Association between arsenic, cadmium, manganese, and lead levels in private wells and birth defects prevalence in North Carolina: a semi-ecologic study

Abstract

Background: Toxic metals including arsenic, cadmium, manganese, and lead are known human developmental toxicants that are able to cross the placental barrier from mother to fetus. In this population-based study, we assess the association between metal concentrations in private well water and birth defect prevalence in North Carolina.

Methods: A semi-ecologic study was conducted including 20,151 infants born between 2003 and 2008 with selected birth defects (cases) identified by the North Carolina Birth Defects Monitoring Program, and 668,381 non-malformed infants (controls). Maternal residences at delivery and over 10,000 well locations measured for metals by the North Carolina Division of Public Health were geocoded. The average level of each metal was calculated among wells sampled within North Carolina census tracts. Individual exposure was assigned as the average metal level of the census tract that contained the geocoded maternal residence. Prevalence ratios (PR) with 95% confidence intervals (CI) were calculated to estimate the association between the prevalence of birth defects in the highest category (≥90th percentile) of average census tract metal levels and compared to the lowest category (≤50th percentile).

Results: Statewide, private well metal levels exceeded the EPA Maximum Contaminant Level (MCL) or secondary MCL for arsenic, cadmium, manganese, and lead in 2.4, 0.1, 20.5, and 3.1 percent of wells tested. Elevated manganese levels were statistically significantly associated with a higher prevalence of conotruncal heart defects (PR: 1.6 95% CI: 1.1-2.5).

Conclusions: These findings suggest an ecologic association between higher manganese concentrations in drinking water and the prevalence of conotruncal heart defects.

Figure 1

Average levels of arsenic (A), cadmium (B), manganese (C), and lead (D) within census tracts across North Carolina. The color gradient represents the calculated exposure percentile categories (≤50^th; 50-75^th; 75-90^th; ≥90^th percentiles). The dark brown category represents exposed tracts and the light yellow category represents unexposed tracts that were used to assess the association with birth defects prevalence. Gray areas had fewer than 10 tested wells and were excluded from the analysis.

Figure 2

Sensitivity analysis plots for selected associations between residence in ecologic units of highest compared to lowest metal level categories and prevalence of birth defects including the following metal-defect pairs: A) manganese and conotruncal heart defects, B) arsenic and conotruncal heart defects, and C) arsenic and cleft lip with or without cleft palate (CP), D) arsenic and hypoplastic left heart syndrome (HLHS), E) cadmium and pyloric stenosis, and F) lead and hypoplastic left heart syndrome (HLHS). Prevalence ratios (PRs) and 95% confidence intervals (CIs) correspond as follows: 1. Crude estimate, 2. Adjusted estimate for maternal age, race and education status, or 3. Estimate restricted to individuals outside public service areas and adjusted for maternal age, race and education status.