| Introduction:
Neurodevelopmental Disorders in Children | Mental
Retardation
Emerging Issues: Attention-Deficit/Hyperactivity Disorder
Neurodevelopmental Disorders in Children
Researchers estimate that between 3 and 8 percent of the babies
born in the United States each year will be affected by neurodevelopmental
disorders such as attention-deficit/ hyperactivity disorder or mental
retardation.51 Neurodevelopmental
disorders are disabilities in the functioning of the brain that
affect a child’s behavior, memory, or ability to learn. These
effects may result from exposure of the fetus or young child to
certain environmental contaminants, though current data do not indicate
the extent to which environmental contaminants contribute to overall
rates of neurodevelopmental disorders in children. A child’s
brain and nervous system are vulnerable to adverse impacts from
pollutants because they go through a long developmental process
beginning shortly after conception and continuing through adolescence.52,53
Studies have found that several widespread environmental contaminants
can damage children’s developing brain and nervous system.
Childhood exposure to lead contributes to learning problems such
as reduced intelligence and cognitive development.54-56
Studies also have found that childhood exposure to lead contributes
to attention-deficit/ hyperactivity disorder57
and hyperactivity and distractibility;58-60
increases the likelihood of dropping out of high school, having
a reading disability, lower vocabulary, and lower class standing
in high school;61 and
increases the risk for antisocial and delinquent behavior.62
Methylmercury also has negative impacts on children’s neurological
development. Studies of children whose mothers had high intakes
of mercury-contaminated seafood prior to conception found adverse
impacts on intelligence63,64
and decreased functioning in the areas of language, attention, and
memory.65 Particularly
high levels of exposure to mercury in the womb have been found to
cause mental retardation.66,67
Several studies of children exposed to elevated levels of polychlorinated
biphenyls (PCBs) have linked these contaminants to neurodevelopmental
effects, including lowered intelligence and behavioral deficits
such as inattention and excessive reaction to stimulation. Most
of these studies find that the effects are associated with exposure
in the womb resulting from the mother having eaten food contaminated
with PCBs.68-73 Adverse
effects on intelligence and behavior also have been found in children
of women who were highly exposed to mixtures of PCBs, chlorinated
dibenzofurans, and other pollutants prior to conception.74-76
Human studies also suggest that exposures to other metals such
as cadmium and arsenic may have adverse effects on neurological
development.58,77-79
Other types of pollutants also have been associated in animal studies
with neurodevelopmental effects. Numerous toxicological studies
link both prenatal and postnatal exposure to organophosphate pesticides
to neurodevelopmental effects.80
A recent study of brominated flame retardants found that two of
these compounds caused adverse effects on behavior, learning, and
memory in animals.81
Mental retardation is a neurodevelopmental disorder that, in some
cases, is related to exposures to environmental contaminants such
as lead. A measure of mental retardation in children is presented
here. A second neurodevelopmental disorder, attention-deficit/hyperactivity
disorder, is discussed in the Emerging
Issues section of this report.
Although the studies described above have related lead, PCBs, mercury,
and perhaps other contaminants to adverse neurodevelopmental effects
in humans, it is not currently possible to determine the extent
to which environmental contaminants contribute to developmental
disorders.
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Mental Retardation
The most commonly used definitions of mental retardation emphasize
subaverage intellectual functioning before the age of 18, usually
defined as an intelligence quotient (IQ) less than 70 and impairments
in life skills such as communication, self-care, home living, and
social or interpersonal skills. Different severity categories, ranging
from mild retardation to severe retardation, are defined on the
basis of IQ scores.82
Researchers have identified many causes of mental retardation,
including genetic disorders, traumatic accidents, and prenatal events
such as maternal infection or exposure to alcohol.82,83
Exposure to lead and exposure to particularly high levels of mercury
also have been shown to cause mental retardation.66,67,84
Furthermore, lead, mercury, and PCBs all have been found to have
adverse effects on intelligence and cognitive functioning in children.
Exposure to these environmental contaminants therefore has the potential
to increase the proportion of the population with IQ less than 70,
thus increasing the incidence of mental retardation in an exposed
population.85
The causes of mental retardation are unknown in 30 to 50 percent
of all cases.83 The causes
are more frequently identified for cases of severe retardation (IQ
less than 50). The cause of mild retardation (IQ between 50 and
70) is unknown in more than 75 percent of cases.86,87
This measure on the prevalence of mental retardation among U.S.
children presents data obtained from the National Health Interview
Survey (NHIS). Although the NHIS provides the best national-level
data available, NHIS data likely underestimate the prevalence of
mental retardation. Reasons for this understatement may include
late identification of affected children and the exclusion of institutionalized
children from the NHIS survey population. Further, the NHIS relies
on parents reporting that their child has been diagnosed with mental
retardation, and accuracy of parental responses could be affected
by cultural and other factors.
Data table for this graph
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- In 1997-2000, about 6 children out of every 1,000 were reported
to have been diagnosed with mental retardation.
- Reported rates of mental retardation were 10 per 1,000 Black
non-Hispanic children, 6 per 1,000 White non-Hispanic children,
and 5 per 1,000 Hispanic children.
- 12 children out of every 1,000 living in families with incomes
below the poverty level were reported to have mental retardation.
Reported rates of mental retardation were lowest for children
living in families with higher incomes.
Related Measures:
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