The present study examined the associations of education and income with SHS exposure among pregnant women in Japan. About one-third of pregnant women were exposed to SHS. Pregnant women with lower educational attainment and/or lower equivalent household income had significantly higher risks of SHS exposure than pregnant women with higher education and/or income. These associations did not differ between non-working women and working women.
Lower education was associated with an increased risk of SHS exposure during pregnancy, independent of equivalent household income. There are several possible explanations for this association. First, education conveys factual health-related knowledge [23], and lower educated people working jobs offering few opportunities for learning may have limited knowledge of the harmful effects of unhealthy behaviors [24]. WHO guidelines recommend that health care providers should provide pregnant women with advice and information about the risks of SHS exposure [25], reflecting the importance of knowledge of the risks of SHS exposure among pregnant women. Second, education raises cognitive skills that foster health-promoting decisions [23, 26]. Hence, education may increase individuals’ understanding of the negative effects of SHS exposure and build their capacity to manage their own SHS exposure [24]. Women’s capacity to manage their SHS exposure is affected by the intersecting influences of gender roles and power [27]. A small number of clinical interventions have suggested that providing brief advice or counseling to non-smoking pregnant women may reduce their SHS exposure [28]. Third, education increases the chances of having a well-educated partner [23]. In Japan, men with lower education have been found to have a higher risk of current smoking, especially among the young population [29], and SHS from partners is a major source of exposure for non-smoking women [6]. However, the present study showed that women’s education was associated with SHS exposure after adjusting for partner’s education. Fourth, social networks, which combine individuals’ resources with those of others [30], may also partially explain the observed association. Having a higher level of education increases the chances of associating with other highly educated individuals, and social networks made up of such individuals communicate health-promoting behaviors, thus widening disparities by educational attainment [23, 24, 30]. Higher levels of education also correspond to a greater likelihood of friends emulating each other’s smoking behavior [31], which could lead to an association between education and SHS exposure.
We also found that lower equivalent household income was associated with an increased risk of SHS exposure during pregnancy, independent of education. This association was observed among non-working women as well as among working women. This result suggests that the partners and other household members of pregnant women with lower equivalent household income might be more likely to smoke, compared with those with higher equivalent household income. The association between lower income and smoking has been consistently shown [32], including among community-dwelling people in Japan [33]. One possible mechanism by which lower income has been hypothesized to be associated with smoking is that people with lower income are more susceptible to the tobacco industry’s marketing strategy of glamorizing smoking by associating it with wealth and success [32]. Banning advertising that gives the impression that smoking is attractive might be useful, although no studies have examined the impact of advertising bans on the association between income and smoking [34]. A higher prevalence of smoking among lower-income individuals may also be explained by several other factors such as the frequent normalization of smoking and earlier smoking initiation, less concern about harm caused by smoking, poorer access to smoking cessation resources, and more difficulty with successfully quitting among this group [32, 34]. Adopting many health behaviors does not require money, but paying for smoking cessation aids such as individual counseling and medications can help individuals to realize their desire to quit [23, 24]. According to WHO guidelines, health care providers should give partners and other household members advice and information about the risks to pregnant women from SHS exposure and, if possible, also provide them with cessation support [25]. Effective interventions to create smoke-free homes alongside smoking cessation for them, perhaps involving financial incentives, need to be developed [35, 36]. On the other hand, we also found that working women, who had higher equivalent household income, were more likely to be exposed to SHS than were non-working women. This suggests that measures against SHS exposure at work, such as totally smoke-free workplace policies, should also be implemented [37].
To our knowledge, this is the first study to demonstrate the associations of education and income with SHS exposure during pregnancy in Japan. By examining education and income simultaneously, we showed independent associations between SHS exposure and both education and income, which have different meanings in society [17]. Although Japan lagged behind Western countries in terms of preventive measures against SHS exposure [16], the Japanese government approved its first national smoking ban for public facilities in July 2018, and this ordinance has come into force in April 2020. This measure is expected to be beneficial for reducing SHS exposure for non-smoking pregnant women [38]. Nevertheless, the association of education and income with SHS exposure in Japan should continue to be monitored after the introduction of this national smoking ban, because smaller magnitudes of declines in SHS exposure for lower socioeconomic groups than for higher socioeconomic groups have been found to increase the disparity in SHS exposure between these groups in Western countries [14].
The present study has several limitations. First, about one-third of the analytic women had missing values in their educational attainment. As they were more likely to be exposed to SHS than those with university education or higher, the observed associations in the present study may have underestimated the true associations between education and SHS exposure. Second, this study was conducted at one of 47 prefectures in Japan; therefore, the generalizability of the present study is limited. However, there is little difference between the prevalence of smoking in Miyagi Prefecture and that reported in a national survey in 2016: 33.4% among men and 9.7% among women in Miyagi Prefecture and 31.1% among men and 9.5% among women in the national survey [39]. Finally, SHS exposure was self-reported. It is possible that some women underreported active smoking and also SHS exposure because active smoking has become more socially unacceptable [40, 41], again leading to an underestimation of the association.