Background OPENACCESS Environmental tobacco smoke (ETS) exposure attributable respiratory illness burden is Citation: Dai S, Chan KCC (2020) Household huge in paediatric population. Understanding the epidemiology of ETS exposure is impor- environmental tobacco smoke exposure in healthy tant to guide health promotion planning. Therefore, we designed this study to determine the young children in Hong Kong: Prevalence and risk prevalence of household ETS exposure in healthy young children under 2 years of age in factors. PLoS ONE 15(1): e0227733. https://doi. org/10.1371/journal.pone.0227733 Hong Kong, and to explore risk factors associated with the exposure. Our secondary goal was to characterise children’s exposure profile to maternal smoking. Editor: Abraham Salinas-Miranda, University of South Florida, UNITED STATES Received: June 27, 2019 Methods Accepted: December 29, 2019 A secondary analysis was performed based on the data collected from our 2013–2014 terri- Published: January 14, 2020 tory-wide cross-sectional pneumococcal carriage surveillance study, with a sample size of 1541. We conducted descriptive analysis for exposure prevalence, univariate and multivari- Copyright:© 2020 Dai, Chan. This is an open access article distributed under the terms of the ate analysis for identification of risk factors. Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original Results author and source are credited. 1541 children (mean age: 11.2± 6.4 months, male: 50.7%) were included in the analysis. Data Availability Statement: Data cannot be The overall prevalence of current household ETS exposure was 31.5%, prevalence of pre- shared publicly because of confidential information involved. Informed consent was not obtained from natal and postnatal maternal smoking was 3.5% and 1.6% respectively. Independent factors individual participant for data sharing. De-identified associated with children’s ETS exposure were: never breastfed (AOR: 1.48, 95% CI: 1.13– data are available upon request for researchers 1.93, p = 0.004); prenatal maternal smoking (AOR: 7.46, 95% CI: 2.73–20.39, p< 0.001); who meet the criteria for access to confidential overcrowding of household living place (AOR: 3.17, 95% CI: 2.02–4.96, P< 0.001); lower data. Data access requests may be made to the following: Miss Juliet Lee, Secretary, Department household income (AOR: 1.34, 95% CI: 1.04–1.72, p = 0.02). Interestingly, children residing of Paediatrics, The Chinese University of Hong in Kowloon (AOR: 1.66, 95% CI: 1.19–2.33, p = 0.003) and New Territories West (AOR: Kong, Email: email@example.com; Joint CUHK- 1.54, 95% CI: 1.11–2.15, p = 0.01) were associated with exposure compared with children NTEC Clinical Research Ethics Committee, 8/F, Lui Che Woo Clinical Sciences Building, Prince of residing in Hong Kong Island. PLOS ONE | https://doi.org/10.1371/journal.pone.0227733 January 14, 2020 1 / 13 Profile of household ETS exposure in young children in Hong Kong Wales Hospital, Shatin, Hong Kong, Email: Conclusion firstname.lastname@example.org. Exposure to household ETS is prevalent among Hong Kong young children, particularly in Funding: The author(s) received no specific children with maternal unfavourable behaviour and lower socioeconomic status. The identi- funding for this work. fied risk factors should be considered while tobacco control interventions and legislations Competing interests: The authors have declared are planned. that no competing interests exist. Introduction ETS is defined as tobacco smoke produced by an active smoker both from the exhalation of smoked tobacco and by the burning end of the cigarette, which is inhaled by non-smokers. There is no safe exposure level of environmental tobacco smoke (ETS) exposure, children and fetus are especially sensitive to the exposure.[1–2] Children’s ETS exposure most commonly takes place in their homes and the main source is parental smoking. The exposure attribut- able disease burden is huge. Lower respiratory tract infection (LRTI) in children younger than 5 years was reported to be the largest global disease burden of ETS , while LRTI is a leading infectious cause of mortality in children younger than 5 years worldwide. Globally, the prevalence of ETS exposure in children was 40% in year 2004. In Hong Kong, the prevalence was quite similar. For young children and infants, the information is relatively scarce. A previous birth cohort study reported a prevalence of 42.1% in 1997. However, many milestones on local tobacco control have been achieved thereafter, such as the public smoke-free legislation since 2007. According to a recent meta-analysis of the impact of public smoking bans on children’s household ETS exposure, majority of the included studies reported a decrease in the post-legislation exposure prevalence, while two studies including one Hong Kong study reported an increase. Ho S. et al. indicated that comprehensive smoke-free legislation without strong support for smoking cessation might have displaced smoking into the homes of school-aged children. However, the condition of young children remained unknown. A review study shows that having smoking parents, lower family socioeconomic status (SES), lower parental education level, and having less negative parental attitudes towards ETS are shown to be risk factors of exposure. However, majority of the existing publications were done in elder children, only three studies explored the risk factors of ETS exposure among children under two years of age, although younger children can actually be more sus- ceptible to the exposure risks. A Thai study found that smoking in the presence of an infant was associated with paternal age of 25–34 or more than 44 years, low parental education level, and a Muslim father. In Iran, lower social status and younger infant’s age were associated with the exposure. More recently, a large-scale American study reported that infants of families having more children and having smoking mother were less likely to have a no-smoke policy and more likely to report that someone smoked inside the home. The household ETS exposure profile has not been comprehensively drawn yet in Hong Kong young children, cultural difference may have significant influence on the epidemiology. Understanding the epidemiology profile of ETS exposure is important to guide health pro- motion planning such as to identify at-risk population and to distribute limited resources to decrease ETS attributable disease burden appropriately and efficiently. Therefore, we designed this study to determine the prevalence of household ETS exposure in healthy young children under 2 years of age in Hong Kong, and to explore risk factors associated with exposure. Our secondary goal was to characterise children’s exposure profile to maternal smoking. PLOS ONE | https://doi.org/10.1371/journal.pone.0227733 January 14, 2020 2 / 13 Profile of household ETS exposure in young children in Hong Kong Materials and methods Study population We carried out this secondary analysis based on the data obtained from our 2013–2014 com- munity-based, territory-wide cross-sectional pneumococcal carriage surveillance study. Chil- dren aged 2 months, 12 months and 18 months, across 4 main regions of Hong Kong were recruited randomly when they attended the health service provided by the Maternal and Child Health Centre. Details of the surveillance study have been published previously. This ret- rospective study was approved by the Joint Chinese University of Hong Kong-New Territories East Cluster Clinical Research Ethics Committee (CRE-2018.452). Data collection In-depth interview was carried out in the previous surveillance study. Caregivers of the young children were asked to complete standardised self-administered questionnaire under supervi- sion by a trained research nurse, and the completed questionnaire was checked by our research nurse at the study site. Furthermore, clarifications would be made over the phone by research nurse if necessary. Data on a range of variables including demographics information, chil- dren’s clinical characteristics and their household tobacco exposure status were collected. Con- cerning the household ETS exposure, information about smoking of any household members, prenatal maternal smoking and postnatal maternal smoking were collected. Variable measurements Binary variables on exposure were current household ETS exposure, prenatal maternal smok- ing exposure and postnatal maternal smoking exposure. The caregiver was asked whether there were any household smokers living with the child (yes vs. no), whether the mother smoked during pregnancy (yes vs. no), and whether the mother was an active smoker (yes vs. no). Exposure level was examined using ordinal categorical variables including number of household smokers at home (0, 1, >1), and daily total cigarettes consumption by all household smokers (0 cigarettes/ day, 1–20 cigarettes/ day, >20 cigarettes/ day). Those potential associ- ated risk factors (demographic and clinical) including children’s age, sex, history of breastfeed- ing (yes vs. no), having siblings (yes vs. no), living region, overcrowding of living area (a living space of <5.5 m /person in accordance with the guideline of the Hong Kong Housing Author- ity), low household income (household monthly income�HK$20,000). Our measures of vari- ables were similar with previous studies.[7, 15] Sample size calculation Based on previous local and global studies, the estimated prevalence of household ETS expo- sure in Hong Kong children was 30%.[4, 7] Our study was powered to estimate ETS exposure prevalence of 30% with 80% power with 2.5% possible error, which required a minimum sam- ple size of 1289 (Epi Info V.7.2). Statistical method Period prevalence of children’s household ETS exposure was calculated. As in previous surveil- lance study, data were not collected at one time, they were collected during Jun 2013 to June 2014. As for the prevalence calculation equation, the denominator is the number of partici- pants in our previous surveillance study, while numerator was the number of children reported to be exposed to each ETS variables. Categorical risk factors of ETS exposure condi- tion were identified by univariate analysis using Chi square test. Variables that were significant PLOS ONE | https://doi.org/10.1371/journal.pone.0227733 January 14, 2020 3 / 13 Profile of household ETS exposure in young children in Hong Kong in the univariate analysis as defined with a p value< 0.05, and those with a p value< 0.1 and could increase the risk of having household ETS exposure from a clinical point of view were further tested by multivariate logistic regression using the forward-conditional model. Adjust- ments were carried out to minimise potential confounding effect. Since the effect of prenatal maternal smoking on children’s current household ETS exposure condition could be dominate , thus different models were built: 1) by ignoring the effect of prenatal maternal smoking; 2) by adjusting to prenatal maternal smoking; 3) by excluding young children with prenatal maternal smoking. Nagelkerke R-square value which stands for the percent of variance could be explained by the model would be stated. Model with the largest R-square value was further interpreted. A p-value of <0.05 was considered to be statistically significant. All statistical anal- ysis was performed by the SPSS statistics package. Results Baseline characteristics A total of 1541 young children (mean age: 11.2 ± 6.4 months, male: 50.7%) were included in this study. Detailed baseline characteristics of the participants were shown in Table 1. The numbers of participants distributed in each gender, age and geographical region group were quite equal. Most of the children were born by vaginal delivery (66.8%) and had been breastfed (79.2%). About half (48.5%) of the young children had siblings. About 29% of the families had low household income and 6% had overcrowding of the household living areas. The exposure prevalence The overall prevalence of household ETS exposure was 31.5%. Prevalence of prenatal and post- natal maternal smoking was 1.6% and 3.5% respectively. As for the exposure strata, the major- ity of the exposed young children (which accounted for 77.9% of the exposed ones) lived with 1 household smoker and the majority of the exposed young children (which accounted for 89.2% of the exposed ones) lived in household environment where the total daily cigarette con- sumption by the household smokers was 1–20 cigarettes. Risk factors of household ETS exposure Characteristics associated with household ETS exposure are shown in Table 2. Current household ETS exposure By univariate analysis (Table 2), factors associated with current household ETS exposure were: never breastfed; prenatal maternal smoking; living in Kowloon and New Territories (NT) West; overcrowding of household living place; lower household income; older in age. With multivariate analysis (Table 3), the results remained similar. According to our analysis results, model 2 had the biggest Nagelkerke R-square value of 0.098, followed by model 1 of 0.081 and model 3 of 0.077. Thus according to model 2, the independent factors associated with the cur- rent exposure were: never breastfed (AOR: 1.48, 95% CI: 1.13–1.93, p = 0.004); prenatal mater- nal smoking (AOR: 7.46, 95% CI: 2.73–20.39, p< 0.001); overcrowding of household living place (AOR:3.17, 95% CI: 2.02–4.96, P< 0.001); and lower household income (AOR: 1.34, 95% CI: 1.04–1.72, p = 0.02). Interestingly, we found some geographical differences of the ETS exposure distribution that young children residing in Kowloon (AOR: 1.66, 95% CI: 1.19–2.33, p = 0.003) and NT West (AOR: 1.54, 95% CI: 1.11–2.15, p = 0.01) were independently and sig- nificantly associated with household ETS exposure compared with children residing in Hong Kong Island. PLOS ONE | https://doi.org/10.1371/journal.pone.0227733 January 14, 2020 4 / 13 Profile of household ETS exposure in young children in Hong Kong Table 1. Demographic characteristics of the children and their ETS exposure condition (N = 1541). Characteristics All infants Mean age (months) 11.2 ± 6.4 Birth history Gestation weeks (weeks) 38.6 ± 1.4 Birth weight (kg) 3.1 ± 0.4 BMI z score 0.2 ± 1.3 Maternal age (yrs.) 32.7 ± 4.6 Male gender (N, %) 782 (50.7) Vaginal delivery (N, %) 1030 (66.8) Have breastfeed ever (N, %) 1220 (79.2) Number of young children (N, %) 2 months 477 (31.0) in 3 age groups 12 months 522 (33.9) 18 months 542 (35.2) Have sibling (N, %) 748 (48.5) Group care attendance ever (N, %) 197 (12.8) Pet Keeping at home (N, %) 234 (15.2) Living region (N, %)П HK Island 338 (21.9) NT East 377 (24.5) KL 432 (28.0) NT West 394 (25.6) Overcrowdingª (N, %) 96 (6.2) Low income# (N, %) 447 (29.0) Household ETS (N, %) Current household ETS exposure 485 (31.5) Postnatal maternal smoking 53 (3.4) Prenatal maternal smokingσ 25 (1.6) Number of household smoker (N, %) 0 household smoker 1052 (68.3) 1 household smoker 381 (24.7) > 1 household smoker 108 (7.0) Total cigarettes consumption amount 0 cigarette/ day 1052 (68.3) of household smoker (s) (N, %) 1–20 cigarettes/ day 436 (28.3) >20 cigarettes/ day 53 (3.4) : Values are mean ± SD ª: Household living area� 5.5 m is regarded as overcrowding, in accordance with the guideline of the Hong Kong Housing Authority #: Family income�20,000HK D is regarded as low income according to the HK statistic and census department 2014 σ: Maternal smoking during pregnancy П: “HK Island” stands for “Hong Kong Island”; “NT East” stands for “New Territories East”; “KL” stands for “Kowloon”; “NT West” stands for “New Territories West” https://doi.org/10.1371/journal.pone.0227733.t001 Postnatal maternal smoking By univariate analysis (Table 2), the identified risk factors were never breastfed, prenatal maternal smoke, living in Kowloon and N.T West regions, and lower household income. According to our multivariate analysis results (Table 4), model 2 had the biggest Nagelkerke R-square value of 0.235, followed by model 1 of 0.076 and model 3 of 0.022. Thus, by multivar- iate analysis of model 2, the independent risk factors were never breastfed (AOR: 2.45, 95% CI: 1.29–4.64, p = 0.006) and prenatal maternal smoking (AOR: 75.9, 95% CI: 30.26–190.53, p< 0.001). PLOS ONE | https://doi.org/10.1371/journal.pone.0227733 January 14, 2020 5 / 13 Profile of household ETS exposure in young children in Hong Kong Table 2. Univariate analysis of risk factors for household ETS exposure (N = 1541). Current household ETS exposure Postnatal maternal smoking Prenatal maternal smoking Exposure OR 95% CI p value Exposure OR 95% CI p value Exposure OR 95% CI p frequency frequency frequency value Age group 2 27.3% 1 1.9% 1 months 12 33.0% 1.31 (1.00, 0.05 4.2% 2.29 (1.04, 0.23 months 1.72) 5.02) 18 33.8% 1.36 (1.04, 0.03 4.1% 2.20 (1.00, 0.33 months 1.78) 4.83) Gender Boy 30.6% 1 4.1% 1 0.16 Girl 32.4% 1.09 (0.88, 0.44 2.8% 1.50 (0.86, 0.16 1.35) 2.62) Never breastfeed 41.4% 1.13 (1.07, <0.001 7.2% 3.06 (1.75, <0.001 3.7% 0.7 (0.32, 0.43 1.20) 5.35) 1.58) Prenatal maternal 80.0% 9.04 (3.37, <0.001 68.0% 87.4 (35.41, <0.001 smoking 24.24) 215.52) Have siblings (Not 33.9% 1.24 (1.0, 0.06 4.0% 1.40 (0.81, 0.26 1.6% 0.98 (0.45, 1.00 first parity) 1.54) 2.44) 2.16) Living region HK 22.8% 1 0.9% 1 0.6% 1 Island NT East 28.1% 1.33 (0.99, 0.1 5.0% 5.93 (1.74, 0.004 2.9% 5.05 (1.11, 0.04 1.86) 20.21) 22.95) KL 35.6% 1.88 (1.31, <0.001 3.7% 4.30 (1.24, 0.21 1.4% 2.37 (0.48, 0.29 2.59) 14.86) 11.80) NT West 37.6% 2.04 (1.47, <0.001 3.8% 4.42 (1.27, 0.02 1.5% 2.6 (0.52, 0.24 2.83) 15.40) 12.96) Overcrowding 63.5% 4.20 (2.73, <0.001 6.3% 1.98 (0.83, 0.14 2.1% 1.32 (0.31, 0.71 6.46) 4.76) 5.66) Low household 40.7% 1.79 (1.42, <0.001 4.9% 1.78 (1.02, 0.04 3.1% 3.18 (1.43, 0.004 income 2.26) 3.10) 7.07) : Age and gender were not considered as potential risk factors of prenatal maternal smoking exposure П: “HK Island” stands for “Hong Kong Island”; “NT East” stands for “New Territories East”; “KL” stands for “Kowloon”; “NT West” stands for “New Territories West” https://doi.org/10.1371/journal.pone.0227733.t002 Table 3. Multivariate analysis of risk factors for current household ETS exposure. Model 1 (N = 1541) Model 2 (N = 1541) Model 3 (N = 1516) AOR 95% CI p value AOR 95% CI p value AOR 95% CI p value Never breastfeed 1.54 (1.18, 2.01) 0.001 1.48 (1.13, 1.93) 0.004 1.51 (1.16, 1.98) 0.003 Have siblings 6-18yrs 1.48 (1.12, 1.94) 0.005 1.44 (1.09, 1.90) 0.01 1.45 (1.10, 1.92) 0.09 Living region HK Island 1 1 1 NT East 1.21 (0.85, 1.70) 0.29 1.16 (0.81, 1.64) 0.42 1.15 (0.81, 1.64) 0.43 KL 1.55 (1.11, 2.15) 0.01 1.54 (1.11, 2.15) 0.01 1.51 (1.08, 2.12) 0.02 NT West 1.66 (1.19, 2.33) 0.003 1.66 (1.19, 2.33) 0.003 1.65 (1.72, 2.31) 0.004 Overcrowding 3.11 (1.99, 4.87) <0.001 3.17 (2.02, 4.96) <0.001 3.15 (2.01, 4.95) <0.001 Low household income 1.38 (1.08, 1.77) 0.01 1.34 (1.04, 1.72) 0.02 1.32 (1.03, 1.70) 0.03 Pregnancy maternal smoking 7.46 (2.73, 20.39) <0.001 : Since the effect of prenatal maternal smoking on children’s ETS exposure condition could be difficult to fully ruled out, 3 different models were built: 1) by ignoring the effect of prenatal maternal smoking; 2) by adjusting to prenatal maternal smoking; 3) by excluding young children with prenatal maternal smoking П: “HK Island” stands for “Hong Kong Island”; “NT East” stands for “New Territories East”; “KL” stands for “Kowloon”; “NT West” stands for “New Territories West” https://doi.org/10.1371/journal.pone.0227733.t003 PLOS ONE | https://doi.org/10.1371/journal.pone.0227733 January 14, 2020 6 / 13 Profile of household ETS exposure in young children in Hong Kong Table 4. Multivariate analysis of risk factors for postnatal maternal smoking. Characteristics Model 1 (N = 1541) Model 2 (N = 1541) Model 3 (N = 1516) AOR 95% CI p value AOR 95% CI p value AOR 95% CI p value Never breastfeed 2.73 (1.55, 4.81) 0.001 2.45 (1.29, 4.64) 0.006 2.55 (1.29, 5.05) 0.007 Have siblings 6-18yrs 2.16 (1.20, 3.88) 0.01 Living region HK Island 1 NT East 5.15 (1.50, 17.65) 0.009 KL 3.62 (1.04, 12.63) 0.04 NT West 3.41 (0.97, 12.01) 0.06 Prenatal maternal smoking 75.9 (30.26, 190.53) <0.001 П: “HK Island” stands for “Hong Kong Island”; “NT East” stands for “New Territories East”; “KL” stands for “Kowloon”; “NT West” stands for “New Territories West” https://doi.org/10.1371/journal.pone.0227733.t004 Prenatal maternal smoking By univariate analysis (Table 2), significant factors associated with prenatal maternal smoking found were living in Kowloon and N.T West regions and lower household income. With mul- tivariate analysis, only lower household income (AOR: 3.18, 95% CI: 1.43–7.05, p<0.001) remained statistically significant in the adjusted model. Distribution of ETS exposure strata We carried out this further analysis to explore the patterns of number of household smokers and total daily cigarette consumption by household smokers (Table 5). The identified risk factors for higher exposure strata were never breastfed, prenatal maternal smoking, postnatal maternal smoking, having sibling, living in Kowloon and N.T West, overcrowding of house- hold living place and lower household income. Comparison with global and previous local data The comparison of current study data with previous global and local data was summarized in Table 6. The overall prevalence of household ETS exposure 31.5% was lower than the 2004 global average figure 40% , and the 1997 local figure 42.1% . Discussion Our study demonstrated the profile of household ETS exposure in Hong Kong healthy young children under 2 years of age. Prevalence of household ETS exposure in young children was 31.5%, prevalence of prenatal and postnatal maternal smoking was 1.6% and 3.4% respectively. Exposure was associated with never breastfed, prenatal maternal smoking, overcrowding of household living place and lower household income. Moreover, a geographical pattern of the exposure was found that young children residing in Kowloon and NT West were indepen- dently and significantly associated with household ETS exposure compared with children residing in Hong Kong Island. The identified risk factors of higher household ETS strata were quite similar with the identified risk factors of current household ETS exposure. Comparison of the results from our study with previous study demonstrated that the overall prevalence of household ETS exposure in children younger than 2 years of age was lower com- pared with the 1997 data. This might reflect the success of the tobacco control milestones among very young children. In contrast, previous studies reported the increase in the exposure prevalence of indoor ETS exposure of school-aged children in recent decades. Therefore, PLOS ONE | https://doi.org/10.1371/journal.pone.0227733 January 14, 2020 7 / 13 Profile of household ETS exposure in young children in Hong Kong Table 5. Household ETS exposure strata under different variables (N = 1541). Characteristics Number of Household smokers Number of total daily cigarettes consumption of household smoker 0 1 >1 0 1–20 >20 N (Frequency) N (Frequency) N (Frequency) P value N (Frequency) N (Frequency) N (Frequency) P value Age group 2 months 347 (72.7%) 99 (20.8%) 31 (6.5%) 0.17 347 (72.7%) 117 (24.5%) 13 (2.7%) 0.08 12 months 350 (67.0%) 137 (26.2%) 35 (6.7%) 350 (67.0%) 156 (29.9%) 16 (3.1%) 18 months 359 (66.2%) 141 (26.0%) 42 (7.7%) 359 (66.2%) 157 (29.0%) 26 (4.8%) Gender Boy 543 (69.4%) 183 (23.4%) 56 (7.2%) 0.61 543 (69.4%) 211 (27.0%) 28 (3.6%) 0.71 Girl 513 (67.6%) 194 (25.6%) 52 (6.9%) 513 (67.6%) 219 (28.9%) 27 (3.6%) Have breastfeed ever 868 (71.1%) 288 (23.6%) 64 (5.2%) <0.001 868 (71.7%) 317 (26.0%) 35 (2.9%) <0.001 Gestation> = 39 weeks 554 (65.9%) 219 (26.0%) 68 (8.1%) 0.03 554 (65.9%) 250 (29.7%) 37 (4.4%) 0.02 Prenatal maternal smoking 5 (20.0%) 7 (28.0%) 13 (52.0%) <0.001 5 (20.0%) 13 (52.0%) 7 (28.0%) <0.001 Postnatal maternal smoking 0 (0.0%) 10 (18.9%) 43 (81.1%) <0.001 0 (0.0%) 39 (73.6%) 14 (26.4%) <0.001 Not the first parity baby 493 (66.1%) 196 (26.3%) 57 (7.6%) 0.14 493 (66.1%) 218 (29.2%) 35 (4.7%) 0.03 Group care attendance 152 (77.2%) 35 (17.8%) 10 (5.1%) 0.02 152 (77.2%) 43 (21.8%) 2 (1.0%) 0.008 Living region HK Island 261 (77.2%) 69 (20.4%) 8 (2.4%) <0.001 261 (77.2%) 71 (21.0%) 6 (1.8%) <0.001 NT East 271 (71.9%) 76 (20.2%) 38 (8.8%) 271 (71.9%) 94 (24.9%) 12 (3.2%) KL 278 (64.4%) 116 (26.9%) 38 (8.8%) 278 (64.4%) 137 (31.7%) 17 (3.9%) NT West 246 (62.4%) 116 (29.4%) 32 (8.1%) 246 (62.4%) 128 (32.5%) 20 (5.1%) Overcrowding 35 (36.5%) 40 (41.7%) 21 (21.9%) <0.001 35 (36.5%) 51 (53.1%) 10 (10.4%) <0.001 Low household income 265 (59.3%) 144 (32.2%) 38 (8.5%) <0.001 265 (59.3%) 163 (36.5%) 19 (4.3%) <0.001 П: “HK Island” stands for “Hong Kong Island”; “NT East” stands for “New Territories East”; “KL” stands for “Kowloon”; “NT West” stands for “New Territories West” https://doi.org/10.1371/journal.pone.0227733.t005 our findings may actually coincide with previous evidence demonstrating the protective effect of having young children at home against household smoking. For the exposure to postnatal and prenatal maternal smoking, the prevalence in our cohort was 3.4% and 1.6% respectively, which was similar with previous figures  and much lower Table 6. Comparison with previous global and local data. Reference Oberg M et al. Leung M et al.  Mak YW et al.  Current study  Region Global Hong Kong Hong Kong Hong Kong Sampling period 2004 1997 1997 2014–2014 Participants 0–14 years old 3, 9, and 18 months 4–5 years old children 2, 12, 18 months children old children old children Prevalence of ETS exposure (%) Overall 40% 42.1% NA, only studied children 31.5% with smoking household Prenatal maternal NA 4.8% NA 1.6% smoking Postnatal maternal NA 2.8% NA 3.4% smoking Exposure strata by number of household smoker 1 household smoker NA NA 79.3% among families with 24.7% smoking household More than 1 NA NA 20.7% among families with 7.0% household smoker smoking household Exposure strata by number of total cigarettes 1–20 cigarettes/ day NA NA 96.7% among all exposed 28.3% consumption amount of household smoker children More than 20 NA NA 3.3% among all exposed 3.4% cigarettes/ day children https://doi.org/10.1371/journal.pone.0227733.t006 PLOS ONE | https://doi.org/10.1371/journal.pone.0227733 January 14, 2020 8 / 13 Profile of household ETS exposure in young children in Hong Kong than the western average figure 15% [20–21]. The prevalence of female smoking have been ris- ing in Western Europe, Australia and the United States since the 20th century, while the figure in Hong Kong is consistently low.[22–23] The low prevalence might because of the efforts of the Hong Kong government and tobacco control advocates in raising tobacco tax, publishing legislation and anti- smoking campaigns etc. in the past decade. Additionally, there is female-specific smoking cessation hotline service in Hong Kong, which is provided by The University of Hong Kong. Maternal smoking matters more than other household smoking despite the generally lower prevalence. Smoking mothers smoke more at home while other household smokers might smoke at work, additionally, mothers comparatively spend more time with the infants, and stay nearer with the baby, which let the exposure condition worse.  There is a lack of population-based smoking cessation interventions targeting Chinese woman smokers. A recent local study demonstrated the effectiveness of one cessation counselling intervention which was given by trained counsellor according to the stage of readi- ness to quit. Continued effort for female-specific tobacco control is needed. Scholars sug- gested that pregnancy is an opportune time to provide smoking cessation interventions for female smokers and their partners with an emphasis on the maintenance of post-partum smoking abstinence. Smoking cessation strategies could be more early accepted by smok- ers at the health care circumstances , and being parents might further motivate the smoker to change. There is a need to further develop female-specific smoking cessation intervention. The construction of the exposure extent in our study was quite comparable to the data reported by Mak YW et al. , The construction did not significantly change over these two decades. It seems our tobacco control effort has not yet effectively relieved the exposure sever- ity of those exposed young children. The identified demographics or clinical risk factors can be grouped into three dimensionali- ties: (1) Lower SES which includes overcrowding of household living area and lower household income; (2) Unfavourable maternal behaviors which include no practice of breastfeeding ever and prenatal maternal smoking; (3) Geographical factor which was residing in Kowloon and NT West regions. In our cohort, young children with lower family SES such as lower household income and overcrowding of household living area was associated with the ETS exposure. Similar observa- tions were reported in several previous studies.[10, 24, 30–31] The underlying mechanism have not been fully investigated and explained. It could be complex and multifactorial. It is well recognized that lower family SES is linked to a series of disadvantaged health conditions and poor health-related behaviours such as tobacco use.[32–34] Families with lower SES could have lower parental education, disadvantaged growing environment for the children and inad- equate parental health-related beliefs and behaviours.[35–36] The identified association between overcrowding and ETS exposure has particular implications as Hong Kong is a densely populated city. Overcrowding of the living area is not only a surrogate of lower SES of the family, it may also mean the children are more susceptible to the tobacco exposure because of the overcrowded environment. Therefore, socially deprived children are at a higher risk of adverse long term health because of higher ETS exposure, worse health-related behaviours, and less access to medical service and optimal nutrition. Our study found that children with smoking mother had a much higher risk of household ETS exposure than those with non-smoking mother. This result was in line with the major finding of previous review study that maternal smoking is a strong predictor of children’s ETS exposure at home. Smoking mother could have weaker consciousness of preventing their children from ETS exposure compared with non-smoking ones. Thus, though the preva- lence of maternal smoking is relatively low in Hong Kong, significant attention should be PLOS ONE | https://doi.org/10.1371/journal.pone.0227733 January 14, 2020 9 / 13 Profile of household ETS exposure in young children in Hong Kong given to this group of smokers. It was found that negative emotions and stress were important factors in both smoking initiation and continued tobacco use among female smokers in Hong Kong. Scholars highlighted that it is vital that healthcare professionals should focus on helping female smokers understand the health hazards of smoking, and counselling them about alternative strategies for coping with negative emotions and stress. No practice of breastfeeding ever was found to be associated with the exposure in a previous local study. Previous research demonstrated that ETS may affect the maternal milk pro- duction, and the alternative explanation was lack of breastfeeding practice is an indicator of low SES, which is closely linked to smoking behaviours and less favourable health practice. Understanding the geographical varying pattern of one disease or one health-related condi- tion could be very significant for the profile draw and related health promotion planning.[39– 40] We found the exposure distribution was not equal among the 4 main regions in Hong Kong. The exposure risk was especially high in Kowloon and NT West regions, the need of ETS exposure control in young children of these regions should be highlighted. The reasons for this geographical pattern are not fully understood and could be complex. One important potential reason is the variation in the SES profile among these four regions [41–42], which in other words, the living region may be a surrogate of family SES, and therefore associated with ETS exposure. A classical English study found the influence of community levels of smoking within towns on passive exposure status of children aged 5–7 years old. However our local data of region-specific smoking prevalence were not available to confirm such postulation. Our study was territory representative, and we provided precious information on children under 2 years of age. However, there remained some limitations. First, for ETS exposure mea- sures, objective measurement such as urinary cotinine and salivary cotinine was not adopted, which could be a shared limitation with the previous local study. Moreover, socially desir- ability bias may reduce the validity of maternal self-reporting on smoking habit. Some groups of people are more reluctant than others to disclose their smoking status and exposure to tobacco, especially pregnant women and parents of young children, whose smoking is often regarded as socially unacceptable. However the general validity of parental self-reported smok- ing condition in Hong Kong has been proven to be valid and reliable previously. Second, the questionnaire we used to collect data was not validated. There is no validated international tool for ETS exposure profile study so far, and our study methods especially the variable measures are similar with previous studies [7, 15]. Third, as the current study used secondary data from our previous surveillance study, which was designed with different objectives, it might render non-specific results under the problem with some confounding factors not evaluated. Information about parental education and occupation was not collected, while these two factors could be important aspects of family SES. The type of tobacco product consumption was not recorded in our questionnaire, how- ever cigarettes were still reported to be the dominant type of tobacco consumption among Hong Kong smokers according to the latest Hong Kong Government’s Thematic Household Survey Report (daily smokers by consumed products daily: cigarettes- 99.1% of all daily smok- ers; electronic cigarettes- 0.9% of all daily smokers; other types-0.6% of all daily smokers). Future studies should be done to evaluate children’s exposure to different kinds of tobacco products. And data of more years, with seasonal trend is warranted to gain a deeper under- standing of the epidemiology profile of ETS exposure among young children. Finally, the previous surveillance study is limited by a moderate response rate about 60%, the major reason was parental refusal to consent for nasopharyngeal swab. As household ETS exposure profile has never been comprehensively drawn in Hong Kong young children previously, and the data of our previous surveillance study could be regarded as representative with its community based nature and territory-wide recruitment. Thus, PLOS ONE | https://doi.org/10.1371/journal.pone.0227733 January 14, 2020 10 / 13 Profile of household ETS exposure in young children in Hong Kong despite the limitations, current study could still provide precious information of household ETS exposure profile in Hong Kong young children. Conclusions Findings of this study add to our knowledge that household ETS exposure is prevalent among Hong Kong young children, especially those with smoking mother and those in socially deprived families. More resources should be put to reduce the ETS exposure in our young chil- dren, particularly those from disadvantaged families. Early identification, early intervention. Author Contributions Conceptualization: Kate Ching Ching Chan. Data curation: Siyu Dai, Kate Ching Ching Chan. Formal analysis: Siyu Dai, Kate Ching Ching Chan. Investigation: Siyu Dai, Kate Ching Ching Chan. Methodology: Siyu Dai, Kate Ching Ching Chan. Project administration: Kate Ching Ching Chan. Resources: Kate Ching Ching Chan. Supervision: Kate Ching Ching Chan. Writing – original draft: Siyu Dai. Writing – review & editing: Siyu Dai, Kate Ching Ching Chan. References 1. US Department of Health and Human Services. 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