Late effects of low blood lead concentrations in children on school performance and cognitive functions

Background: Although it is known that lead is a neurotoxin that negatively impacts cognitive functions at low blood concentrations (B-Pb), little is known about the impact of early exposure on later cognitive functions.

Objectives: This study assesses the effects of very low lead exposure in early childhood on teenage cognitive performance.

Methods: Using data collected between 1978 and 2007, we analyzed B-Pb (median 30 μg/L; six-fold decrease over time) in 3176 Swedish children (age 7-12). School performance in grade 9 (age 16; boys and girls) and over-all IQs measured during conscription examinations (age 18-19; mainly boys) were obtained from registers. In multivariate models, potential confounders (age at blood sampling, sex, parents' education, family economy, and country of birth of child and parents) and effect modifiers (socioeconomic; father's IQ at conscription examination) were included.

Results: There were statistically significant adjusted negative associations between school performance (Grades up to 1991: P<0.0001; Merits 1992-2007:P<0.0001) and IQ (P=0.03) and B-Pb. The dose-response relationships were non-linear. Effects were more pronounced for B-Pb≤50 μg/L than for higher levels. In the B-Pb range 5-50 μg/L, the average IQ loss corresponded to about 5 IU. There was no significant effect modification associated with socioeconomic factors.

Conclusions: Lead causes neurotoxic effects at very low exposures (B-Pb<50 μg/L) in childhood and these effects remain for many years.