Chemistry Reference
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arithmetic, approximately 1-point in reading, 0.1-point
in nonverbal reasoning, and 0.5-point in short-term
memory. An inverse relationship between blood lead
concentration and arithmetic and reading scores was
observed for children with blood lead concentrations
lower than 50
it was found that the blood lead concentration was
inversely associated with IQ, with an extrapolated
decline of 1.29 points in total IQ for 10
µ
g/L increase in
blood concentration.
Coscia
et al
. (2003) examined the effect of lead expo-
sure on cognitive growth patterns in 196 children at
ages 6.5, 11, and 15 years. Children with higher lead lev-
els, measured at 15 years of age, showed lower verbal
comprehension scores over time and greater decline in
their rate of vocabulary development. Lead exposure
was not signifi cantly correlated with growth in percep-
tual organization test scores. Moreover, socioeconomic
status and maternal intelligence were statistically sig-
nifi cantly associated with growth patterns for both test
scores, independently from the effects of lead.
Counter
et al
. (2005) assessed the effects of PbB
concentration on nonverbal intelligence in Andean
children working in glass manufacturing and chroni-
cally exposed to lead. A signifi cant negative associa-
tion (
r
= −0.331;
P
< 0.0001) was noted between PbB
level and a non-language-based test of cognitive func-
tion—an approximate two-point decrease in IQ for
each 100
g/L.
Other recent studies of neurobehavioral effects
and cognitive function of children exposed postna-
tally to lead include one by Tong
et al
. (1996), who
examined 375 children born near the lead smelt-
ing town of Port Pirie, Australia, for an association
between environmental lead exposure and children's
intelligence at age 11-13 years. Verbal, performance,
and full-scale IQ were inversely related to PbB, with
no apparent threshold. The expected mean full scale
IQ declined by 3.0 points (95% CI, 0.07-5.93) for an
increase in lifetime average blood lead concentration
from 100-200
µ
g/L.
Walkowiak
et al
. (1998) investigated the effects of
low-level lead exposure (mean, 43
µ
µ
g/L; 95th percen-
tile, 89
g/L) on cognitive functioning in a cohort of
6-7-year-old German children. Impaired attention was
demonstrated, whereas visual perception, visual mem-
ory, fi ngertapping, and reaction time were unaffected.
Wasserman
et al
. (1998) studied 379 3-year-old chil-
dren born from mothers living near a lead smelter in
Yugoslavia (geometric mean PbB of 409
µ
g/L increase in PbB.
Koller
et al
. (2004) reviewed the studies of Lanphear
et al
. (2000) and of Canfi eld
et al
. (2004) along with oth-
ers in the fi eld of low-level lead effects on children's
cognitive development (Al-Saleh
et al
., 2001; Calderon
et al
., 2001; Garcia Vargas
et al
., 2001; Prpic-Majic
et al
.,
2000; Rahbar
et al
., 2002; Wang
et al
., 2002) and con-
cluded that these fi ndings are scientifi cally important
and that efforts to reduce childhood exposure to lead
should continue.
Lanphear
et al
. (2005) carried out pooled analysis to
examine the association of intelligence test scores and
blood lead concentration, especially for children who
had maximal measured blood lead levels <100
µ
µ
g/L in the
exposed subjects versus 98
g/L in controls). At age
3, small but signifi cant decrements were found in
destructive and withdrawn scores, somatic and sleep
problems, and anxious-depressed subscale, after
checking for confounders. Wasserman
et al
. (2000) con-
ducted a subsequent investigation of 442 children from
the same Yugoslavian lead-polluted area, comparing
the relative contribution of prenatal blood lead with
that of relative increases in PbB in either the early (0-2
years) or the later (from 2 years on) postnatal period to
child intelligence measured at ages 3 and 4. This study
confi rmed that elevations in both prenatal and postna-
tal PbB were associated with small decrements in the
children's intelligence.
Canfi eld
et al
. (2003) measured blood lead concen-
tration in 172 children at 6, 12, 18, 24, 36, 48, and 60
months of age and compared it with the Stanford-Binet
Intelligence Scale at the ages of 3 and 5 years. Blood
lead concentration was inversely associated with IQ.
Estimated by a linear model, each increase of 100
µ
g/L.
They collected data from 1333 subjects from seven pop-
ulation-based longitudinal cohort studies, followed
from birth or infancy until 5-10 years of age. The full-
scale IQ score was the primary outcome measure. The
geometric mean PbB peaked at 178
µ
µ
g/L and declined
to 94
g/dL by 5-7 years of age. After adjustment for
covariates, the estimated IQ point decrements associ-
ated with an increase in blood lead from 24-100
µ
µ
g/
L, 100-200
g/L were 3.9, 1.9, and
1.1, respectively. This suggests that the dose-response
curve is steeper at the lower levels compared with the
higher ones.
As alluded to previously, no clear and generally
accepted threshold value has been identifi ed. The
current WHO/CDC blood level of concern stands at
100
µ
g/L, and 200-300
µ
g/L
in the lifetime average blood lead concentration was
associated with a 4.6-point decrease in IQ (
P
= 0.004).
When estimated in a nonlinear model, IQ declined by
7.4 points as lifetime average blood lead concentra-
tions increased from 10 to 100
µ
g/L.
Carta
et al
. (2003) assessed 64 adolescents living in
a polluted area in Italy. By applying a linear model,
µ
g/L but is being challenged by the recent studies
showing adverse effects below this level. Meta-analy-
sis on both cross-sectional and prospective studies
µ
