Association of Childhood Blood Lead Levels With Cognitive Function and Socioeconomic Status at Age 38 Years and With IQ Change and Socioeconomic Mobility Between Childhood and Adulthood

Abstract

Importance: Many children in the United States and around the world are exposed to lead, a developmental neurotoxin. The long-term cognitive and socioeconomic consequences of lead exposure are uncertain.

Objective: To test the hypothesis that childhood lead exposure is associated with cognitive function and socioeconomic status in adulthood and with changes in IQ and socioeconomic mobility between childhood and midlife.

Design, setting, and participants: A prospective cohort study based on a population-representative 1972-1973 birth cohort from New Zealand; the Dunedin Multidisciplinary Health and Development Study observed participants to age 38 years (until December 2012).

Exposures: Childhood lead exposure ascertained as blood lead levels measured at age 11 years. High blood lead levels were observed among children from all socioeconomic status levels in this cohort.

Main outcomes and measures: The IQ (primary outcome) and indexes of Verbal Comprehension, Perceptual Reasoning, Working Memory, and Processing Speed (secondary outcomes) were assessed at age 38 years using the Wechsler Adult Intelligence Scale-IV (WAIS-IV; IQ range, 40-160). Socioeconomic status (primary outcome) was assessed at age 38 years using the New Zealand Socioeconomic Index-2006 (NZSEI-06; range, 10 [lowest]-90 [highest]).

Results: Of 1037 original participants, 1007 were alive at age 38 years, of whom 565 (56%) had been lead tested at age 11 years (54% male; 93% white). Mean (SD) blood lead level at age 11 years was 10.99 (4.63) µg/dL. Among blood-tested participants included at age 38 years, mean WAIS-IV score was 101.16 (14.82) and mean NZSEI-06 score was 49.75 (17.12). After adjusting for maternal IQ, childhood IQ, and childhood socioeconomic status, each 5-µg/dL higher level of blood lead in childhood was associated with a 1.61-point lower score (95% CI, -2.48 to -0.74) in adult IQ, a 2.07-point lower score (95% CI, -3.14 to -1.01) in perceptual reasoning, and a 1.26-point lower score (95% CI, -2.38 to -0.14) in working memory. Associations of childhood blood lead level with deficits in verbal comprehension and processing speed were not statistically significant. After adjusting for confounders, each 5-µg/dL higher level of blood lead in childhood was associated with a 1.79-unit lower score (95% CI, -3.17 to -0.40) in socioeconomic status. An association between greater blood lead levels and a decline in IQ and socioeconomic status from childhood to adulthood was observed with 40% of the association with downward mobility mediated by cognitive decline from childhood.

Conclusions and relevance: In this cohort born in New Zealand in 1972-1973, childhood lead exposure was associated with lower cognitive function and socioeconomic status at age 38 years and with declines in IQ and with downward social mobility. Childhood lead exposure may have long-term ramifications.

Figure 1. Distribution of blood-lead levels at age 11 years in Dunedin cohort children grouped by socioeconomic status

Note. Histograms and box plots depicting the distribution of childhood blood-lead levels for participants from low, middle, and high socioeconomic status families based on the 6-point Elley-Irving scale coding participants’ parents’ occupations and their associated income and education levels. Low childhood family socioeconomic status includes categories 1 and 2 on the 6-point scale; middle status includes categories 3 and 4; high status includes categories 5 and 6. Histogram interval bins represent whole integers of blood-lead level. Shown in each box plot are the median value (white line), the 25th and 75th percentiles (box outer borders), and a lower-bound value equal to the 25th percentile minus 150% of the interquartile range and an upper-bound value equal to the 75th percentile plus 150% of the interquartile range (whiskers). High blood-lead levels were observed in all status groups. N=563 (two participants were not assigned a childhood socioeconomic status score).

Figure 2. Association of childhood blood-lead level with WAIS-IV IQ (Panel A) and socioeconomic status (Panel B) in adulthood (unadjusted for covariates)

Note. Mean outcomes in adulthood with 95% confidence intervals (error bars) for each 5μg/dL higher level of blood-lead in childhood. Each 5μg/dL higher level of blood-lead in childhood was associated with an additional 1.97-point lower score (95%CI: −3.34, −0.59, P=.005) in adult WAIS-IV full-scale IQ and an additional 1.94-unit lower score (95%CI: −3.50, −0.37, P=.02) in adult socioeconomic status (see Table 3). Socioeconomic status was assessed at age 38 years using the New Zealand Socioeconomic Index-2006 (NZSEI-06; range 10 = lowest – 90 = highest).

Figure 3. Association of childhood blood-lead level with cognitive decline (Panel A) and downward socioeconomic mobility (Panel B) into adulthood (unadjusted for covariates)

Note. Mean change in outcome from childhood to adulthood with 95% confidence intervals (error bars) for each 5μg/dL higher level of blood-lead in childhood. To create IQ change scores, childhood IQ was subtracted from adulthood IQ where both IQs were measured on matched scales (WISC-R for child IQ and WAIS-IV for adult IQ). To create socioeconomic status change scores, childhood (i.e., parental) socioeconomic status was subtracted from adult socioeconomic status where both status variables were measured on comparable 6-category scales (the Elley-Irving scale for childhood and the New Zealand Socioeconomic Index for adulthood) assessing socioeconomic status in New Zealand by assigning occupations into one of six socioeconomic status groups (6 = professional; 1 = unskilled laborer). Each 5μg/dL higher level of blood-lead in childhood was associated with a 1.61-point decline (95%CI: −2.48, −0.74, P<.001) in full-scale IQ and with a 1.79-unit decline (95%CI: −3.17, −0.40, P=.01) in socioeconomic status (see Table 3).