Recent Findings on the Effects of Lead Exposure
The following summaries are excerpted from the Franklin Institute.
Lead Lowers Intelligence – Study
In 1995, the American Academy of Pediatrics reviewed 18 scientific studies on the correlation between children's mental abilities and lead in their blood. "The relationship between lead levels and IQ deficits was found to be remarkably consistent," the Academy said. "A number of studies have found that for every 10 ug/dL (microgram per deciliter) increase in blood lead levels, there was a lowering of mean IQ in children by 4 to 7 points." (That's less than a thousandth of a gram of lead.)
Lead Promotes Delinquent and Antisocial Behavior – Study
Lead exposure is probably the most preventable cause of antisocial behavior, according to the leading expert on lead exposure and behavior. Herbert L. Needleman, M.D., a professor of child psychiatry and pediatrics at the University of Pittsburgh, has documented how low levels of lead in boys are associated with aggressive behavior, delinquency, and attention disorders - all predictors of criminal behavior later in life.
His groundbreaking studies were instrumental in nationwide government bans on lead from paint, gasoline and food and beverage cans. In 1979, using measurements of lead in children's teeth, Needleman concluded that children with high lead levels in their teeth (but no outward signs of lead poisoning), had lower IQ scores, shorter attention spans, and poorer language skills.
Needleman's Antisocial Behavior – Study
Needleman's 1996 study published in the Journal of the American Medical Association examined 301 boys in Pittsburgh public schools. Behaviors reported by the boys' teachers and parents - and by the boys themselves - were correlated with the lead measured in their bones.
The boys' behavior was measured at age 7 and again at age 11. Those who had more lead in their bones consistently had more reports of aggressive and delinquent behavior, and problems paying attention.
They were more likely to engage in antisocial activities like bullying, vandalism, truancy, and shoplifting. Furthermore, their behavior got worse as they grew older. In contrast, behavior did not change among boys with less lead in their bones.
Lead and Delinquency – Study
A 2000 study examined the lead levels in 216 youths convicted in the
Juvenile Court of Allegheny County, PA - and 201 non-delinquent controls
from high schools in Pittsburgh. Convicted male juveniles were nearly
twice as likely to have high bone-lead levels, compared to those with
no convictions.
Delinquency was associated with higher lead levels even when sex, race, and education,
and the neighborhood crime rate were taken into account. About 16% of juvenile
delinquent behavior can be attributed to lead exposure.
"This study provides further evidence that delinquent behavior
can be caused, in part, by childhood exposure to lead," said Needleman. "Of
all the causes of juvenile delinquency, lead exposure is perhaps the
most preventable." It is part of a growing body of evidence linking
lead to cognitive and behavioral problems in children.
Some British doctors have called for routine blood screening of children with
learning and behavioral problems. They found that problem youngsters had higher,
sometimes toxic, levels of lead. "Our results suggest that children with
developmental and/or behavioral problems are more likely to have higher blood
concentrations than the general childhood population."
Lead is a Risk Factor for Alzheimer's Disease and Senile Dementia – Studies
A U.S. study found that exposure to lead may be an important environmental link to one of the most dreaded forms of brain degeneration, Alzheimer's disease. Research presented at the May 2000 meeting of the American Academy of Neurology revealed a dramatic correlation between on-the-job lead exposure and the risk of developing Alzheimer's disease in later years.
The occupational histories of 185 people with Alzheimer's disease were compared to 303 people without the disease. Results showed that individuals were up to 3.4 times more likely to develop Alzheimer's if they had worked in jobs exposing them to high levels of lead - either by breathing lead dust or from direct skin contact.
Although most of us need not worry about being poisoned by lead from jobs that require smelting, welding, and the manufacture of lead products - like those represented in this study - we should still be cautious. "Lead exposure remains a major public concern because of its adverse effects on brain development and health in general, even with low exposure levels," says Elisabeth Koss, Ph.D., the study's lead author at Case Western Reserve University.
Early Exposure to Lead May Set the Stage for Alzheimer's Disease – Study
This early exposure to lead may be setting the stage for later onset of Alzheimer's disease, according to a report published in the November 1998 issue of Epidemiology. According to Dr. Prince at the London School of Hygiene and Tropical Medicine and the Institute of Psychiatry, England:
"Few environmental risk factors for Alzheimer's disease have been identified. This lack of information may reflect the fact that salient factors affect most of the population in developed countries.
Furthermore, the critical period of exposure may be earlier than hitherto suspected, during the first years of life, as the brain differentiates and develops. Exposure to lead at levels lower than those associated with evident toxicity causes mild intellectual impairment in childhood. I hypothesize that this may be one of the childhood exposures that also confers an additional risk for the onset of Alzheimer's disease."
Myelin - A Link Between Lead and Senile Dementia – Study
Studies suggest that lead may be involved in senile dementia. Myelin-producing cells seem to be particularly vulnerable to lead, and myelin damage is associated with Alzheimer's disease.
Lead inhibits myelination, the formation of the white fatty myelin sheath that surrounds and protects nerve fibers. In animal studies, lead was also shown to interfere with the assembly of microtubules (the tiny pipe-like structures that transport brain chemicals within a neuron) and to form fibers in the brain - structural brain changes associated with dementia.
Early research observed that "the
changes in myelin from humans with Alzheimer's disease are more
pronounced than in normal aging. These changes might represent
severe or accelerated aging."
A Swedish study of the autopsied brains of people with Alzheimer's
disease and vascular dementia determined that the altered fatty
acid composition in white matter suggested that "the myelin
sheath is the primary lesion site." The same researchers at
the University of Goteborg later concluded that membrane lipids
selectively diminished in Alzheimer's brains indicate that demyelination
is a primary event in late-onset form Alzheimer's disease.
Lead Causes Progressive Mental Decline – Study
"We know there's a decline in brain power as we get older - generally we call this 'normal aging,'" said Brian Schwartz, MD, of Johns Hopkins School of Hygiene and Public Health in Baltimore. "Most of the research has been about how chemicals, like lead, affect kids. This is the first study to explore long-term problems caused by exposure to chemicals as adults. Some of what we have been calling 'normal aging' may in fact be due to past exposures to chemicals or other agents that can affect the central nervous system. This is potentially a very important health problem."
A four-year study looked at 535 former chemical manufacturing employees, who had an average of eight years of occupational exposure to lead and an average of 16 years since last working with lead. Blood and bone levels of lead were measured, and neurological tests were given - and compared with 118 non-exposed people from the same neighborhoods.
The lead workers not only had greater declines in test scores, but also in normal age-related declines in brain functions. "The effects of the average level of bone lead found in former lead workers was like five more years of aging on the brain," said Dr. Schwartz.
Significant differences were discovered between the former workers and other participants in tests involved in visual construction, verbal memory and learning, visual memory, planning and organizational ability, and manual dexterity.
"The higher the peak level of lead determined in former lead workers, the greater the decline in brain functions," Schwartz said. "Since these declines were seen long after exposure to lead had stopped, it suggests that the effect of lead on the brain is progressive."
Lead in the Brain-Physical Damage – Study
Once in the brain, lead-induced damage occurs primarily in the prefrontal cortex , cerebellum, and hippocampus . It adversely affects many biological activities at the molecular, cellular, and intracellular levels.
Investigating lead's effects on the brains of children, Israeli researchers report that, lead disrupts the main structural components of the blood-brain barrier, by damaging its capillaries and by injuring the glial cells that protect neurons.
Lead Blocks Neurotransmitter Development – Study
Exposure to lead alters the developmental processes of synapse formation, resulting in a less efficient brain and cognitive deficits. It blocks the action of calcium atoms in the synthesis of serotonin and dopamine, neurotransmitters essential to normal impulse control and suppression of violent behavior.
A vast amount of evidence accumulated over many years has shown that lead disrupts processes that are regulated by calcium. Researchers at the Kennedy Krieger Research Institute in Baltimore found that "many of the neurotoxic effects of lead appear related to the ability of lead to mimic or in some cases inhibit the action of calcium as a regulator of cell function."
Lead Can Cause Myelin Damage – Study
One of the mechanisms by which lead damages the brain involves myelin, the white fatty protective substance that insulates nerves and allows them to carry signals quickly and accurately. If the myelin sheath is structurally damaged, then a nerve's electrochemical impulse can become abnormal and uncoordinated. Consequently, the information being conveyed by this nerve is scrambled or cut off.
Myelin-producing cells seem to be particularly vulnerable to lead. Animal studies have shown that lead inhibits myelination (myelin formation), and prolonged lead toxicity causes significant changes in the structure of myelin cells.
An overview of research concluded that the effect of lead poisoning is one of "hypomyelination as seen from the prominent reduction in the content of cerebral myelin. . . Furthermore, the toxin specifically hampers the process of myelin membrane assembly."