Background:
Secondhand smoke (SHS) contributes to significant morbidity and mortality worldwide including Hong Kong, a densely-populated city with the lowest daily smoking prevalence (10.5%) in the developed world. Most Hong Kong citizens reside in multiunit housing where SHS can transmit from one unit to another because of the shared ventilation system and crowded living environment. We explored the prevalence of SHS infiltration and its sociodemographic correlates in the Hong Kong general population.

Methods:
A cross-sectional sample of 5151 randomly selected Hong Kong residents aged 15+ were interviewed by telephone (response rate = 85.8%) for the Hong Kong Tobacco Control Policy-related Survey in 2016. Participants reported the number of days experiencing SHS infiltration from neighbour at home in the past 7 days, and their sociodemographic characteristics and smoking status. Data were weighted by the Hong Kong official data on age, gender and smoking status distributions. Chi-square tests and multivariable logistic regression examined the associations of home SHS infiltration with sociodemographic and smoking status.

Results:
Overall, the prevalence (95% CI) of ever and daily SHS infiltration at home in the past 7 days were 22.2% (20.3 to 24.2%) and 5.8% (4.8 to 7.1%) respectively. SHS infiltration was individually associated with female (P = 0.002), younger age (P < 0.001) and never smoking status (P < 0.001) but not with indices of socioeconomic status including highest education level (P = 0.07), family income (P = 0.43), employment (0.89) or immigrant (P = 0.09) statuses. Multivariable regression analyses showed that current smokers were less likely to report home SHS infiltration (OR = 0.43, 95% CI 0.32-0.58).

Conclusions:
In Hong Kong, the prevalence of home SHS infiltration was very high given the low prevalence of smoking and did not vary with socioeconomic status. Implementation of smoke-free policy in multiunit housing is imperative to safeguard its residents from passive smoking.