development is a complex process. At
each stage in a child’s developmental path unique biological processes occur.
The physiological and morphological milestones that mark a child’s
development equate to “windows of vulnerability” or critical periods during
which interference with the developmental process by exposure to an
environmental contaminant may result in irreversible changes. The outcome of exposure varies depending
upon the timing of exposure in a child’s life.
|·In utero exposure of the embryo or fetus
to environmental contaminants, many of which are able to cross the placenta,
may permanently alter the course of development, even at very subtle exposure
levels. The prenatal stages of
development of major organs, body structures, nervous and reproductive
systems, represent especially sensitive times.
|·Postnatally, certain organ systems and body
structures are also more vulnerable to effects from exposure because they
undergo continued differentiation before adulthood. The brain, nervous system
and lungs undergo extensive growth after birth and are particularly sensitive
throughout much of childhood as a result. For example, we now know that lead
exposure prior to age 2 has marked effects on nervous system development.
Endocrine disruptors and air pollutants may operate at an early stage in
development, predisposing children to health effects later on in life.
|·At the same time, the fact that several other
body systems are immature in the infant and child renders them particularly
ill equipped to handle toxic contaminants. The digestive tract and skin are
extremely permeable and the developing lungs present large surface areas
through which chemicals may be easily absorbed. The physiological mechanisms that normally
help protect the body from chemicals that do invade it, such as the immune,
excretory and de-toxifying systems are also underdeveloped in the earliest
stages of life.