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

AX6-6
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
Latin America
Schnaas et al. (2000)
Mexico
Schnaas et al. (2006)
Mexico
Gomaa et al. (2002)
Mexico
112 children followed since birth with complete
psychometric data from the Mexico City
Prospective Study were examined. Intellectual
status indexed by General Cognitive Index (GCI)
from McCarthy Scales of Children’s Abilities
(MSCA). Purpose of the study was to determine if
magnitude of the effect of postnatal blood Pb levels
on cognition varies with time between blood Pb
and cognitive assessments.
From the Mexico City Prospective Study, 150
children followed since birth with complete data
for all covariates were examined. Intelligence from
age 6 to 10 yrs was assessed using the WISC-R.
Blood Pb measurements from various time points,
starting from maternal blood Pb levels during the
2nd trimester to postnatal Pb levels at age 10 yrs.
197 two yr-old children residing in Mexico City
followed since birth. Bayley Scales of Infant
Development Mental Development Index (MDI)
used to index intellectual status. Extensive
assessment of medical and sociodemographic
covariates.
Serial postnatal blood Pb
Avg blood Pb
24-36 mos 9.7
(range 3-48) µg/dL
Serial prenatal (maternal) and
postnatal blood Pb
Geometric mean blood Pb
During pregnancy 8.0
(range 1-33) µg/dL
Age 1-5 yrs 9.8
(range 2.8-36.4) µg/dL
Age 6-10 yrs 6.2
(range 2.2-18.6) µg/dL
Umbilical cord and serial
postnatal blood Pb
Umbilical cord blood
Pb 6.7 (SD 3.4) µg/dL
Blood Pb at 2 yrs 8.4
(SD 4.6) µg/dL
Maternal tibial and patellar
bone Pb
Patellar (trabecular)
bone Pb 17.9 (SD 15.2) µg/g
A number of significant interactions observed between
blood Pb levels and age of assessment. Greatest effect
observed at 48 mos, when a 5.8 deficit in adjusted GCI
scores was observed for each natural log increment in
blood Pb. Authors concluded that four to five yrs of age
appears to be a critical period for manifestation of earlier
postnatal blood Pb level effects on cognition.
Among all the Pb variables at the various time points, only
log-transformed blood Pb levels during the 3rd trimester
were significantly associated with full scale IQ at ages 6 to
10 yrs, after adjusting for potential confounders. A 3.44
point deficit in full scale IQ was observed for each natural
log increment in blood Pb.
The authors note that, given the modest sample size and
relatively low power of this study, they do not claim that
Pb exposure from other developmental period has no effect
on child IQ.
Umbilical cord blood Pb and patellar (trabecular) bone Pb
significantly associated with lower Bayley MDI scores.
Maternal trabecular bone Pb levels predicted poorer
sensorimotor functioning at two yrs independent of
concentration Pb measured in cord blood. Increase in cord
blood Pb level from 5 to 10 µg/dL was associated with a
3.1 point decrement in adjusted MDI scores. In relation to
lowest quartile of trabecular bone Pb, the 2nd, 3rd, and 4th
quartiles were associated with 5.4, 7.2, and 6.5 decrement
in MDI following covariate adjustment. Authors
concluded that higher maternal trabecular bone Pb levels
constitute an independent risk factor for impaired mental
development in infancy, likely due to the mobilization of
maternal bone Pb stores over gestation.