Table 2 shows study characteristics, frequency of exposure to passive smoking, and prevalences of respiratory and allergic disorders. Bronchitis was reported most frequently, prevalences exceeding 30% in four of the studies. Hay fever and “woken by wheeze in the last 12 months” were reported least frequently, with prevalences under 10% in most studies.
Table 2Study characteristics, prevalences of children's passive smoking and respiratory symptoms
Numbers living with a reported current smoker ranged from 45.8% in Germany to 70.9% in Bulgaria. Passive smoking during the first two years of life ranged from 18.7% in Germany (first year of life only), then from 46.3% in Russia, to 74.8% in Poland. Reported prenatal exposure tended to be rarer in Eastern countries and ranged from 4.4% in Russia to 28.7% in Holland.
The exposures were correlated, as anticipated. Coefficients of correlation (r) between current exposure and exposure during the first two years ranged from 0.22 in Germany to 0.75 in the Czech Republic. Between prenatal exposure and exposure in the first two years, r ranged from 0.16 in Bulgaria to 0.42 in North America and the Netherlands, and between current and prenatal exposure, r ranged from 0.14 in Bulgaria to 0.42 in North America.
Confounder adjusted results
Wheeze and nocturnal cough were associated with all three smoking measures, with harmful effects seen in nearly all studies (fig 1). Mean odds ratios for wheeze ranged from 1.17 (95% confidence interval (CI) 1.02 to 1.33) for the effect of smoking during the first two years, to 1.25 (95% CI 1.14 to 1.37) for smoking during pregnancy (table 3).
Figure 1Forest plots of study‐specific odds ratios, and mean odds ratios, for effects of “current smoker” (top row), “smoking during pregnancy” (middle row) and “smoking during first two years of (more ...)
Table 3Mean odds ratios (95% confidence intervals) for associations between smoking exposures and each outcome
There was evidence of heterogeneity (within a strong predominance of positive results) among results for both current smoking and smoking during the first two years. Mean odds of nocturnal cough were raised by around 12% for all three smoking variables, with no evidence of heterogeneity.
Asthma was clearly related to smoking during pregnancy, with a mean odds ratio of 1.18 (95% CI 1.08 to 1.28). Odds ratios for the other periods of exposure were also above one in nine of the 12 countries, with no evidence of heterogeneity.
Associations between current smoking and “woken by wheeze” were predominantly positive, with a mean odds ratio of 1.12 (95% CI 0.99 to 1.25). For the other two smoking measures results were less clear—mean associations were positive, but with considerable heterogeneity.
Bronchitis was related to current smoking, and to smoking during the first two years, with mean odds ratios of about 1.10 (fig 2). Associations of bronchitis with smoking during pregnancy were heterogeneous, within no clear pattern.
Figure 2Forest plots of study‐specific odds ratios, and mean odds ratios, for the effects of “current smoker” (top row), “smoking during pregnancy” (middle row), and “smoking during first two years (more ...)
For all three exposures, effect estimates for hay fever were predominantly negative, with Russia the main exception (fig 2). Mean odds ratios for both the effect of smoking during the first two years and of smoking during pregnancy were around 0.90. The Russian estimate for current smoking was large, giving rise to some between study heterogeneity (p
Effect estimates of current smoking on “sensitivity to inhaled allergens” showed some apparently protective effect, with mean odds ratio of 0.96 (95% CI 0.91 to 1.02). For smoking during pregnancy, and smoking during the first two years, estimates tended to be small and mean odds ratios essentially null.
Mean effect estimates for morning cough were positive for all exposures, up to 1.12 (95% CI 0.99 to 1.27) for smoking during the first two years, but with heterogeneity between studies within unclear patterns of association.
Few consistent associations were seen between study specific results and potential sources of heterogeneity tested, and differences between groups of studies were generally small. Odds ratios tended to be inversely related to the proportion of younger children in the study. For smoking during pregnancy, estimates were higher in the later studies for most outcomes, and also in the studies with lower response rates (categorisations which overlapped considerably).
Independent effects of the exposure periods
For most of the outcomes strongly associated with smoking during pregnancy (wheeze, asthma, hay fever), associations remained robust to all adjustments. For nocturnal cough, the association became of borderline statistical significance. Mean, fully‐adjusted odds ratios were 0.89 (95% CI 0.80 to 1.00) for hay fever, and around 1.15 for these other four outcomes (table 3).
Of those outcomes most strongly associated with smoking during the first two years of life (wheeze, asthma, bronchitis, nocturnal cough, hay fever), associations with bronchitis, wheeze and nocturnal cough were independent of smoking during pregnancy (data not shown), but were weakened in the fully adjusted model.
For current smoking, associations with bronchitis and “woken by wheeze” remained robust after all adjustments (table 3). Those for wheeze and nocturnal cough were independent of smoking during pregnancy (data not shown), and odds ratios remained raised after all adjustments, albeit not statistically significant. The mean odds ratio for asthma was considerably reduced by these adjustments.
There was no consistent effect modification by age within the individual studies.
In more crowded households than less crowded ones mean effects were greater for wheeze, bronchitis, and both coughs, but lower for asthma and sensitivity to inhaled allergens. The strongest interaction was found for morning cough: a mean odds ratio 17% higher (95% CI −1% to 39%) in crowded households than in less crowded ones.
In those studies for which parental illness data were available, parental allergy did not confound associations between child's “sensitivity to inhaled allergens” and parental smoking, nor parental lung disease for nocturnal cough, nor parental asthma for child's asthma. For hay fever, some odds ratios changed a little towards the null on adjusting for parental allergy, and similarly for bronchitis and morning cough, on adjusting for parental lung disease (data not shown). Neither attendance at kindergarten nor breastfeeding had any confounding effect.
Child's sex was not a strong modifier of effects of maternal smoking in pregnancy. Contrary to Alati's findings,31
the mean odds ratio for asthma was higher in boys: 1.24 (95% CI 1.12 to 1.38) versus 1.10 (95% CI 0.95 to 1.27) in girls. However, outcome‐ and country‐specific interactions were generally small and varied in direction. There was little evidence of breastfeeding modifying the effect of either postnatal exposure, particularly for smoking in the first two years of life. For current exposure, the strongest evidence was seen for wheeze where, in six of nine countries, stronger effects of exposure were seen among children who were breastfed (the great majority of the children) than in those who were not. (This pattern was little changed by controlling for other smoking measures, or the interaction between breastfeeding and smoking in the first two years of life.) The interaction was statistically significant in Bulgaria and Germany, and the mean of the nine interaction terms was also statistically significant. In the German dataset, the existing, apparently protective effect became more pronounced among children not breastfed, with a null effect among breastfed children. We tested these results by restricting the analysis to children who were breastfed, and testing for a trend in environmental tobacco smoke (ETS) effect across length of breastfeeding. The resulting interaction terms bore no correspondence to the interactions seen with the yes/no breastfeeding variable.