) were presented for each parameter, and the difference with no overlap in CIs was identified to be statistically significant between subgroups. All analyses were conducted by SAS software (version 9.4, SAS Institute Inc., Cary, USA).

In this study, 70,778 and 72,169 female students in junior high and senior high schools, respectively, accomplished the questionnaire, of which 62.8% (61.5, 64.1) on average overall [60.8% (59.3, 62.1) from junior high and 65.3% (63.7, 67.0) from senior high school] were exposed to SHS in any place — with 31.7% (30.6, 32.8) at home, 46.5% (45.4, 47.6) in indoor public places, 48.8% (47.5, 50.0) in outdoor public places, and 22.1% (21.1, 23.1) on public transportation. The prevalence of SHS exposure in indoor public and outdoor public places was significantly higher than that at home and in public transportation — despite smoking or not and across different types of schools. In terms of geographic location, be it at home or in public places, there was no significant difference observed in the rate of SHS exposure between urban and rural areas. However, more girls in rural areas were exposed to SHS in public transportation than those in urban areas. For female non-smokers, the SHS exposure rate of senior high school students [64.8% (63.1, 66.4)] was higher than that of junior high school students [60.2% (58.8, 61.6)], but no significant difference between schools in SHS exposure was observed for female smokers [91.6% (88.2, 95.0) vs. 91.7% (89.1, 94.3)]. (Table 1)

The null model in multilevel modeling showed a statistically significant random part of level-2 accounting for 12 percent of the total variance (P<0.0001) and indicated the hierarchical structure of the data in this study. As a result, the introduction of Multilevel modeling was needed.

The results of the Multilevel modeling showed that higher grade levels [grade one as reference, grade two: odds ratio (OR)=1.438; grade three: OR=1.456], senior high school status (junior high school as reference: OR=1.158), urban areas (rural as reference: OR=1.302), more pocket money (OR=1.334), current smoking behaviors (no as reference: OR=2.441), exposure to tobacco advertisements (none as reference, one venue: OR=2.047; two venues: OR=2.222; three venues: OR=3.639), parental smoking behaviors (no one as reference: OR=2.098), close friends’ smoking behaviors (no one as reference: OR=1.950), use of e-cigarettes (no as reference: OR=1.654), and believing SHS exposure is detrimental to health (no as reference, maybe not: OR=1.504; maybe yes: OR=1.952; certainly: OR=2.430) were factors associated with higher SHS exposure (Table 2).