1. In this prospective cohort study, New Zealand children with increased lead levels in their blood had significantly lower cognitive skills and socioeconomic status in adulthood compared to children with lower lead levels.
2. The link between childhood lead levels and lower adulthood socioeconomic status was primarily mediated through cognitive decline.
Evidence Rating Level: Good
Study Rundown: Environmental protection policies are being increasingly scrutinized in the United States, especially since the events concerning lead contamination in Flint, Michigan. While it’s known that childhood lead exposure leads to serious neurological deficits, few studies have assessed the impact of this exposure into adulthood. In a prospective cohort study, children from New Zealand were followed until 38 years old, and IQ and socioeconomic status were recorded. The New Zealand cohort is especially informative because high lead levels were driven by automobile emissions and childhood blood lead concentrations were independent of socioeconomic status (SES) and IQ. At 38 years of age, higher childhood lead levels were proportionately associated with reduced SES and cognitive abilities. IQ was demonstrated to be a significant mediator of the association between SES change and childhood lead levels, and a sensitivity analysis and further adjustments for covariates were unable to attenuate this findings.
The natural design of this study, in that children were exposed to lead at levels independent of SES, increases the weight of these findings. While effect sizes were comparatively small, they were proportional to other childhood risk factors, like very low birthweight. The economic impact of higher lead levels are hard to assess, but the pervasiveness of these findings should be especially noted by health care providers and policy makers.
In-Depth [prospective cohort study]: A total of 565 children from New Zealand were tested for lead levels at age 11 and followed until age 38. Adult participants were then assessed for IQ and SES. Because lead was used in gasoline during the 70’s and 80’s in New Zealand, children of all SES were affected nearly equally in terms of lead exposure, making this cohort distinct from those of other developed nations where blood lead concentrations were correlated with childhood SES. IQ was assessed with the Wechsler Adult Intelligence Scale-IV (WAIS-IV) and SES by the New Zealand Socioeconomic Index-2006 (NZSEI-06). For each 5mg/dl increase in lead exposure, IQ decreased by 1.61 points (CI95 -2.48 to -0.74) and SES by 1.79 (CI95 -3.17 to -0.40), after controlling for childhood IQ, childhood SES, and mother’s IQ. Additionally, those with lead levels below 10mg/dL showed an increase in IQ of 1.22 while those above showed a decreased IQ of 1.68, resulting in a significant difference of 2.10 between groups (CI95 0.19 to 3.99). IQ decline accounted for 40% of the association between childhood lead levels and SES change (p = 0.002).
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