Appendix 3.  Detail on Prevalence and Risk

What Is the Prevalence of Elevated Lead in Children? 

The prevalence of elevated blood lead levels in the U.S. continues to decline sharply, due primarily to marked reductions in lead in gasoline, air, dietary sources, and residential paint.⁴  In a 1999-2002 national survey of children aged 1-5 years, 1.6% had blood lead levels >10 µg/dL, compared to 9% in a similar survey in 1988-1991.⁵ Although the nationwide prevalence of elevated blood lead levels among children ages 1-5 years declined dramatically from 1991-94 through 1999-2002, the prevalence still varies substantially among different communities and populations, and an estimated 310,000 children remain at risk for exposure to harmful levels of lead.⁴

What Is the Prevalence of Elevated Lead in Asymptomatic Pregnant Women?

Blood lead levels and blood umbilical cord lead levels are frequently used to assess both the mother's and fetus's level of lead exposure and risk. In 1992, two large surveys of low-income pregnant women found 0% and 6%¹ with blood lead levels >15 µg/dL.  A study of all women who enrolled in prenatal clinics in Mahoning County, Ohio, from 1990 to1992 found that 13% of prenatal patients had blood lead levels > 10 µg/dL, with 1% having blood lead levels greater than 15 µg/dL.⁶

Population mean blood lead levels in women of childbearing age and pregnant women have fallen over the past two decades. Although it was estimated in 1990 that 4.4 million women of childbearing age, and over 400,000 pregnant women, had blood lead levels of >10 µg/dl,⁷ a longitudinal study of pregnant women in Boston demonstrated that umbilical cord blood lead levels declined 82% between 1980 and 1990.⁸   A recent study of 1109 infants in Quebec, Canada, found a mean cord blood lead of 1.5 µg/dL (0.076 µmol/l; 95% CI = 0.074, 0.079).⁹ and in a recent review of NHANES data of 4394 women of child-bearing age, the GM blood lead level was 1.78 µg/dL.¹⁰

Are There Population-level Risk Factors That Identify Children at Higher Risk for Elevated Lead Levels (i.e. geography, race/ethnicity, socioeconomic status, age)?

The highest geometric mean blood lead levels (GM blood lead levels) in the U.S. occur in children aged 1-5 years (GM 1.9 µg/dL) and in adults > 60 years of age (GM 2.2 µg/dL), with the lowest in youth aged 6-19 years (GM 1.1 µg/dL).⁴  Children under five are at greater risk for elevated blood lead levels and lead toxicity because of increased hand to mouth activity, increased lead absorption from the gastrointestinal tract, and the greater vulnerability of a developing central nervous system.¹¹  Geometric mean levels are significantly higher in males than in females except among children aged 1-5 years.⁴ Correlates of higher blood lead levels at all ages include minority race/ethnicity, urban residence, low income, low educational attainment, older (pre-1950) housing, and recent immigration.^4,12-16  These factors are associated with increased exposure to important lead sources, including dilapidated housing containing lead-based paint, lead-soldered pipes, household lead dust, and lead in dust and soil from heavy traffic and industry.^1,17  There have been major reductions in the number of US homes with lead-based paint from the estimated 64 million in 1990, but approximately 24 million housing units still contain substantial lead hazards, with 1.2 million of these units occupied by low-income families with young children.^4,18

Less frequent sources of household lead exposure include contaminated clothing or materials brought home by workers in lead-using industries, lead-using home businesses or hobbies, lead-based paint and dust contamination in pre-1978 housing that is undergoing remodeling or renovation,¹⁶ dietary intake from lead-contaminated consumer products, drinking water, and lead-based pottery, and traditional ethnic remedies.^4,19-22

Geometric mean blood lead levels among African-American children (2.8 µg/dL) remain significantly higher than Mexican-American children (1.9 µg/dL) and non-Hispanic whites (1.8 µg/dL). Even among low income families, however, GM blood lead levels declined significantly from 1991-1994 (3.7 µg/dL) to 1999-2002 (2.5 µg/dL).⁴

A woman of childbearing age with a high blood lead level risks transmitting lead to her unborn child.²³  Ethnic background, country of origin, and immigrant status of birth mothers, pica behavior, as well as lifestyle and work patterns of pregnant women and age have shown to be associated with prenatal lead exposure in newborns. Multivariate analyses of pregnant women in Quebec, Canada revealed that both cigarette smoking (15% increase) and alcohol intake (17% increase) make significant and independent contributions to cord blood lead concentrations.²⁴  In a survey of 10 Quebec hospitals, umbilical cord blood samples were obtained from 1109 newborns. Although blood lead levels were considered low, a statistically significant relationship was observed between maternal age, and smoking during pregnancy, in cord blood lead concentrations.⁹

One hundred and fifty nine mother-infant pairs from a cohort of women receiving prenatal care in Pittsburgh, PA provided blood samples at delivery for lead determination. Alcohol use was associated with relatively greater cord blood lead compared with maternal Blood lead.  No association was found with cord blood lead or maternal blood lead with smoking, physical exertion, or calcium consumption.²⁵

A recent study in NYC of pregnant women in their third trimester with an incident blood lead level (blood lead levels) of 20 µg/dL or greater showed they had newborns with a median incident blood lead level of 12 µg/dL. In addition, maternal blood lead levels were directly associated with gestational age and pica behavior. These cases were more than twice as likely to be foreign-born women.²⁶

Neurotoxic Effects of Lead Exposure in Children

High levels of lead can produce serious central and peripheral neurological complications, including acute encephalopathy which can result in coma, death or long-term impairment.^1,27,28  Prospective cohort studies across several child populations have suggested that a rise in blood lead from 10 to 20 µg/dL is associated with a likely decrement of 2-3 points (reported range -6 to +1) in intelligence test scores (IQ).¹  The variety of test instruments that have been used, and differences in adjustment for important covariates, make direct comparison of these studies difficult but a consistent negative effect on intellectual development is reported.

Significant associations have been demonstrated between umbilical blood lead levels and neurodevelopmental testing at two years of age although the association was not significant at later ages. Blood lead levels at two years of age, however, were associated with neurocognitive performance at 10 years of age.¹¹  A recent analysis of school-aged children demonstrated a stronger cross-sectional inverse association of IQ with contemporary blood lead levels (mean BLL = 8 mcg/dL at age 7 years) than with baseline blood levels (mean BLL = 26 mcg/dL at 24 months old) suggesting an ongoing adverse effect of lead on cognitive performance among school-aged children.²⁹

Prior cross-sectional studies¹ consistently reported small, inverse associations between blood or tooth lead and reaction (attentional) performance, but studies evaluating the effect of mildly elevated lead levels on other measures of neurodevelopmental function (e.g., behavior, learning disorders, auditory function) produced inconclusive results. These outcomes have been less thoroughly evaluated than IQ, and more recent studies bolster an association between childhood lead exposure and disorders of attention and learning, and aggressive and delinquent behavior.^11,27,30,31

A growing number of human epidemiology studies have reported associations between neurotoxic effects and blood lead levels once thought to be harmless. Several recent studies have demonstrated an inverse relationship between historical blood lead levels and subsequent measures of intellectual and cognitive performance at blood lead levels of <10 µg/dL. The shape of the dose response curve at levels below 10 µg/dL is uncertain although data suggests that lead associated cognitive changes may be greater with incremental changes in blood lead levels in this range.^11,27,31-35   A recent meta-analysis of seven prospective international cohort studies found evidence of deficits on standard IQ testing among children with maximal blood lead levels <7.5 mcg/dL.  A decline of 6.2 IQ points (95% CI, 3.8-8.6) was observed as blood lead levels increased from 1 to 10 µg/dL.³⁶

Lead associated effects on neurobehavioral functioning must be considered relative to other important covariates such as socioeconomic status, home and parenting environment, and genetic factors.³²   The contribution of childhood lead exposure to the observed variance in cognitive ability (IQ testing) is believed to be in the range of 1-4%, while social and caregiving factors may be responsible for 40% or more.^30,32 Blood lead levels, however, appear to be associated with a substantial proportion of the known, modifiable variance in children's cognitive ability and incur a substantial social and economic burden among those affected and on the nation.^37,38

Reproductive Effects of Lead Exposure

The effects of high blood lead levels on reproductive outcomes have been well described.¹  High paternal blood lead levels (>40 µg/dL or prolonged levels greater than 25 µg/dL) are associated with impaired fertility, spontaneous abortion, and fetal growth abnormalities (preterm deliver and low birth weight). Maternal blood lead levels as low as 10 µg/dL have been associated with pregnancy hypertension, spontaneous abortion, and neurobehavioral effects in offspring. Studies evaluating potential associations between parental lead exposure and congenital malformations in offspring have not demonstrated consistent patterns of defects or magnitude of risk, and often lack biological indices of exposure at developmentally significant times.³

The Mexico City Prospective Lead Study examined the association of maternal prenatal blood lead level during pregnancy (range 7.5-9.0 µg/dl [0.36-0.43 µmol/l]) and child postnatal blood lead level (range of median blood lead level from birth to 48 months 7.0-10.0 µg/dl [0.34-0.48 µmol/l]) with head circumference in a sample of Latino immigrants living in Los Angeles. Multiple regression modeling showed significant negative associations (p<0.05, two-tailed) between 6-month head circumference and 36-week maternal blood lead level, and 36-month head circumference and 12-month blood lead level but these were the only significant associations among the over fifty assessed in this study.³⁹

In 272 mother-infant pairs, tibia bone lead was the only lead biomarker clearly related to birth weight (other significant birth weight predictors included maternal nutritional status, parity, education, gestational age, and smoking during pregnancy). Findings suggest that bone lead might be a better biomarker of lead body burden than blood lead.⁴⁰

Neurodevelopmental and Cognitive Measures and Lead Effects

Recent observational studies (prospective cohort and cross-sectional) provide limited, preliminary data that prenatal blood lead levels may be associated with neurodevelopmental delay or impairment. Study design and measurement issues, however, limit interpretation of these studies.

A prospective study of 103 African American neonates with low-level (<5 µg/dL) parental lead exposure included a battery of 16 neonatal behavioral assessments at 1 to 2 days after birth. No differences were found in 15 of the 16 domains studied, with neonates in the higher exposure group receiving lower scores on the hand-to-mouth motor activity than did those infants in the lower exposure group (p<0.05).⁴¹  A sample of 79 African-American infants with low-level prenatal parent lead exposure were given the Fagan test of Infant Intelligence (FTII) battery at 7 months of age.⁴²  Excluding all but infants with scores in the 5th and 95th percentiles of the FTII (n=5 in both groups) revealed that subjects rated at high risk for impairment on the FTII (lower 5th percentile) were 6 times more likely to be in the highest maternal blood lead level quartile (p<.004).  Infants scoring in the lower 15th percentile on FTII score (n=12), were 2 times more likely to be in the high maternal blood lead level quartile, though significance dropped to p<0.056.⁴²  The difference between the mean blood lead levels in the infants with lowest and highest FTII scores (5th and 95th percentiles) was very small, however (0.44 vs. 0.94 mcg/dL).  Recent evidence suggests that children may demonstrate differences in evoked visual and auditory potentials associated with increased levels of prenatal lead exposure.^43,44

Other Adverse Effects of Lead Exposure

Higher levels of blood lead (>40 µg/dL) exert detrimental effects on neurological, cardiovascular, renal, and hepatic function.¹ Subclinical effects on renal function can be observed at lower levels of exposure and children may be more vulnerable.^45,46

In a cohort of women in their third trimester, immigrant women were more likely to have elevated blood lead levels and elevated blood pressures, compared to non-immigrant women. An association between elevated blood level and blood pressure was significant only in the immigrant group.⁴⁷  Past lead exposure was associated with hypertension and elevated blood pressure during pregnancy.  Bone lead concentration, however, was not shown to be related to hypertension or elevated blood lead in pregnancy.⁴⁸

Among 110 women in their third trimester, gestational hypertension cases showed significantly higher blood lead levels than normotensives, and blood lead was significantly related to blood pressure, even after correcting for the body mass indices and age. The lead:ionized-calcium ratio showed a stronger association with blood pressure, than lead alone.⁴⁹  A cross-sectional study of 39 pregnant women in the third trimester of pregnancy compared red blood cell (RBC) levels of lead (Pb) and blood pressure. The study population included 20 women with normal pregnancies, 15 with mild hypertension, and 4 with severe hypertension and preeclampsia. Preeclamptic pregnancies were more likely to have an elevated RBC Pb.  Rank correlation showed a significant effect of RBC Pb level on blood pressure.⁵⁰

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