LEAD POISONING AND INTELLECTUAL/BEHAVIORAL IMPAIRMENT - RESEARCH REVIEW
LOW LEVEL LEAD EXPOSURE AND INTELLECTUAL DEFICITS
♦ Bellinger
Lead is a confirmed neurotoxin, but questions remain about lead-associated intellectual deficits at blood lead levels < 10 mcg/dl and whether lower exposures are, for a given change in exposure, associated with greater deficits. Bellinger studied the association of intelligence test scores and blood lead concentration, especially for children who had maximal measured blood lead levels < 10 mcg/dl. Data was collected from 1,333 children who participated in seven international population-based longitudinal cohort studies, followed from birth or infancy until 5 to10 years of age. The full-scale IQ score was the primary outcome measure.
After adjustment for covariates, an inverse relationship between blood lead concentration and IQ score was found. Using a log-linear model, a 6.9 IQ point decrement [95% confidence interval}was associated with an increase in concurrent blood lead levels from 2.4 to 30 mcg/dl. The estimated IQ point decrements associated with an increase in blood lead from 2.4 to 10 mcg/dl, 10 to 20 mcg/dl, and 20 to 30mcg/dL were 3.9, 1.9 and 1.1 respectively. It was concluded that environmental lead exposure in children who have maximal blood lead levels < 7.5 mcg/dL is associated with intellectual deficits.
♦Pl Landrigan, Mt.SinaiSchool of Medicine From Pediatric Lead Poisoning: Is There A Threshold
The lowest blood lead concentration (BLL) associated with deficits in cognitive function and academic achievement is poorly defined. This study examined low BLL, 10 mcg/dl, and performance in tests of cognitive function in U.S. children and adolescence. After control of confounding variables, in a multivariate analysis of 4853 children an inverse relationship between BLL and scores of four measures of cognitive function was found. For every 1mcg/dl increase in BLL there was a 0.7 point decrement in arithmetic scores, 1 point in reading scores, 0.5 in short term memory and a 0.1 decrease in non-verbal reasoning. The authors feel these findings need to be replicated before becoming the basis of public health action but caution against complacency regarding the BLL of 10 mcg/dl being a bright line below which pediatricians need not worry.
♦ Low-Level Environmental Lead Exposure and Children’s Intellectual Function:
An International Pooled Analysis - Environ Health Perspectives, 2005 July
In the last several years researchers have begun to suspect that even lower concentrations, < 10 mcg/dl may impair cognition. Now a reevaluation of data from seven international longitudinal studies involving 1,400 children confirms this suspicion. The studies conducted in Boston, Cincinnati, Cleveland, Rochester, Port Pirie (Australia), Mexico City, and Yugoslavia looked at children known to be at risk for lead poisoning.
The research team calculated, across the seven studies, how much of the difference in IQ scores was related to lead alone by controlling for other factors that influence IQ scores, including child birth weight, birth order, prenatal exposure to tobacco smoke and alcohol, and mother’s IQ.
This looks at the same group of studies as above and concludes similarly:
• On a population basis, an increase in blood lead level from 2.4 to 10 mcg/dL at the time of testing was associated with a decrease of 3.9 IQ points.
•At lower blood lead levels, a small increase in blood lead made a bigger difference in IQ than the same size increase did at higher concentrations. Consistent with a study published in the May 2005 issue of EHP, blood lead level at the time of IQ testing was generally a stronger predictor of effects on IQ - than was previously believed - blood lead level at age 2. These new findings, along with those from previous human and animal studies, point to the importance of eliminating nonessential uses of lead and lowering allowable levels of lead in air emissions, house dust, soil, water, and consumer products.
♦ A Rationale For Lowering The Blood Lead Action Level From 10 To 2 Mcg/dl
Steven G. Gilberta, Institute of Neurotoxicology & Neurological Disorders, Seattle, WA
Bernard Weiss, Department of Environmental Medicine, University of Rochester, School of Medicine and Dentistry, Rochester, NY, United States - January 2006
In 1991, the U.S. Centers for Disease Control and Prevention (CDC) established 10mcg/dl as the lowest level of concern for children's BLLs. A growing body of evidence reveals that blood lead levels below 10 mcg/dl may impair neurobehavioral development however, there remains uncertainty about the effects of lower levels of lead exposure in children. There is now sufficient and compelling scientific evidence for the CDC to lower the blood lead action level in children. This review argues that a level of 2mcg/dl is a useful and feasible replacement. Although it can be argued, in turn, that no threshold for the health effects of lead is demonstrable, analytically a blood level of 2 mcg/dl is readily and accurately measured and provides a benchmark for successful prevention. Lowering the level of concern would also offer economic advantages because of the coupling between lead, educational attainment, earnings and anti-social conduct.
♦ Declining Blood Lead Levels and Changes in Cognitive Function During Childhood: Port Pirie , Australia Cohort Study.
Tong S, Baghurst PA, Sawyer MG, Burns J, & McMichael AJ.
Many studies have found a significant inverse association between early exposure to environmental lead and cognitive function in childhood. Whether these effects are reversible when exposure is reduced is not clear. This study looked at the reversibility of the apparent effects of lead on cognitive abilities in early childhood by testing whether declines in blood lead concentrations beyond the age of 2 years are associated with improvements in cognition.
Three hundred and seventy five children were followed up from birth to the age of 11 to 13 years using a prospective cohort study. The Bayley Mental Development Index at age 2 years, the McCarthy General Cognitive Index at age 4 years, and IQs from the Wechsler Intelligence Scale at ages 7 and 11 to 13 years were administered.
Mean blood lead concentrations in the children decreased from 21 .2 mcg/dl at age 2 years to 7.9 mcg/dl at age 11 to 13 years, but cognitive scores in children whose blood lead concentration declined most were generally not improved relative to the scores of children whose blood lead levels declined least. Changes in IQ and declines in blood lead levels that occurred between the ages of 7 and 11 to 13 years (r= 0.12, P= .09) suggested slightly better cognition among children whose blood lead levels declined most. The cognitive deficits associated with exposure to environmental lead in early childhood appear to be only partially reversed by a subsequent decline in blood lead level.
LINKS BETWEEN CRIMINAL BEHAVIOR, AGRESSION, ADHD AND LEAD TOXICITY IN CHILDREN
♦ Professor Deborah Denno, of FordhamUniversity, work shows a highly significant link between observed lead toxicity and the likelihood of criminal activity. The rich nature of the data allowed her to explore some 3,000 factors for links to criminal behavior in 1000 children who were followed from birth to age 22. The researchers found that the best predictor (when the children are 7 years old) for aggressive behavior in school, juvenile delinquency, and eventual criminal violence is the degree of lead poisoning. The second best predictor was anemia itself is a common symptom of lead poisoning.
This researched found a significant dose-response of higher BLL and ADHD when compared to children at the lowest quintile of BLL. Children with levels > 2 mcg/dl had a 4 fold increased risk of ADHD.
EPA records of toxic metal releases and FBI data on violent crime found a strong correlation between violent crime and toxic metal pollution. It is believed violent crime may be attributable to brain damage from neurotoxins that breakdowns the inhibition mechanism in the frontal lobe of the brain that controls behavior. Behavior of children diagnosed with ADHD looks very much like the uninhibited, impulsive behavior of the children studied for ties between aggression and lead toxicity.
♦In a study published in the May 2000 Environmental Research Journal, Rick Nevin performed a statistical analysis of childhood lead exposure and violent crime rates. Childhood lead exposure explained 88% of the variation in the violent crime rates from 1960 to 1998. In May of 2001, the American Medical Association published a study titled "The Relationship Between Lead Exposure and Homicide" by Paul B. Stretesky, PhD. and Michael J. Lynch, PhD. (Arch Pediatric Adolesc Med. 2001which reported:
Numerous behavioral, neuropsychological, and biological studies suggest that sufficient exposure to lead, a metallic neurotoxin, can promote brain dysfunction. This happens when lead exposure alters neurotransmitter and hormonal systems and may induce aggressive and violent behavior. It is not possible to say that the observed relationship is causal. Nevertheless, the finding of an association between lead exposure and homicide is consistent with the few individual-level studies examining the role of lead exposure in delinquency and aggression. In recent years, research has pointed toward the neurotoxicity and behavioral effects of lead at sub-clinical levels.
♦ A National Academy of Sciences report "Understanding and Preventing Violence”, linked lead exposure to both ADHD and juvenile delinquency. Investigators seeking the bio-chemical origins of criminal behavior frequently point to the clear involvement of lead poisoning as a contributor to, if not a cause of, Attention Deficit Hyperactivity Disorder (ADHD). The report reads:
“It has recently been found that lead poisoning during childhood can have long-term detrimental effects on behavior. Exposure to lead, usually from a young child consuming lead-based paints, has been associated with ADHD. As previously mentioned, ADHD is a well-established risk factor for later antisocial behavior.
♦ Robert Tuthill, Archives of Environmental Health, Vol. 51, No. 3, May-June 1996.
A new study by Robert Tuthill links even slightly elevated lead levels to attention deficit disorder. Tuthill analyzed lead levels in the hair of 277 first-graders from eight Massachusetts schools, and found a striking dose-response relationship between hair lead levels and symptoms of attention deficit disorder as reported by teachers. This relationship remained strong, he says, even after age, ethnicity, gender, and socioeconomic status were taken into account. "An even stronger relationship existed between physician-diagnosed attention-deficit hyperactivity disorder and hair lead in the same children.” This means the more certain they were of ADHD the more certain they were that lead was involved. ADHD may simply be symptoms of brain damage from exposure to lead in the early years of a child's life.
Tuthill also found that "there was no apparent `safe' threshold for lead," with even low exposure increasing the likelihood of attention deficits - a disturbing finding, since approximately three million children in the United States are estimated to have at least mildly elevated lead levels. ("Hair lead levels related to children's classroom attention-deficit behavior,"
♦Michael Naylor, Behavioral Scientist, SydneyUniversitybelieves evidence demonstrates strong links between various learning and behavioral problems and lead exposure. Some problems observed in the classroom that have been associated with lead poisoning include a mild decrease in IQ, slower response time, central auditory processing problems, word finding difficulties, sequencing difficulties, and perceptual integration problems.
Even at low BLLs exposure has been associated with deficits in the early developmental years. Canfield et al.’s study (2003) on the effects of low level lead poisoning
(< 10mg/dl) on cognitive functioning in children and infants, found that IQ declined 7.4 points as average blood lead concentrations increased from 1 m g/dl to 10 m g/dl, and then declined 4.6 points for every 10 m g/dl increase after that.
♦ Lead exposure has also been linked to behavioral problems. In studies, as far back as 1988, Needleman measured dentine Pb levels in first-and second-grade students and administered neurobehavioral tests. The results showed that behavioral and performance problems in primary school children were more prevalent with increasing dentine (teeth) lead levels. These behavioral problems included hyperactivity, distractibility,impulsivity, disorganization, non-persistence, inability to follow simple instructions and overall poor functioning. When these children were re-examined at age 18 results showed increased dentine levels were associated with increased risk of not graduating high school, reading difficulties, low class standing in high school, increased absenteeism, lower vocabulary and grammatical reasoning scores and slower reaction time. Lead levels were also inversely related to self-reports of minor delinquent activity
These and other studies such as those by Needleman (2002), in which elevated body bone lead concentrations were found to be associated with elevated risk for adjudicated delinquency, and Dietrich et al (2001)
♦ Exposure to Environmental Toxicants and Attention Deficit Hyperactivity Disorder (ADHD): Association with Low Blood Levels
The prevalence of ADHD is about 3-8% and it puts kids at high risk for conduct disorders, anti-social behavior and drug abuse later in life. There are both genetic and environmental factors contributing to ADHD and the exact mechanism of their interplay is not well understood. Environmental lead exposure, determined by high blood and dentine levels, has been associated with inattention and impulsivity (documented and corroborated in studies as far back as 1993). In older studies the samples were small and they were based on higher mean Lead levels than currently seen.
Studies conducted in 2003 and 2005 linked IQ deficits at lead levels < 10mcg/dl, the current action level set by CDC. This link is complicated because neurobehavioral outcomes appear to differ by gender with boys having a much higher prevalence.
The diagnosis was defined by parental report that a health professional had told them their child had ADHD and stimulant medication use. The primary analysis used both criteria. The study examined multiple covariates and potential confounders plus demographics. In the secondary analysis they looked at levels < 5mcg with ADHD children from age 4 to 15. Blood Lead Level (BLL) was a significant predictor of ADHD and had a dose-response relationship. It looks different between genders.
In the secondary analysis BLL of <5 mcg/dl showed a significant association. Children in the higher quintile of the 2 – 5 mcg/dl range showed a 4 fold higher risk for ADHD while children in the lowest quintile showed no detected increase. The authors estimate that 21.1% of ADHD cases among children ages 4 to 15 where attributable to having BLL >2.0mcg/dl; an excess of 290,000 cases.
This analysis for the first time demonstrates a significant dose-response relationship between childhood lead exposure and ADHD which is consistent with previous findings of an association between BLL or Dentine lead levels and behavior problems(Sood 2001, Wasserman, 1998 & 2001).