Secondhand Smoke From Multiple Sources, Thirdhand Smoke and Respiratory Symptoms in Hong Kong Adolescents.

Introduction: Reports on involuntary tobacco smoke exposure in children have focused mostly on secondhand smoke (SHS) from smoking inside the home. We studied the separate and combined prevalence of SHS exposure from multiple sources and thirdhand smoke (THS) and the associations with respiratory symptoms in Hong Kong adolescents.
Methods: In 2010-2011, 61 810 Secondary 1 (US Grade 7) to seven students reported their smoking status, respiratory symptoms, and exposure to four sources of tobacco smoke in the past 7 days. Weighted prevalence of exposure was calculated. Associations with respiratory symptoms were analyzed in 50 762 never smokers using logistic regression.
Results: Tobacco smoke exposure at home was 23.2% considering SHS exposure from inside the home, but increased to 33.2% including SHS from neighbors and 36.2% further including THS. Including SHS outside home (55.3%), 63.3% of adolescents were exposed to SHS anywhere or THS at home. In never smokers, SHS from each source and THS at home were linearly associated with respiratory symptoms. Exposure to more sources yielded stronger associations with respiratory symptoms (p for trend<.001). The adjusted odds ratios (95% CI) were 1.04 (0.97-1.11), 1.12 (1.03-1.22), 1.40 (1.26-1.56) and 1.99 (1.74-2.28) for 1, 2, 3, and 4 sources, respectively. Conclusions: Although Hong Kong's smoking prevalence is among the lowest in the developed world, over 60% of its adolescents were involuntarily exposed to tobacco smoke from one or more sources with a linear association with respiratory symptoms in never smokers. More stringent policies are needed to protect adolescents from tobacco smoke. Implications: In a high-density urban setting, involuntary exposure to tobacco smoke in adolescents can be much higher than the smoking prevalence of the general population, especially if SHS exposure from multiple sources and THS are also considered. Such exposures have important health implications as demonstrated by their linear associations with respiratory symptoms. Tobacco control measures effective in reducing smoking prevalence may have little effect in reducing adolescent exposure to tobacco smoke, especially in the private home, in which other public health strategies are urgently needed.