INTRODUCTION

Secondhand smoke exposure (SHSE) increases risks to many diseases, such as cardiovascular diseases and cancers in adults, and respiratory diseases and poor mental health outcomes in both children and adults¹. Globally, SHSE causes an estimated 0.88 million deaths, of which 75% are non-smokers². Hence, combating SHSE, particularly in workplaces and public spaces, is a key component of worldwide tobacco control policies. The World Health Organization (WHO) Framework Convention on Tobacco Control (FCTC) requires member states to enact comprehensive measures that protect the general population from SHSE at work (SHSE_work) and public spaces³, such as legislating smoking bans in indoor workplaces, indoor public places, and public transport. This is necessary as workplaces without smoking restrictions increase the risk to SHSE, with some workplaces constituting higher risk than others^4,5. Previous studies have reported that some groups of workers, such as males, ethnic minorities, younger workers, and those with lower education level^4,5 have greater odds of SHSE_work.

FCTC campaigns actively advocate for the adoption of smoke-free homes, as research shows children are at greater risk to SHSE than adults⁶, and more than 40% of youths globally are exposed to secondhand smoke (SHS) at home and have parents who smoke at home⁷. Risk of SHSE increases among children living with a smoker, and they have poorer health outcomes both during childhood and in the future as adults¹. In particular, children living with a father or sibling who smoked at home is associated with earlier and/or greater odds of smoking initiation and having positive attitudes towards smoking and other tobacco-related health outcomes^8,9. This suggests childhood SHSE and identity of the smoker at home may play a role in other future tobacco-related situations and outcomes, such as willingness to be exposed to SHS at work in adulthood. In addition, studies have shown that among people who are exposed to SHS at home, a vast majority are also exposed to SHS at work¹⁰. Overtime, SHSE at home (SHSE_home) and at work (SHSE_work) have decreased in tandem¹¹. This suggests an association between SHSE at different physical settings.

To the best of our knowledge, the relationship between SHSE in different temporal and physical settings, especially within multi-ethnic Asian populations, are not fully understood. Singapore is a highly urbanized city-state in Asia with strict tobacco regulations, and had managed to lower smoking prevalence from almost 25% in the 1970s¹² to 10% today¹³. Simultaneously, there is evidence that public perception and attitudes towards smoking has become less positive¹⁴, suggesting a shift towards social norms more hostile towards tobacco use, which is also observed in other developed economies^15,16. As other Asian societies also become increasingly urban and cosmopolitan, Singapore’s multi-ethnic population provides an opportunity for this investigation. Studies from Western countries may be limited, given the need to contextualize the impact of culture and social norms on tobacco-related outcomes and behaviors in different societies¹⁷. We investigate the associations of childhood (age ≤18 years) and current SHSE_home with current SHSE_work, among healthy adults who are never smokers, in Singapore, and whether these associations could be further explained by the sources of daily smokers at home.

METHODS

RESULTS

Figure 1 is the flow chart of participants’ selection that included 925 never smokers, employed, and healthy participants with complete information used in this analysis. Table 1 presents the characteristics of the participants. Overall, the majority of participants were female (66.8%), of Chinese ethnicity (45.3%), had pre-university education (37.0%), lived in HDB 4-room flats (40.3%), and were married (68.1%). The average age of participants was 38.7 years (SD=10.5). In total, 144 (15.6%) were exposed to SHS at work, while 217 (23.5%) were currently exposed to SHS at home; and among them the two most common sources of SHSE_home were the spouse (47.5%) and parent/in-law (28.1%). Similarly, 483 participants (52.2%) were exposed to daily SHSE_home during childhood. Among those respondents, the two most common sources of exposure were the father (86.7%) and siblings (21.9%). Having at least one sibling who smoked daily at home during childhood was significantly associated with current SHSE_work. For current SHSE_home, having at least one parent or parent in-law and at least one other source who smoked daily at home was significantly associated with reporting current SHSE_work. Participants who were younger, males, Indians, with pre-university education, never married, those who reported a lower consumption of fresh fruits, lower HDL and lower HbA1c, were more likely to report current SHSE_work.

Figure 1

Study participants selection flow chart

Figure 2 presents the associations between characteristics of SHSE_home and current SHSE_work, after adjusting for the covariates that enter the model through model building (i.e. age, gender, education level, ethnicity, marital status, ever consumed an alcohol beverage, consumption levels of fruits, and HbA1c levels). Association for childhood SHSE_home (Model 1: AOR=1.43; 95% CI: 0.93–2.19) and current SHSE_home (Model 1: AOR=2.05; 95% CI: 1.28–3.29) with current SHSE_work were both positive, but only the latter was statistically significant. When sources of daily smokers at home were entered into Model 1 through model building, the association for childhood SHSE_home remained non-significant (Model 2: AOR=0.54; 95% CI: 0.20–1.44) while the association for current SHSE_home (Model 2: AOR=1.78; 95% CI: 1.08–2.95) remained significant and positive.

Figure 2

Associations between characteristics of secondhand smoke exposure (SHSE) at home (SHSE_home) and current SHSE at work (SHSE_work)

Two out of the five sources of exposure to SHS during childhood were then entered into the model (i.e. father and sibling). Specifically, living with a father (Model 2: AOR=2.30; 95% CI: 0.94–5.64) and at least one sibling (Model 2: AOR=2.97; 95% CI: 1.55–5.68) who smoked daily at home during childhood was positively associated with current SHSE_work, where the latter was significant. Only one out of the four sources from current enter the model (i.e. child). Specifically, living with at least one child who currently smokes daily at home was associated with current SHSE_work (Model 2: AOR=3.56; 95% CI: 1.19–10.64). We did not find a significant interaction between childhood and current SHSE_home in Model 1 (p=0.914) and Model 2 (p=0.755). Similarly, interactions between the two sources of exposure from childhood that enter the model were not significant (p=0.626).

Based on the sources that enter Model 2, we classified participants into subgroups that contain SHSE_home profiles with the same adjusted odds ratio. Since the interaction analyses suggest the associations of childhood and current SHSE_home are independent, Table 2 presents the AORs of subgroups at each temporal setting with the reference subgroup corresponding to no SHSE_home in the same temporal setting. The AOR between the subgroup with no childhood SHSE_home and each of the subgroups experiencing childhood SHSE_home were not significantly different except for the subgroup with combinations of SHS sources that include both father and sibling as daily smokers at home (Model 2: AOR=3.70; 95% CI: 1.88–7.30). This subgroup contains combinations, such as father and sibling only, and father and sibling with one or more of the following three sources of exposure: mother, grandparent, and other.

For current SHSE_home, the subgroup experiencing current SHSE_home with combinations of sources that exclude children as the current daily smoker at home (i.e. combinations with one or more of these three sources: spouse, parent/in-law, and other) had significantly higher odds of reporting exposure than the subgroup with no current SHSE_home (Model 2: AOR=1.78; 95% CI: 1.08–2.95). When the combinations of sources include also children in the home, the odds further increased (Model 2: AOR=6.35; 95% CI: 2.19–18.40). Similar results were obtained when we represented these subgroups of SHSE_home profiles from two temporal settings as two categorical variables and include them as predictors into one model with adjustment for the same covariates as Model 2 (Model 3).

DISCUSSION

We investigate associations of childhood SHSE_home and current SHSE_home with current SHSE_work, among healthy and employed adults who were never smokers, in Singapore. Although it is only the association between current SHSE_home and SHSE_work that is significant and positive, the associations of childhood and current SHSE_home with SHSE_work are further explained by the sources of daily smokers at home. For childhood SHSE_home, the two sources were the father and siblings. Although both sources increase the odds of current SHSE_work, only exposure via siblings was significant. However, when compared to no childhood SHSE_home, the odds were only significantly higher among exposed participants who lived with their fathers and siblings who smoked daily at home during their childhood. For current SHSE_home, regardless of the identity of daily smokers at home, the odds among the exposed is significantly higher than the odds among the unexposed. But participants living with their children who are current daily smokers at home led to significantly elevated odds.

Our results highlight the need to consider sources of smokers at home to gain deeper insights into the impact of SHSE_home on current SHSE_work, indicating the importance of familial relationships and dynamics. Considering only the occurrence of SHSE without factoring in the other underlying characteristics of SHSE could prematurely lead to false negative findings for childhood SHSE_home. Existing studies found social relationships are closely linked with tobacco use in general²¹, and hence could potentially be linked with other tobacco-related issues, such as SHSE in different physical settings and throughout one’s life-course. Parents’, siblings’ and peers’ attitudes and behaviors play significant roles in shaping children’s and adolescents’ attitudes towards tobacco smoking^8,9, and evidence supports that the roles played by these different social relations are independent. Similarly, our results suggest that although living with either father or sibling who smoked increase the odds of current SHSE_work independently, it is the sibling identity that may play a greater role.

When the sources of childhood daily smokers at home include both father and sibling, the odds of current SHSE_work is significantly higher than those with no childhood SHSE_home. The smoking behavior of father and sibling(s) at home could have established social norms by signaling that smoking and exposing others to SHS is acceptable¹⁶. It has been posited that social norms are one of the important processes through which one’s environment can affect behaviours²². Within Asian households²³, fathers are, generally, in a position of authority, and siblings could be viewed as both family members and peers to the individual^8,24. Their collective behaviors at home could enhance the prominence of the normative behaviour²⁵ of smoking in the presence of others. When such behaviors are more commonly observed, individuals could be more susceptible to other factors that also affect tobacco-related health behaviors.

Efforts to promote smoke-free homes by focusing on the household level have been proven to be effective in lowering SHSE in children²⁶. Given the potential impact of various sources of smokers during childhood on tobacco-related behavior and health outcomes in adulthood, efforts to alleviate SHSE_home should take the family context into consideration, ensuring the involvement of family members who are daily smokers at home. This is particularly pertinent during one’s childhood, as children typically have limited power to change the behavior of adults they are living with.

In contrast to findings in childhood SHSE_home, the association of current SHSE_home was positive and significant. This could suggest current norms in the home environment is spread to the workplace or vice-versa²⁷, given that SHSE_home and SHSE_work are happening at the same period. The prevailing social and environmental norms are likely acting on both the home and workplace setting^28,29, hence smoke-free policies have the potential to enhance changes in social norms surrounding tobacco-related behavior at the broader, community level³⁰. When the current daily smokers at home include the child identity, the odds of current SHSE_work are elevated more than for the subgroups that include the other current sources only. Prior evidence has shown that parents of smokers tend to have non-negative attitudes towards smoking^31,32.

Overall, this study implies that different smoker sources at different phases of an individual’s life may have an impact on accepting, and hence reporting SHSE_work. We examined different sources at different temporal settings, reflecting the changing life-course of the participant. Although we can distinguish between the specific parent who smoked daily at home during childhood, we are unable to do so for current SHSE_home, as they are grouped with the in-laws in the survey. The changing impact of family dynamics³³ on SHSE_home as one ages could be further studied using qualitative methods.

CONCLUSIONS

In general, our study found positive associations between SHSE_home and SHSE_work, with the sources of daily smokers at home playing an important role among working adults who are healthy, never smokers, in Singapore. Current SHSE_home is a risk factor for current SHSE_work and the subgroup with the highest risk consists of individuals who are currently living with their children who smoke daily at home. Childhood SHSE_home may have an impact on current SHSE_work, particularly among individuals who lived with their fathers and siblings who smoked daily at home, during their childhood. These results provide additional evidence on the importance of living in a smoke-free home environment because it reduces exposure to SHS at work. Deformalizing smoking in the presence of others at home, in different temporal phases of an individual’s life-course, could lead to meaningful changes in the individual’s exposure to SHS at work.