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Environmental Pollutants and their Influence on
Brain Development: What We Know
Ted Schettler, MD
Excerpted
from
http://www.protectingourhealth.org/newscience/learning/learningknow.htm
with additions and edits by Elise Miller, M.Ed. For more information,
please see www.cheforhealth.org
and www.iceh.org regarding the
national Learning and Developmental Disabilities Initiative (LDDI),
and
www.healthychildrenproject.org, the new web site launched by the
Learning Disabilities Association of America.
There are many ways that something can go awry in the brain, which
can impair our ability to learn, think, move, feel, perceive and/or
behave appropriately. The brain starts developing early in embryonic
life and continues developing into adolescence.
The developing brain is extraordinarily sensitive to environmental
agents' exposure levels that have no lasting effect on an adultıs
brain can have dramatic effects on the developing brain before birth
or during childhood.
What Causes Learning Disabilities, Emotional/Behavioral Disorders,
and Other Brain Disorders?
Learning and developmental disabilities are estimated to affect one
in six children in the U.S. under the age of 18 and appears to be
increasing.
Genetics, environmental, and social factors interact in complex ways
to determine how the brain develops and functions. They may be
inherited, or caused by factors that impact brain development before,
during, or after birth (such as drugs or poor nutrition before or
after birth, German measles before birth, or oxygen deprivation
during birth. Or, they can be related to brain trauma or brain
tumors. Though heredity, gene expression, social environments and
nutrition play critical roles in brain development, toxic chemicals
are currently the least researched and the most preventable.
How can environmental agents affect the brain?
Recent scientific studies suggest that the developing fetus and
children are particularly vulnerable to environmental hazards. Of the
80,000 chemicals currently registered on the marketplace, however,
only 12 neurotoxicants have been reasonably well studied. These
include lead, mercury, pesticides, polychlorinated biphenyls (PCBs),
and solvents. Exposures to these neurotoxicants during certain
developmental windows may negatively impact the developing brain and
contribute to learning and developmental disabilities (LDDs). In
addition, those who already have LDDs may have their conditions
further exacerbated by additional exposures later in life.
One specific way that environmental agents disrupt brain development
is by interfering with the processes that control gene activity. New
research is finding, for example, that the compound bisphenol A,
which is found in many plastic containers, affects when genes
important for controlling brain development are turned on.
By altering gene expression, environmental agents can interfere with
any one of many steps of brain development, as brain cells divide,
differentiate into specialized cell types, establish connections with
other brain cells, and die. Environmental agents can also interfere
with chemical messengers in the brain that help transmit nerve
impulses.
Interfering with any one step may impact later steps of brain
development. The timing can be as important as the amount of
exposure. Thus, even a brief exposure to an environmental agent at an
important step in brain development can have significant consequences
later in life.
Lead, alcohol, and nicotine are known to interfere at several points
in brain development. Both lead and alcohol interfere with the
differentiation of nerve cells, as well as the transmission of nerve
impulses, for example.
What role do environmental agents play in learning disabilities,
behavioral/emotional disorders, and other brain disorders?
A few chemicals have been studied intensively for their impacts on
the brain. However, the vast majority of chemicals to which people
are commonly exposed have never been examined at all for their
impacts on the developing brain. Among those that have been studied
incompletely, evidence coming from a wide array of experiments in the
lab point to possible impacts on people. There are many scientific
steps between what we understand today and reaching scientific
certainty, but we already can make some targeted recommendations
about ways that exposure reduction might reduce risks.
Below are some examples of environmental agents that affect the brain
and can play a role in brain disorders:
* Lead exposures during infancy and childhood can cause
attention problems, hyperactivity, impulsive behavior, reduced IQ,
poor school performance, aggression, and delinquent behavior. Lead
paint and lead-contaminated water and soil are the major sources of
lead exposure to children. The more lead is studied, the more
evidence we have showing that levels previously thought ³safe²
actually cause harm to the developing brain.
* Mercury easily crosses the placenta and disrupts many steps
in brain development. Even exposures at relatively low levels to a
pregnant woman can impair the IQ, language development,
visual-spatial skills, memory, and attention of her child. Like with
lead, the ³safe² level of mercury keeps dropping the more mercury is
studied.
* Manganese is essential to health at low levels in the diet,
but elevated levels of manganese in hair are associated with ADHD,
and laboratory experiments in animals link manganese with
hyperactivity. It is also associated with Parkinsons.
* (polychlorinated biphenyls), industrial chemicals now banned
but which persist in the environment, especially in fatty tissue, can
impair reflexes and IQ, delay mental development and the development
of motor skills, and cause hyperactivity. These effects have been
demonstrated in children born to women who consumed fish contaminated
with PCBs, as well as in laboratory animals.
* Tobacco smoke and nicotine are among the best studied agents
for their effects on the developing brain. Children born to women who
smoke during pregnancy are at risk for IQ deficits, learning
disorders, and attention deficits. Children born to women who are
passively exposed to cigarette smoke are also at risk for impaired
speech, language skills, and intelligence. Children exposed to
tobacco smoke after birth also are at risk for various behavioral
problems.
* Alcohol crosses the placenta and disrupts many steps in
brain development. Depending on the timing and amount of exposure to
a pregnant woman, the exposed fetus may develop into a child with
hyperactivity, learning problems, lowered IQ, or in more serious
cases, mental retardation.
* Bisphenol A alters the expression of genes that are
important for long-term memory formation and for early brain
development. Exposing fetal mice to extremely low doses of bisphenol
causes changes in their adult behavior.
* Perchlorate, a rocket fuel that now contaminates drinking
water in many communities in the US west, interferes with thyroid
hormone control of brain development in mice.
* Solvents like toluene cause learning, speech, and motor skill
problems in children. These effects were discovered in studies of
children borne to mothers who sniffed glue during pregnancy. The
impacts on the developing brain of other solvents have not been
studied in humans, but studies in animals indicate that they can also
impair normal brain development.
* Prions an infectious form of a type of protein, are believed
to be the agents that cause a rare brain disorder called variant
Creutzfeldt-Jakob Disease (CJD) in humans, as well as mad cow disease
in cattle, chronic wasting syndrome in deer and elk, scrapie in
sheep, and other similar brain diseases in other species. People can
be exposed to prions by eating contaminated food or other products
made from animals with diseases such as mad cow disease.
Why isnıt more known?
There are a number of reasons, including:
* The brain is complex and can be affected in many ways.
* Interactions between multiple factors (environmental, social,
genetic) make it hard to pinpoint the contribution of one factor
* Lag time between exposure and effects makes it difficult to measure
or estimate exposure.
* It is difficult to use experimental animals to study how
environmental agents affect higher mental abilities involving
learning and behavior since these are often unique to humans.
* Companies that produce these chemicals often resist regulations as
well as scientific studies that may indicate their products are toxic
and should be taken off the market.
* Current chemical laws are 30 years old and haven't kept pace with
the latest scientific research
Where does that leave us now?
* Learning disabilities, behavioral disorders such as autism and
ADHD, and some other brain disorders appear to be on the increase but
for the most part, hard numbers are lacking.
* The largest study of children in history, the National Childrenıs
Study, will track more than 100,000 children from before birth until
age 21, examining how inheritance and a variety of environment,
social, cultural, dietary, and biological factors influence health
and development. Behavioral disorders like ADHD and autism are high
on the study's research agenda.
* New scientific results on the impact of lead, mercury, microwave
radiation, bisphenol A, brominated flame retardants, PAHs (polycyclic
aromatic hydrocarbons) and other agents provide intriguing clues that
need follow-up.
* Few of the chemicals in widespread use today--even those regularly
found in human tissue, including umbilical cord blood and amniotic
fluid--have been submitted to any testing to determine their possible
impact on neurological development.
* A "better safe than sorry" approach based on the precautionary
principle should be taken when dealing with chemicals that may impact
the developing brain. While such an approach is most effective when
it is taken by government and industry, there fortunately are many
common sense steps that individuals and families can take as well.
Elise Miller, M.Ed
Executive Director
Institute for Children's Environmental Health
1646 Dow Road
Freeland, WA 98249
ph: 360-331-7904; fax: 360-331-7908
emiller@iceh.org;
www.iceh.org;
www.partnersforchildren.org
"Working for a Healthy, Just and Sustainable Future for all Children
and the Planet |
