Kehl et al. BMC Public Health 2010, 10:424 http://www.biomedcentral.com/1471-2458/10/424 Table 4 Household smoking, adjusted for infants’ sex and age and for study day, and infant cotinine-to-creatinine ratio Household smoking Smoking outside the home Smoking inside the home Infant’s sex and age Male Female Age (month) Study day B Ref. 1.42 Ref. .79 -.03 .03 Cotinine-to-creatinine ratio (μg/mmol) with log e transformation SE z p > |z| 95% CI 0.33 0.26 0.01 0.03 4.32 3.01 -2.02 1.02 < .001 < .01 < .05 .31 .77 .28 -.06 -.03 Page 6 of 8 2.07 1.31 -.00 .08 Generalized estimation equation model to predict the infants’ cotinine-to-creatinine ratio (CCR), taking into account cluster correlation within subjects across study days. Model is based on an exchangeable correlation structure for repeated observations within subjects. Predictor variable household smoking was adjusted for infants’ age and sex and for study day. Wald chi2 (4) = 40.53, scale parameter = .64. Explained variance of the GEE model was 67.5 percent. in boys. Also, older infants had lower CCR. The data also revealed that there was no significant increase of CCR over time. The final GEE model accounted for 68% of the variance in the CCR. To assess the variability of CCR of all infants over the seven-day study period, correlation coefficients were cal- culated. All 21 coefficients in the correlation matrix, for study day one through seven, ranged between .61 (3/7 day) and .96 (1/6 day) and were significant at a p-value of ≤.001; only one had a p-value of < .05. In the correlation matrix for Monday to Sunday, all 21 coeffi- cients ranged between .69 (Wed/Sun and Thu/Sat) to .95 (Fri/Sun) and were significant at a p- level of ≤.001. On weekends vs. weekdays was no absolute difference of CCR. The intra-class-correlation over the seven-day study period was .88, p <.001. Discussion There are three main findings of this study. First, smok- ing outside the home, compared to inside the home, was related with CCR. Second, female and younger infants revealed higher CCRs than their male and older counterparts. Third, significant fluctuations in daily CCR were not apparent in infants over a seven day time period. Infants from homes with indoor smoking had higher CCRs than infants from homes where smoking occurred on the balcony or in other places outside the home. The data support preexisting cross-sectional findings that suggest a home smoking ban may reduce infants’ coti- nine levels [4,5]. According to dose-response relation- ships, the present data are somewhat contradictory. The cross-sectional data clearly revealed a dose-response relationship, but the longitudinal data did not. The mean CCR was higher (57.9 μg/mmol) in infants with two smokers in the home compared to infants with one smoker in the home (24.9 μg/mmol). This confirms findings from the German Environmental Survey IV that also revealed a higher geometrical mean of cotinine among infants in households with more than one smo- ker than in households with only one smoker [20]. In another study urine cotinine levels of infants’ aged 3 to 27 month were correlated with the number of smokers in the home, however in this study also households with no smokers were included [4]. Our cross-sectional data clearly revealed a significant dose-response relationship between CCR and time infants spend in rooms in which smoking occurs in respect to study day 1, rs = .46, p = .01 (one-tailed), however our longitudinal data did not. Furthermore a positive but not significant relation- ship was revealed between CCR and cpd parents smoked in the presence of the infant in respect to study day 1, rs = .32, p = .07 (one-tailed). Second, the present data revealed that sex and age of the infant predicted CCR. When these two predictors were included, in addition to smoking inside home, 68% of the CCR variance was explained. Only 22% of the variance was explained when only the information about smoking inside the home was used. CCRs were greater among girls than among boys. Mean creatinine was lower in girls than in boys. Whether total creatinine excretion over a 24 hours period was lower in girls is unknown. This would have required 24 hour urine volume measurements. The mean cotinine value tended to be higher in girls than in boys. This confirms former evidence about higher mean cotinine values in girls compared to boys [13]. Our data revealed that mean cotinine and CCRs were higher and mean creatinine values were lower in younger than in older infants. This is in accordance with the expectation that the concentration of the same amount of tobacco smoke is higher among younger than among older infants due to less body tissue of the younger [21-23]. We do not know, whether total creati- nine excretion over a 24 hours period was lower in younger than in older infants. However, our finding