Neurological Development
 Developmental
neurotoxicity and behavioural effects are of particular concern
in children’s environmental health. The window of susceptibility
to neurotoxic effects in children is broad because of the lengthy,
complicated development that the brain and nervous system undergoes
from the embryonic stage, through childhood and into adolescence.
The CNS cells also do not readily repair themselves after damage;
as a result, the brain and nervous system represent unusually vulnerable
target organs from exposure to contaminants.
- The earliest critical window for neural development occurs in
utero during the first trimester, from about weeks
3 to 5. This represents the time during which the
basic structures of the central nervous system are initially
formed and, hence, adverse contaminant exposures during this period
are more likely to result in major (anatomical) malformations
of the CNS.1
- The brain undergoes enormous growth in size from its
differentiation in the embryo into the postnatal period. At birth,
the brain is about 25% of adult size and it expands in size well
into childhood. The brain reaches its maximum volume when a child
is about age ten.2
- Development of the brain and CNS structures is highly intricate
and involves a number of essential processes (such as nerve cell
proliferation, migration and differentiation, synapse formation,
myelination and apoptosis). The entire sequence of events is rather
rigidly programmed and unfolds at prescribed times and rates that
differ depending on the area of the brain.3
- For the most part, the greatest vulnerability to disruption
of any of the processes is in the fetal and infant stages
of brain development. Timing of exposures in early life
is a critical consideration in determining the potential effects
of neurotoxic substances. Of note, the blood-brain barrier,
which partly protects the brain from toxic substances, does not
fully develop until about six months postnatally.4
- Exposure of a child’s brain and nervous system tissues can occur
transplacentally (i.e., in utero) and depending
upon the specific neurotoxin, from inhalation, dermal absorption
and ingestion postnatally.
- Children’s behavioural differences throughout their lives
frequently bring them into greater contact with neurotoxins.
For example, their hand-to-mouth activity, tendency to explore
their environments and the fact that they spend a lot of time
crawling or close to the ground put infants and young children
at greater risk of exposure to contaminants.
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