Guxens et al. were measured in maternal urine collected in the third trimester of pregnancy, using the Cotinine Micro-Plate EIA Kit (Ora Sure Technologies, Inc., Bethlehem, PA, USA). Total lead levels were analyzed in cord blood, using thermal decomposition, amalgamation, and atomic absorption spectrometry with a detection limit of 2 μg/ L. Almost all cohort members had lead levels below the limit of detection (95.4%). Information related to the child’s gestational age, sex, anthropometric measures, type of delivery, and Apgar score at birth was obtained from clinical records. In a subsequent interview when the child was 14 months of age (range, 11–23 months), information on changes in residence since the third trimester of pregnancy, main caregiver, nursery attendance, maternal working status, use of gas stove at home, and child second- hand smoke exposure was collected. All ques- tionnaires were administered face to face by trained interviewers. Statistical analysis. Statistical analyses included all subjects with complete data on mental development and air pollution (n = 2,138). Preterm births (< 37 weeks of gesta- tion; n = 81), children with unknown ges- tational age (n = 12), children with specific pathologies (n = 18), and children with low- quality neuropsychological tests (n = 138) were excluded. Among the included subjects (n = 1,889), multiple imputation of missing values for the covariables was performed using chained equations where 20 completed data sets were generated and analyzed using the standard combination rules for multiple impu- tation [see Supplemental Material, Tables 1, 2 (http://dx.doi.org/10.1289/ehp.1103469)] (Spratt et al. 2010; Sterne et al. 2009). We used generalized additive models to assess the linearity of the relationship between NO2 and benzene and infant mental develop- ment by graphical examination and likelihood ratio tests. Benzene was not linearly related with infant mental development (p-value for gain in linearity = 0.059), and benzene and NO2 were therefore log2 transformed. Multivariable linear regression models were performed to examine the relationship between log-transformed NO2 and benzene and infant mental development as continuous normal variables. First, to assess the effect of potential mediators and confounders on coefficients for associations with each air pollution vari- able, a series of models were run to assess the effect of additionally adjusting (forward selection) and eliminating (backward elimi- nation) one by one each of the individual parental and child characteristics of interest. Separate models were run for each region. Final multivariable-adjusted models simul- taneously incorporated all covariates either associated with mental scores (p < 0.20) [see Supplemental Material, Figure 1 (http:// dx.doi.org/10.1289/ehp.1103469)] or that modified the coefficient for the air pollutant variables by > 15% in any region. In a second step, we performed a meta- analysis using random-effects models com- bining the estimates in each region of the association between each air pollutant and infant mental development. Exposure– response slopes derived for each region were Table 1. Distribution of characteristics of interest by region. Overall Valencia Sabadell Asturias Gipuzkoa Characteristic (n = 1,889) (n = 608) (n = 471) (n = 345) (n = 465) Mental development score 99.9 ± 15.2 100.7 ± 15.5 99.1 ± 14.6 101.6 ± 15.6 98.5 ± 15.2 Age at mental development assessment (months) 14.8 ± 2.6 12.4 ± 0.8 14.6 ± 0.7 19.4 ± 1.3 14.5 ± 0.8 NO2 exposure during pregnancy (μg/m3) 29.0 ± 11.2 36.8 ± 11.0 32.1 ± 8.8 23.2 ± 7.1 20.1 ± 6.5 Benzene exposure during pregnancy (μg/m3) 1.5 ± 0.9 2.2 ± 0.6 0.8 ± 0.3 2.3 ± 1.3 1.0 ± 0.3 Maternal fruit and vegetable consumption, first trimestera Low tertile (≤ 405 g/day) Medium/high tertile (> 405 g/day) Breast-feeding duration Maternal circulating vitamin D levelsb None < 6 months ≥ 6 months Low Medium High Parental social classc I/II managers/technicians III skilled manual/nonmanual IV/V semiskilled/unskilled Maternal education level Primary or less Secondary University degree 33.5 66.5 14.7 40.5 44.8 33.6 33.0 33.4 32.8 25.8 41.4 22.8 41.2 36.0 39.8 60.2 16.0 39.9 44.1 22.7 34.0 43.3 23.5 28.1 48.4 31.7 43.3 25.0 33.0 67.0 6.7 42.6 50.7 40.4 27.1 32.5 32.0 28.7 29.3 26.3 42.5 31.2 30.5 69.5 29.0 43.2 27.8 40.0 35.5 24.5 32.6 24.1 43.3 15.6 43.2 41.2 27.6 72.4 10.5 36.9 52.6 35.7 35.9 28.4 45.8 20.9 33.3 12.7 35.7 51.6 plotted together with the summary slope from the meta-analysis using forest plots of β-coefficients with 95% confidence intervals (CIs). We assessed heterogeneity in risk esti- mates using the Q-test. In a third step, effect modification of air pollution associations by levels of antioxidants and detoxification factors (fruits and vegetables, breast-feeding, circulating vitamin D), as well as by socioeconomic variables (parental social class and maternal education), was assessed using stratified analysis and interaction terms. Models for each region were fitted stratifying by or including an interaction term for each variable of interest. Results for each stratum were then combined using meta-analysis. We performed various sensitivity analyses to assess the robustness of our results. First, multivariable-adjusted models were further adjusted for other environmental pollutants and socioeconomic variables to minimize the likelihood of residual confounding. Moreover, because fetal growth restriction might be an intermediate factor of the assessed relation (Slama et al. 2008), the possible mediation of birth outcomes was tested by including them in the multivariable-adjusted models. We also repeated the multivariable-adjusted models in specific population subgroups (including country of origin, social class, parental educa- tion, and subjects with stable residences) to reduce residual confounding. Finally, to assess the consistency of evidence for effect modi- fication, we examined the stratified analyses and the interaction terms in models run sepa- rately for each region and in pooled analy- ses, excluding the Gipuzkoa region because heterogeneity across regions was mainly attributable to this region. Statistical analy- ses were conducted using STATA (version 10.1; StataCorp, College Station, TX, USA). Statistical tests of hypotheses were two tailed, with significance set at p < 0.05. Results A total of 2,644 pregnant women were recruited during the first trimester of preg- nancy [see Supplemental Material, Figure 2 (http://dx.doi.org/10.1289/ehp.1103469)]. A total of 2,505 (94.7%) children were enrolled at birth, 2,348 (88.8%) were assessed in the second year of life, and 2,138 (80.9%) had complete mental development and air pollu- tion data. Our analysis is based on 1,889 chil- dren after some exclusions (preterm births, children with unknown gestational age, chil- dren with specific pathologies, and children with low-quality neuropsychological tests). No statistically significant differences between the 1,889 children in the analysis sample and the original 2,505 were observed in terms of maternal alcohol consumption during preg- nancy, maternal prepregnancy body mass index, paternal education and age, parental Values are percentages for categorical variables and mean ± SD for continuous variables. aLow versus medium/high tertile of maternal fruit and vegetable consumption. bSeason-specific tertiles of maternal cir- culating vitamin D levels (see “Materials and Methods”). cSocial class data from Instituto Nacional de Estadística (1994). 146 ovlume 120 | number 1 | January 2012 • Environmental Health Perspectives