Air Quality Criteria for Lead Volume II of II - (NEPIS)(EPA) - US ...

AX6-4
Table AX6-2.1 (cont’d). Prospective Longitudinal Cohort Studies of Neurocognitive Ability in Children
Reference, Study
Location, and Period Study Description Pb Measurement Findings, Interpretation
United States (cont’d)
Bellinger and
Needleman (2003)
U.S.
Chen et al. (2005)
U.S.
Reanalysis of data from the Boston Prospective
Study focusing on 48 subjects at 10 yrs of age
whose blood Pb levels never exceeded 10 µg/dL.
WISCR was used to index intellectual status. See
also Bellinger et al. (1992)
Repeat measure psychometric data on 780 children
enrolled in Treatment of Pb-Exposed Children
(TLC) clinical trial were analyzed to determine if
blood Pb concentrations at 2 yrs of age constitute a
critical period of exposure for expression of later
neurodevelopmental deficits. Data for placebo and
active drug groups were combined in these
analyses, which spanned ~2 to 7 yrs of age.
Measures of intellectual status included the Bayley
Mental Development Index (MDI) and full scale IQ
derived from age-appropriate Wechsler scales.
Serial postnatal blood Pb
Blood Pb at 2 yrs 6.5
(SD 4.9) µg/dL
Blood Pb
Range 20-44 µg/dL
Baseline blood Pb 26
(SD 26.5) µg/dL in both drug
and placebo groups
Blood Pb at 7 yrs 8.0
(SD 4.0) µg/dL
IQ was inversely related to two-yr blood Pb levels
following covariate adjustment. Blood Pb coefficient
(!1.56) was greater than that derived from analyses
including children with concentrations above 10 µg/dL
(!0.58). Authors conclude that children’s IQ scores are
reduced at Pb levels still prevalent in U.S.
Association between blood Pb and psychometric
intelligence increased in strength as children became older,
whereas the relation between baseline (2 yr) blood Pb and
IQ attenuated. Peak blood Pb concentration thus does not
fully account for the observed association in older children
between their lower blood Pb concentrations and IQ. The
effect of concurrent blood Pb on IQ may thusly be greater
than currently believed. Authors conclude that these data
(a) support the idea that Pb exposure continues to be toxic
to children as they reach school age and (b) do not lend
support to the interpretation that majority of the damage is
done by the time the child reaches 2 to 3 yrs of age.