Research | Children’s Health Associations of Early Childhood Manganese and Lead Coexposure with Neurodevelopment Birgit Claus Henn,1 Lourdes Schnaas,2 Adrienne S. Ettinger,1,3 Joel Schwartz,1,4 Héctor Lamadrid-Figueroa,5 Mauricio Hernández-Avila,6 Chitra Amarasiriwardena,4 Howard Hu,7 David C. Bellinger,1,8 Robert O. Wright,1,4,7,9* and Martha María Téllez-Rojo5* 1Department of Environmental Health, Harvard School of Public Health, Boston, Massachusetts, USA; 2Department of Developmental Neurobiology, Instituto Nacional de Perinatologia, Mexico City, Mexico; 3Center for Perinatal, Pediatric and Environmental Epidemiology, Department of Epidemiology and Public Health, Yale University School of Medicine, New Haven, Connecticut, USA; 4Channing Laboratory, Department of Medicine, Brigham and Women’s Hospital, Harvard Medical School, Boston, Massachusetts, USA; 5Division of Statistics, Center for Evaluation Research and Surveys, National Institute of Public Health, Cuernavaca, Morelos, Mexico; 6Ministry of Health, Mexico City, Mexico; 7Department of Environmental Health Sciences, University of Michigan School of Public Health, Ann Arbor, Michigan, USA; 8Department of Neurology, Harvard Medical School and Children’s Hospital Boston, Boston, Massachusetts, USA; 9Department of Emergency Medicine, Children’s Hospital Boston, Boston, Massachusetts, USA Background : Most toxicologic studies focus on a single agent, although this does not reflect real-world scenarios in which humans are exposed to multiple chemicals. o Bjectivs : We prospectively studied manganese–lead interactions in early childhood to examine whether manganese–lead coexposure is associated with neurodevelopmental deficiencies that are more severe than expected based on effects of exposure to each metal alone. Methods : Four hundred fifty-five children were enrolled at birth in an longitudinal cohort study in Mexico City, provided blood samples, and were followed until 36 months of age. We measured lead and manganese at 12 and 24 months and assessed neurodevelopment at 6-month intervals from 12 to 36 months of age using Bayley Scales of Infant Development–II. r esult : Mean (± SD) blood concentrations at 12 and 24 months were, respectively, 24.7 ± 5.9 gμ/L and 21.5 ± 7.4 gμ/L for manganese and 5.1 ± 2.6 gμ/dL and 5.0 ± 2.9 gμ/dL for lead. Mixed- effects models, including Bayley scores at five time points, showed a significant interaction over time: highest manganese quintile × continuous lead; mental development score, β = –1.27 [95% confidence interval (CI): –2.18, –0.37]; psychomotor development score, β = –0.92 (95% CI: –1.76, –0.09). Slopes for the estimated 12-month lead effect on 18-month mental development and 24- through 36-month psychomotor development scores were steeper for children with high man- ganese than for children with midrange manganese levels. c onclusi : We observed evidence of synergism between lead and manganese, whereby lead tox- icity was increased among children with high manganese coexposure. Findings highlight the impor- tance of understanding health effects of mixed exposures, particularly during potentially sensitive developmental stages such as early childhood. k ey words : coexposure, early childhood, lead, manganese, metals, neurodevelopment. Environ Health Perspect 120:126–131 (2012). http://dx.doi.org/10.1289/ehp.1003300 [Online 1 September 2011] disruption of calcium metabolism (Bressler and Goldstein 1991). Biological mechanisms for manganese toxicity are also hypothesized to involve calcium metabolism, oxidative dam- age to neuronal cells, and impaired dopamine neurotransmission (Aschner and Aschner 1991; Normandin and Hazell 2002; Verity 1999). With the same target organ, potentially similar biological mechanisms, and a realistic potential for joint exposure, manganese and lead are important chemicals to examine for interactive effects. Our study examined manganese–lead interaction in early childhood, a period of potentially heightened susceptibility to neuro- toxicants. Among 1- to 3-year-old children in Mexico City, we prospectively examined whether coexposure to manganese and lead causes neurodevelopmental deficiencies that are more severe than expected based on effects of exposure to each metal alone. Materials and Methods Study population. The population of chil- dren analyzed in this study has been described previously (Claus Henn et al. 2010). Briefly, we enrolled women just before or during Address correspondence to B. Claus Henn, Harvard School of Public Health, Landmark Center East, 3rd Floor, 401 Park Dr., Boston, MA 02215 USA. Telephone: (617) 721-2421. Fax: (617) 384-8994. E-mail: bclaus@hsph.harvard.edu *These authors shared senior authorship. Supplemental Material is available online (http:// dx.doi.org/10.1289/ehp.1003300). The research described in this article was funded in part by National Institute of Environmental Health Sciences grants R01-ES007821, R01-ES014930, R01- ES013744, and P30-ES000002. This publication was developed under STAR (Science to Achieve Results) research assistance agreement FP-91690001 awarded by the U.S. Environmental Protection Agency (EPA). This research has not been formally reviewed by the U.S. EPA. The views expressed in this document are solely those of the authors, and the U.S. EPA does not endorse any products or commercial ser- vices mentioned in this publication. The authors declare they have no actual or potential competing financial interests. Received 6 December 2010; accepted 1 September 2011. ovlume 120 | number 1 | January 2012 • Environmental Health Perspectives Most toxicologic studies focus on a single agent and either do not measure or do not adjust for potential confounding or modifying effects of other chemicals. Although such an approach has identified toxicities associated with vari- ous chemicals, it does not reflect real-world scenarios in which humans are exposed to mul- tiple chemicals (Cory-Slechta 2005; Simmons 1995). Human exposure to chemical mixtures is particularly widespread in socioeconomically disadvantaged populations (Naess et al. 2007) and among populations living near hazardous waste sites (Hu et al. 2007). The customary approach of examining chemicals in isolation may constrain our ability to understand neuro- logic sequelae (Bellinger 2008a; Cory-Slechta et al. 2008). Despite the importance of examin- ing joint exposures to toxicants, few epidemio- logic studies have done so. Metals, in particular, are common neurotoxicants occurring within the environments of children. Concomitant exposure to several metals may have more severe (i.e., synergistic) effects on cognition 126 than expected based on effects of exposure to each metal alone (Wright et al. 2006). In this article, we focus on interactions between lead and manganese because these metals share the central nervous system as their primary toxicity target in children. Adverse effects of lead on neurodevelopment are well described (Bellinger et al. 1991; Canfield et al. 2003; Chen et al. 2007; Tellez-Rojo et al. 2006). Cognitive deficits are major concerns, with declines of about 1.1–3.9 IQ points, for example, estimated for a 10-gμ/dL increase in blood lead (Lanphear et al. 2005). Evidence for the neurotoxic effects of environmen- tal manganese on children is mounting and includes associations with cognition, mem- ory, behavior, and motor function (Bouchard et al. 2007, 2011; Claus Henn et al. 2010; Menezes-Filho et al. 2011; Riojas-Rodriguez et al. 2010; Takser et al. 2003; Wasserman et al. 2006; Wright et al. 2006). The mecha- nisms for lead neurotoxicity include increased spontaneous release of neurotransmitters and