Prenatal lead and schizophrenia: further evidence development and schizophrenia. When we adjusted our ﬁndings for covariates, they pro- vide support for previously reported results, with consistent effects across individual sites and in the pooled analysis. Despite some differ- ences in sample and site characteristics, the effect sizes ranges from 1.6 to 1.9 (Table 2). In our samples, elevated prenatal levels of δ-ALA, the proxy for Pb exposure, is associated with almost a 2-fold increase in risk for schizophre- nia spectrum disorder in adulthood. Although these cohorts differed in some respects, there were remarkable similarities, including the birth years, comparability of serum storage procedures, and methods of case ascertainment. Nonetheless, our analysis faced challenges similar to those encountered in meta-analyses—for example, the need to pool results from different studies while taking into account the differences in source populations and methodologies. Although factoring in these differences, our analysis also had to pre- serve the matching structure of each study sample. The matching criteria for both sam- ples were similar in some respects, but not identical: Most notably, the NE-NCPP was matched on race, whereas the PDS study was not. It was also extremely important for us not to use any method that would result in the loss of any subjects or matched sets, thus negating the use of extremely limited archived serum samples. The multilevel, random inter- cept logistic regression model that we ﬁnally employed met these requirements accounting for both site- and matched-set correlations. A limitation of the previous study, which also applies here, is the use of δ-ALA to approx- imate Pb exposure. Because most of the Pb in blood is contained in erythrocytes, which are absent in sera, we have measured Pb exposure indirectly by quantifying the amount of δ-ALA in each serum sample. δ-ALA, a metabolite in the heme pathway, normally dimerizes rapidly in the presence of δ-ALA dehydrase to form porphobilinogen. However, in Pb-exposed individuals, δ-ALA levels in serum will be abnormally high because Pb, a potent inhibitor of erythrocyte δ-ALA dehydrase activity, pre- vents dimerization (Secchi et al. 1974). The laboratory methods we have used are adapted from standard techniques described by several researchers (Endo et al. 1993; Oishi et al. 1996) to determine levels of δ-ALA in plasma and fur- ther adapted by Tomokuni et al. (1993) for use on archived serum samples. We found approximately the same per- centage of exposed subjects in both samples, indicating that between 42% and 45% have estimated BPb levels of ≥ 15 µg/dL. This is consistent with the somewhat limited litera- ture on BPb concentrations in women from samples that are contemporary with the NE-NCPP and PDS study. A study of prox- imity to freeways in Los Angeles County, California, found mean BPb levels of 16.7 ± 7.0 µg/dL for exposed women and 9.9 ± 4.9 µg/dL for unexposed women (Thomas et al. 1967). The NE-NCPP sample had slightly higher exposure levels than the PDS sample, as reﬂected in the slightly higher mean δ-ALA level. Although it might be suspected that this is attributable to older housing stock built before the 1950s, thus predating the removal of Pb-based paint, housing that is likely to contain Pb-based paint was prevalent (and continues to be so) both in New England and in Oakland, California. In particular, the city of Oakland and Alameda County are known to contain a very high percentage of houses with either exterior or interior Pb-based paint (Centers for Disease Control and Prevention 1992). The work of Meyer et al. (2003) shows that 50% of existing housing contain- ing Pb-based paint within the United States was located in seven states—including California and Massachusetts. Other unmeas- ured variables could contribute to differences in exposure levels between the two sites, including the state of the home environment maintenance conditions (Sargent et al. 1997) and levels of automobile traffic (Rabinowitz et al. 1984), because these subjects were recruited before the ban on leaded gasoline. More detailed information regarding cer- tain covariates might have been beneﬁcial, but it is doubtful whether their inclusion would have made a significant difference in the results. For example, more specific informa- tion on the level of exposure to urban environ- ments (sometimes referred to as “urbanicity”) would have been desirable. Urbanicity is a sus- pected risk factor for schizophrenia as well as a determinant of Pb exposure (Pedersen et al. 2004). Although chemical exposures and ambient pollutants are more prevalent in urban settings (Marcelis et al. 1998), work by Pedersen and Mortensen (2006) suggests that the association between urbanicity and schizo- phrenia may not be mediated by exposure to toxins. A review by Krabbendam and van Os (2005) argues that because urban-environ- ment–related cases of schizophrenia are not associated with neuropsychological impair- ments or obstetric complications, neurodevel- opmental mechanisms are less likely to be involved in the relationship. Although we cannot entirely rule out the possibility that our estimates of prenatal Pb exposure are acting as a marker of the number of years spent in urban environments before pregnancy, there are several reasons that maternal urbanicity is not a likely factor in our results. Both the NE-NCPP and PDS samples are derived from cohorts based in urban centers. Consequently, the level of vari- ability is likely to be limited, and very few subjects or parents within either sample are likely to be classified as anything but urban residents. Parental socioeconomic status is another variable we would have liked to analyze in more detail. Like urban residence, it is also associated with elevated prenatal and postna- tal Pb exposure and has been associated with schizophrenia in some settings. However, when analyzed in our previous study, it proved to have no impact on the results (Opler et al. 2004). Other potential covariates that were unavailable for this analysis included family history of schizophrenia and obstetric complications. Thus far, we have hypothesized that in utero exposure and subsequent changes in neurodevelopment were the most likely causal pathway to explain our results. Guilarte (2004) suggested that Pb exposure is a biologically plausible risk factor for schizophrenia that may act via the N-methyl D-aspartate (NMDA) receptor; indeed, Nihei and Guilarte (2001) demonstrated experimentally that Pb exposure may cause changes in the expression and func- tion of the NMDA receptor. This receptor is particularly critical for learning and memory and synaptogenesis during development, and evidence implicating the role of NMDA and glutamate in schizophrenia has been mounting and has now led to the development of new psychopharmacologic agents (Imre 2007). Although our results provide additional support for the theory that Pb may exert developmental effects via changes in expression and function of NMDA receptor, new studies will be required to conﬁrm our ﬁndings. Because in utero expo- sures are often correlated with exposures during the life course, these future studies will need to Table 2. Estimated ORs relating δ-ALA (categorized as ≥ 9.05 and < 9.05 ng/mL) and schizophrenia spectrum disorder using four statistical models. Cohort NE-NCPP (Boston/Providence) PDS (California) PDS and NE-NCPP (combined) Cases Controls 27 44 71 54 75 129 Model Estimated OR (95% CI) p-Value Conditional logistic regression, unadjusted for covariates Conditional logistic regression, unadjusted for covariates Conditional logistic regression, unadjusted for covariates Random intercept logistic regression, unadjusted for covariates Conditional logistic regression, adjusted for maternal age and maternal education Random intercept logistic regression, adjusted for maternal age and maternal education 1.58 (0.55–4.51) 1.89 (0.86–4.11) 1.77 (0.94–3.32) 1.72 (0.96–3.09) 2.17 (1.12–4.17) 1.92 (1.05–3.52) Environmental Health Perspectives • VOLUME 116 | NUMBER 11 | November 2008 0.394 0.109 0.074 0.071 0.020 0.035 1589