Background:
Being able to smoke at home may facilitate adolescent smoking and nicotine dependence. However, this has rarely been studied. We studied the prevalence of smoking at home, its risk factors and potential effect on nicotine dependence in Hong Kong adolescent smokers.

Methods:
In a cross-sectional school-based survey in 2014/15, 10932 secondary school students (mean age 14.8 years; 46.8% boys) indicated whether they smoked at home during 8 periods on weekdays and weekends, analysed as yes (any period) vs no. Also measured were urges to smoke (UTS, range 0-5, 5=greatest urge) based on the frequencies and strength of smoking urges; cigarettes smoked per day (CPD); past 30-day (current) smoking; and sociodemographic and other smoking-related characteristics. We investigated smoking at home's (1) risk factors and (2) associations with UTS and CPD (outcomes) in current smokers.

Results:
Of 461 current smokers, 50.3% (95% CI 45.7-54.9) smoked at home, with “weekend mornings” being the most common (20.6%). Smoking at home was associated with having 2 or more co-residing smokers (vs none) and home thirdhand smoke exposure for 4-7 days/week (vs 0) with adjusted odds ratios of 1.38 (95% CI 1.09, 1.76) and 1.66 (1.23, 2.23). Smoking at home was also associated with UTS and CPD with adjusted β coefficients of 1.78 (1.17, 2.39) and 2.63 (1.27, 3.99).

Conclusions:
About half the Hong Kong adolescent smokers smoked at home. Smoking at home was associated with living with smokers and a home environment with residual tobacco smoke, which probably make adolescent smoking difficult to detect. Smoking at home was also associated with higher nicotine dependence. Whether smoking cessation of family members and home smoking bans can reduce adolescent smoking at home and nicotine dependence should be further studied.