Marks et al. (Bradman et al. 2005). Urine specimens were typically collected at the time of interview/ neurodevelopmental assessment and were ali- quoted and stored at –80°C until shipment on dry ice to the Centers for Disease Control and Prevention (CDC) for analysis. DAPs were measured using gas chromatography-tandem mass spectrometry and quantified using iso- tope dilution calibration (Bravo et al. 2002). We measured six DAP metabolites in maternal and child urine: a) three dimethyl phosphate (DM) metabolites (dimethylphos- phate, dimethylthiophosphate, dimethyldi- thiophosphate), representing breakdown of O,O-dimethyl-substituted OP pesticides such as malathion, oxydemeton-methyl, and dime- thoate, and b) three diethyl phosphate (DE) metabolites (diethylphosphate, diethylthiophos- phate, and diethyldithiophosphate) derived Table 1. Demographic characteristics of CHAMACOS study population, Salinas Valley, California, 2000–2001 (n = 348). Characteristic Mother during pregnancy Parity 0 ≥ 1 Married or living as married Yes No Education ≤ Grade 6 Grades 7–12 ≥ High school graduate Country of origin United States Mexico Other Years in the United States ≤ 5 6–10 ≥ 11 Entire life Smoking Yes No Depressive symptoms (CES-D) Yes No Yes No Yes No Yes No Yes No 1770 Poverty status At or below poverty threshold Above poverty threshold At child follow-up Child sex Male Female Child care ≥ 15 hr/week (3.5 years) Child care ≥ 15 hr/week (5 years) Any preschool by 5 years of age Kindergarten by 5 years of age n (%) 114 (32.8) 234 (67.2) 285 (81.9) 63 (18.1) 149 (42.8) 126 (36.2) 73 (21.0) 44 (12.6) 298 (85.6) 6 (1.7) 162 (46.6) 90 (25.9) 60 (17.2) 36 (10.3) 14 (4.0) 334 (96.0) 144 (43.8) 185 (56.2) 205 (62.7) 122 (37.3) 163 (46.8) 185 (53.2) 177 (53.3) 155 (46.7) 106 (32.8) 217 (67.2) 228 (69.3) 101 (30.7) 96 (29.2) 233 (70.8) from O,O-diethyl-substituted OP pesticides such as chlorpyrifos and diazinon (Bradman et al. 2005). These six metabolites cannot be traced back to individual pesticides but, taken together, represent the breakdown products of approximately 80% of the total OP pesticides used in the Salinas Valley. Values below the limit of detection (LOD) were assigned a value of LOD/√2. Urinary creatinine concentrations were determined using a commercially avail- able diagnostic enzyme method (Vitros CREA slides; Ortho Clinical Diagnostics, Raritan, NJ). Lead, which we considered a potential con- founder, was measured in cord blood by the California State Department of Public Health (Sacramento, CA) and in children’s blood at age 2 years by the Monterey County Public Health Laboratory (Salinas, CA) using graphite furnace atomic absorption spectrophotometry. Lead levels were abstracted from laboratory and clinic medical records by the study staff. Data analysis. We performed analyses using Stata version 10.1 (StataCorp, College Station, TX). DAPs (nanomoles per liter) were summed and transformed to the log (base 10) scale; coefficients and odds ratios (ORs) thus repre- sent the change in mean behavioral scores or relative odds of an outcome for each 10-fold increase in DAP concentration. Pregnancy DM, DE, and total DAP values were created by averaging the two log-trans- formed pregnancy measures. For 19 women with only one DAP measurement in preg- nancy, the single measure was used. Several children were missing DAP measures at 3.5 years (n = 58) and 5 years (n = 14). Separate models were run with maternal DAPs only, child DAPs only, and the two together in the same model. We constructed separate linear regres- sion models for continuous outcomes (e.g., NEPSY Visual Attention, CBCL scale scores) and logistic regression models for categori- cal outcomes (e.g., borderline clinical-range CBCL scores, markedly atypical K-CPT scores). Values of OR > 1.0 or β > 0.0 indicate positive associations between DAP levels and increased risk of adverse outcome, except for the NEPSY visual attention subtest, where a β > 0.0 indicates better performance. p-Values < 0.05 were considered statistically significant for tests of main effects. Model diagnostics were performed and found acceptable. We included the same covariates in all models, except that a variable for psychometri- cian was not included for the CBCL measures. Covariates were selected for these analyses if they were related to conditions of testing or related to neurobehavior in the literature and/ or associated with more than one outcome (p < 0.10 or changes to the main effect coef- ficient by ≥ 10%). We included in the models psychometrician (n = 2 at 3.5 years, n = 4 at 5 years), exact age at assessment, sex, mater- nal education, depressive symptoms, PPVT (continuous), ≥ 15 hr out-of-home child care/ week, and breast feeding duration (months). In addition to these covariates, we examined the potential confounding effects of a number of other variables that did not markedly alter the observed associations and were therefore not used in models, including maternal age, parity, marital status, active/passive smoking exposure and regular alcohol use during preg- nancy, and presence of father in home, mater- nal work status, and household income at the time of assessment. Household income above or below poverty was determined by com- paring total household income with the fed- eral poverty threshold for a household of that size (U.S. Census Bureau 2000). Covariates in final models were categorized as noted in Table 1, unless otherwise specified above. Maternal depression was missing for 5% of observations and maternal PPVT for < 1%. Values of maternal depression measured when the child was 1 year old were available and substituted for 13 instances where maternal depression when the child was 3.5 years was missing. To preserve the size of the analytic population, remaining missing covariate val- ues (six instances for maternal depression and three for maternal PPVT) were imputed by simple random selection of a value from par- ticipants with nonmissing values. We conducted a number of sensitivity analyses. We re-ran all models using creati- nine-adjusted DAP metabolite concentrations. In addition, we added to final models some factors potentially on the causal pathway (i.e., birth weight, gestational duration). We also considered whether controlling for lead (log10- transformed), a known neurotoxicant, altered results for DAP concentrations in the subsam- ples with cord (n = 229) or 2-year (n = 296) lead values. Finally, because rates of ADHD vary considerably by sex in the general popula- tion (Pastor and Reuben 2008) and effects of other toxicants have been found to vary by sex (Delaney-Black et al. 2004), we tested for interactions between DAP concentrations and child sex, using p < 0.15 for the interaction term to determine whether associations of DAP concentrations with measures of atten- tion differed for boys and girls. Results Demographic characteristics. Demographic characteristics of the study population are pre- sented in Table 1. Mean (± SD) maternal age was 26.5 ± 5.2 years at delivery. Most moth- ers were married or living as married (82%), spoke only Spanish at home (90%), were born in Mexico (86%), had not completed high school (79%), were nonsmokers (96%), and lived in a low-income household (97% within 200% of the federal threshold for poverty; ovlume 118 | number 12 | December 2010 • Environmental Health Perspectives