Vrijheid et al. The most difficult issue in our attempt to combine data from the different studies was the use of very different criteria for definition and classification of congenital anomaly sub- groups. Moreover, we found differences in inclusion and exclusion of cases with other anomalies in the same and other organ sys- tems, with chromosomal abnormalities, and with other syndromic conditions. We com- pared the different groupings and exclusions used [see Supplemental Material, Table 1 (doi:10.1289/ehp.1002946)] but found that in some instances it is not possible to deduce from the articles which inclusions and exclu- sions were made. For our meta-analyses we selected a few groups of relatively comparable anomaly groups. However, even for these, studies differed in their approach to classifica- tion. VSDs, for example, were treated as one anomaly group by some studies (Dadvand et al. 2011a, 2011b; Gilboa et al. 2005; Hansen et al. 2009; Rankin et al. 2009; Ritz et al. 2002), as four different anomalies by another (Strickland et al. 2009), and were excluded in yet another (Dolk et al. 2010). Notably, we found the largest evidence for heterogeneity for VSDs. In general, congenital heart defects form a very heterogeneous set of conditions, notoriously difficult to classify (Botto et al. 2007; Strickland et al. 2008). Several classi- fication systems have been proposed (Botto et al. 2007; Jacobs et al. 2007), but diagnostics information in routine registries may often not be specific enough to apply these. Further international harmonization efforts in this area [e.g., those undertaken as part of EUROCAT (2010) and the International Clearinghouse for Birth Defects (2010)] will be of great value. Classification of orofacial clefts is more straightforward than that of cardiac anomalies, but even here differences in exclusions have resulted in somewhat nonhomogeneous case groups for the meta-analyses. We encourage future studies to base their classifications and exclusions on those of previous studies in the same field, where possible. The meta-analysis results should be inter- preted with caution because they are based on few studies, and because some were subject to some degree of statistical heterogeneity. On the other hand, the total numbers of cases included in the meta-analyses were large, rang- ing from 500 to > 3,700, depending on the anomaly group. We found significant results for some pollutant–cardiac anomaly com- binations, but only two of these (NO2 and tetralogy of Fallot, and SO2 and coarctation of the aorta) were robust to the exclusion of the study with the largest weight, and we could not confirm significant findings from the con- tinuous exposure analyses with high- versus low-exposure analyses. It is not clear which of these latter analyses is the most appropri- ate: In continuous analyses one assumes a (log) linear relationship between exposure and outcome; in the field of congenital anomaly, linearity cannot automatically be assumed, because selective survival of more viable fetuses related to the same exposure may lead to non- linearity (Ritz 2010). On the other hand, the high versus low analysis combines categories of very different exposure levels across studies and has lower power because of smaller num- bers of cases in these more extreme exposure categories. We recommend that studies report both types of analyses, in annexes of sensitiv- ity analyses where appropriate, in order to aid future meta-analyses. Heterogeneity in the studies we reviewed may arise from inherent differences between Table 3. Summary of meta-analysis of studies on air pollutant exposures and orofacial clefts. number Continuous exposurec High versus low exposure Pollutant Total and anomaly Studies Heterogeneity Summary OR Heterogeneity Summary OR combination includeda of casesb p-value (95% CI)d p-value (95% CI)d CO per 1 ppm Cleft lip/palatee 1, 2, 3, 6, 8 1,498 0.03 0.89 (0.66–1.20) 0.002 0.99 (0.79–1.24) Cleft palate 1, 2, 6, 8 697 0.009 0.68 (0.36–1.25) 0.06 0.78 (0.55–1.12) NO2 per 10 ppb Cleft lip/palate 2, 6, 7, 8 1,423 0.44 0.99 (0.90–1.10) 0.25 1.06 (0.88–1.28) Cleft palate 2, 6, 7, 8 809 0.29 0.98 (0.85–1.13) 0.23 0.79 (0.62–1.01) O per 10 ppb 3 Cleft lip/palate 1, 2, 3, 6, 8 1,950 0.20 1.10 (0.99–1.21) 0.22 1.06 (0.96–1.17) Cleft palate 1, 2, 6, 8 702 0.79 0.99 (0.84–1.18) 0.42 1.00 (0.79–1.26) PM10 per 10 μg/m3 Cleft lip/palate 2, 3, 6, 7, 8 2,072 0.70 1.01 (0.96–1.06) 0.45 1.02 (0.94–1.11) Cleft palate 2, 6, 7, 8 803 0.10 0.97 (0.82–1.16) 0.15 0.86 (0.70–1.07) SO2 per 1 ppb Cleft lip/palate 2, 3, 6, 7, 8 1,976 0.06 0.99 (0.95–1.04) 0.01 1.06 (0.87–1.29) 2, 3, 4, 6, 7, 8f 0.02 1.06 (0.89–1.27) Cleft palate 2, 6, 7, 8 764 0.30 1.01 (0.99–1.03) 0.17 0.94 (0.81–1.10) aReferences are as follows: 1, Ritz et al. (2002); 2, Gilboa et al. (2005); 3, Hwang and Jaakkola (2008); 4, Rankin et al. (2009); 6, Hansen et al. (2009); 7, Dolk et al. (2010); 8, Marshall et al. (2010). bNumber of cases included in the continuous exposure analysis. cConversion factors: CO, 1 ppb = 1.15 μg/m3; NO2, 1 ppb = 1.88 μg/m3; O3, 1 ppb = 1.96 μg/m3; SO2, 1 ppb = 2.62 μg/m3. dWhen heterogeneity p-value is < 0.10 the OR from random effect model is shown; otherwise, the OR from the fixed-effect model is shown. eCleft lip with or without cleft palate. fSensitivity analysis including Rankin 2009 (no continuous exposure estimate available). the study settings, as well as from differences in study designs and analysis methods. First, the study areas were different with respect to exposure levels and ranges, pollutant mixtures, and underlying anomaly risks; this may have given rise to different dose–response relation- ships. With respect to study design, we consid- ered the assessment of exposure to be relatively similar: Studies assessed mostly the same pol- lutants, same exposure windows, and similar distance-based exposure indices. Heterogeneity in our analyses was not particularly related to studies that deviated from the rest with respect to their approach to exposure assessment such as the Strickland et al. (2009) study with a purely temporal exposure model and the Dolk et al. (2010) study with a purely spatial model. As described above, ascertainment and classifi- cation of congenital anomalies differed greatly among studies, and this may well have played a role in the observed heterogeneity. Because all reviewed studies used routinely registered data, selection and recall biases likely did not play a large role in most of the reviewed studies. However, two of the reviewed studies (Gilboa et al. 2005; Hansen et al. 2009) matched con- trols to cases by, respectively, mother’s county of residence and mother’s distance of resi- dence from an air pollution monitor. This may introduce bias by reducing exposure contrast between cases and controls. Covariates included in analyses differed and residual confounding structures may differ among the studies, thus leading to heterogeneity in study results. However, the number of known risk factors for congenital anomalies is extremely small, and none of the suspected risk factors, such as smoking, alcohol, folate deficiency, or socioeconomic status, are likely to explain a large propor- tion of cases. Maternal smoking, for example, has been established as a risk factor for oro- facial clefts with relative risk estimates around 1.2–1.3 (Lie et al. 2008; Little et al. 2004; Meyer et al. 2004), but evidence for an associ- ation with risk of other congenital anomalies is weak (EUROCAT 2004). Only two of the reviewed studies controlled for tobacco smok- ing (Gilboa et al. 2005; Marshall et al. 2010). Possible mechanisms of teratogenicity of air pollutants at this stage remain speculative, but several mechanisms have been hypothe- sized for effects of PM on fetal growth, including oxidative stress, placental inflamma- tion, and changes in coagulation, as reviewed by Kannan et al. (2006). Congenital anoma- lies may be induced through similar effects on early fetal growth, as well as by air pollutants influencing the migration and differentiation of neural crest cells, by interactions with the metabolism and detoxification of other xeno- biotics, or by indirect effects through mater- nal immunologic reactions, such as infection or asthma, or medication related to these 604 ovlume 119 | number 5 | May 2011 • Environmental Health Perspectives