In this study, we found that current smoking and ≥ 4 pack-years of smoking were independently positively related to wheeze. These findings are in accordance with results from previous cross-sectional studies that reported a positive association between ever smoking and the prevalence of wheeze in adults [12
]. A population-based cross-sectional study in Denmark showed a significantly positive dose-response relationship between the amount of tobacco used per day and the prevalence of wheeze [18
]. In a cross-sectional study among male farmers, current smoking was associated with an increased prevalence of wheeze [12
]. Between asthma and smoking status, on the other hand, we found no association. The relationship between smoking status and asthma in adults remains controversial because the results of previous studies have been inconclusive; some studies have found an increased risk of asthma in relation to smoking [2
], and others have failed to find such a correlation [2
]. In a population-based cohort study among men in Japan, current smokers at baseline had a significantly increased risk of asthma compared with those who had never smoked, though no such statistically significant association was observed among women [2
]. A cross-sectional study of pregnant Japanese women showed that current smoking was associated with an increased prevalence of asthma after the age of 18 years [4
]. In a US cross-sectional study, former smoking was positively associated with the prevalence of asthma among males, whereas no association was observed between current smoking and the prevalence of asthma [6
]. Among females in the same study, however, there was no association between former or current smoking and the prevalence of asthma [6
The lack of association between smoking status and asthma observed in the present study might be explained by the "healthy smoker effect" [28
]. Current smoking might be associated with better health, as healthier individuals are better able to tolerate the negative health effects of smoking. Thus, current smokers may be less likely to have or develop asthma. Moreover, the fact that there were only 38 females with asthma who had ever smoked in our study might have led to low statistical power for detecting a possible association between ever smoking and asthma.
Previous studies on smoking and eczema and/or rhinoconjunctivitis in adults have been limited and have provided contradictory results. Our results regarding eczema are in agreement with those of a previously cited Japanese cross-sectional study demonstrating no association between cigarette smoking and atopic eczema [4
]. A case-control study in Taiwan, in contrast, found that smoking was associated with increased risk of adult-onset atopic eczema [11
]. No association between smoking status and rhinoconjunctivitis was found in the present study. The same conclusion has also been reached in some previous studies [4
], while other previous studies have observed inverse associations between smoking and rhinitis, hay fever, or cedar pollinosis [7
]. It should be noted that the various studies mentioned here have used different definitions of outcomes, study populations and designs, smoking exposure assessment methods, and confounding factors, thus limiting the feasibility of inter-study comparisons.
In keeping with other previous studies [14
], we found that ETS exposure was associated with an increased prevalence of wheeze. A cross-sectional study in Denmark has shown that daily ETS exposure for 5 or more hours is positively associated with the prevalence of wheeze among those who had never smoked [18
]. A positive association between exposure to ETS and the prevalence of positive history of wheezing was found among male police officers in Hong Kong [15
]. In contrast, a prospective study of subjects aged 15 to 40 years of age who had never smoked showed no association between ETS exposure and the risk of wheeze [13
In our study, we found that the positive association between wheeze and exposure to ETS was more evident when exposure occurred at home rather than at work. A cross-sectional study among Italian women who had never smoked, on the other hand, revealed that ETS exposure at work was significantly positively associated with wheeze, whereas there was no association between wheeze and exposure to ETS due to their husbands' smoking [14
]. A cross-sectional study performed in Finland, Estonia, and Sweden also showed a significant positive association between ETS exposure outside of the home, but not at home, and the prevalence of wheeze [17
]. These results are at variance with our findings. In Japan, a law that restricts smoking in public places came into effect in 2004. Since then, awareness about the effects of ETS has been increasing, and many workplaces now have smoking restrictions or bans. In recent years, exposure to ETS at work is probably becoming less common. In the present study, exposure to ETS both at home and at work yielded a higher OR for wheeze than did exposure in only one location. Subjects who are exposed in both places are likely to receive a higher level of ETS exposure compared to those who are exposed in only one place.
A limited number of studies have addressed the effect of ETS exposure on asthma in adults [14
]. A population-based incident case-control study in Finland found that both workplace and home ETS exposure during the past 12 months were significantly related to an increased risk of new development of asthma in 21- to 63-year-old adults [16
]. A significant positive association between ETS exposure and asthma has also been reported in other cross-sectional studies [14
]. In contrast, our results failed to reveal a positive association between ETS exposure and asthma. This may be due to a tendency among patients with asthma to voluntarily avoid exposure to ETS. Also, family members in a household that includes persons with asthma and coworkers of persons with asthma may be more likely to stop or curtail their smoking. In addition, it may be difficult to detect a clear positive association between ETS exposure and asthma in a population containing few patients with asthma.
To our knowledge, there have been two epidemiological studies addressing the association between ETS exposure and eczema in adults [4
]. A case-control study in Taiwan showed that exposure to ETS at home prior to 20 years of age was positively associated with an increased risk of adult-onset atopic eczema among persons who had never smoked. A previously cited cross-sectional study among pregnant Japanese women found no association between ETS exposure at home or at work and the prevalence of eczema [4
]. The findings of the latter study are in agreement with our own.
We found that exposure to ETS only at home and both at home and at work, but not only at work, was associated with an increased prevalence of rhinoconjunctivitis. Consistent with our results, several previously mentioned cross-sectional studies have found positive associations between ETS exposure and rhinoconjunctivitis [4
] or rhinitis symptoms [8
]. A cross-sectional study among Italian women who had never smoked showed a positive association between exposure to smoke from their husbands' smoking and rhinoconjunctivitis; this association was more evident than the relationship between ETS at work and rhinoconjunctivitis [14
]. Our results were at variance with those of other studies that have found no association between ETS exposure and rhinitis [20
This study had certain methodological strengths. Study subjects were homogeneous in that all were pregnant, which likely reduces the potential for confounding resulting from unmeasured factors related to pregnancy. Definitions of wheeze and asthma were based on the questions in the European Community Respiratory Health Survey. The prevalence values for eczema and rhinoconjunctivitis were estimated based on the questions in the International Study of Asthma and Allergies in Childhood, although validation tests for these questions have not been performed for Japanese adults. Using a standardized methodology allows for comparisons with results from other epidemiological studies using the same methodology. Potential confounders were adjusted for with extensive data. However, it is possible that our results remain confounded by other potentially important factors, such as aeroallergens and air pollution.
There were certain limitations of this study as well. The participation rate could not be calculated because the exact number of eligible pregnant women who were provided with a set of leaflets explaining the KOMCHS, an application form, and a self-addressed and stamped return envelope by the 423 collaborating obstetric hospitals is not available. We were not able to assess the differences between participants and non-participants, because information on personal characteristics such as age, socioeconomic status, and history of allergic disorders among non-participants is not available. Our subjects were probably not a representative sample of Japanese women in the general population, given that all were pregnant women. In addition, educational levels in the current study population were higher than in the general population. According to the 2000 population census of Japan, the proportions of women aged 30 to 34 years in Fukuoka Prefecture with < 13, 13-14, ≥ 15, and an unknown number of years of education were 52.0%, 31.5%, 11.8%, and 4.8%, respectively [30
]. The corresponding figures for the current study were 24.6%, 33.1%, 42.4%, and 0.0%, respectively. The present population might therefore have had a greater awareness about health than the general population. Nevertheless, cigarette-smoking status in our study population was likely to be similar to that in the general population. In the National Health and Nutrition Survey in Japan in 2007, the percentages of currently-smoking, formerly-smoking, and non-smoking women aged 30 to 39 years were 17.2%, 11.4%, and 71.4%, respectively, although data specific to pregnant women were not available [31
]. The corresponding figures for the present subjects were 3.3%, 28.9%, and 67.8%, respectively. Many of the pregnant women in the current study may have given up smoking upon becoming pregnant, thus increasing the percentage of former smokers.
Other limitations may also have influenced the interpretation of the current results. As this study is cross-sectional, the temporal nature of the association between smoking status and ETS exposure and allergic diseases could not be examined. Assessment of exposure was based on the subjects' questionnaire responses and was not validated by objective measurements, such as salivary, serum, or hair cotinine levels. Using questionnaires may result in misclassification due to recall bias. If present, however, this bias would tend to reduce the observed association between smoking and allergic diseases. We did not measure exposure to ETS at social settings outside the home and work. Given the difficulties in accurately measuring ETS exposure, the association between ETS exposure and allergic diseases observed here is likely to be an underestimate.
The interface between allergy/immunology and pregnancy should be discussed, as it may have an influence on the association of interest. It has been suggested that pregnancy involves a shift to the Th2 side of the immune response [32
], although Chaouat et al. have pointed out the importance of the role of natural killer cells and IL-12, IL-15, and IL-18 tripods in successful or failed pregnancies in humans beyond the Th1/Th2 paradigm [33
]. The hormonal changes in pregnancy are often invoked to explain the apparent association between rhinitis symptoms and pregnancy. However, rhinitis ascribed solely to pregnancy may not be a distinct entity because most pregnant women do not have significant nasal symptoms [32