Infants and children are not little adults, and they are often more vulnerable to the adverse effects of drugs, chemicals, radiation or disease. In spite of this fact, only one law, that covering pesticides, requires regulators to base safety judgments explicitly on risks faced by the fetus, infant and child, and even this statute has not been rigorously implemented. For most toxic substances in commerce and consumer products, there are few if any health studies at all. Those that are done are typically done on adult (sexually mature) test animals.

The fetus, infants and children are more vulnerable to toxic substances:

Nitrate - Prolonged exposure to tap water with 20 ppm nitrate can kill an infant, but will have no observable effect on an adult.

Mercury - Exposure in the womb at 100 parts per billion will significantly increase the likelihood of learning deficits in childhood, while the same dose during adulthood has no measurable effect.

Radiation - Children exposed to radiation have a much higher incidence of cancer than adults exposed to the same dose.

PCBs - Levels of fetal PCB exposure that cause learning deficits that persist through adolescence, have no measurable effects on adults.

Ritalin - This commonly prescribed drug acts as a depressant and calming agent in children, but has the opposite effect in adults.

Honey - Infants under one year of age can contract a potentially fatal paralytic form of botulism from eating honey. In older children and adults the digestive system is more acidic and it destroys the botulism toxins present in the honey.

Lead - Doses at age 2 that cause IQ deficits throughout childhood produce no effect in adults.

Researchers at a 1997 EPA sponsored national conference on children's environmental health summarize children's vulnerability in the context of chemical exposures (Landrigan et al 1998):

"Many organ systems in young children - the nervous system in particular, but also the lungs, the immune system, and the reproductive organs - undergo extensive growth and development throughout pregnancy and in the first months and years of extra-uterine life. During this period, structures are developed and vital connections established. These systems are not well adapted to repair any damage that may be caused by environmental toxicants. Thus, if cells in the developing brain, immune system, or reproductive organs are destroyed by neurotoxicants, or if development is diverted by endocrine disruptors, there is high risk that the resulting dysfunction will be permanent and irreversible. Depending on the organ damaged, the consequences can include loss of intelligence, immune dysfunction, or reproductive impairment."

Two important factors make the young more vulnerable.

1. The developing human body, particularly the fetus, has critical windows of vulnerability

Many studies have shown that the peak incidence of birth defects coincides with the timing of key developmental events. This means that maternal exposures to toxic substances on a particular day of pregnancy might cause devastating effects to a baby, while exposures on the very next day would cause no discernible effects whatsoever. For example, implantation of the egg occurs on gestational day 6 to 7; organs begin forming on days 21 through 56; the neural plate forms between days 18 and 20; arm buds appear on days 29 to 30, and leg buds follow shortly after on days 31 to 32; testes differentiation occurs on day 43, and the palate closes between days 56 and 58.

The brain is a particularly sensitive target for early-life exposures. The blood brain barrier, which protects the developing brain from some chemical exposures, is not fully developed at birth. Chemical substances have been shown to be three to ten times as toxic to newborns as adults, or in some cases to damage the newborn brain and not the adult brain, in part due to differences in the stages of development of the blood-brain barrier (Klassen 1996).

The human brain develops in overlapping phases, each presenting unique opportunities for chemical-induced damage, beginning in the womb within days of conception, and continuing through childhood. The right chemical at the right time has the potential to disrupt any of these processes.

• Making brain cells (neurulation and neurogensis)
• Moving the cells to their proper location (cell migration)
• Growing axons and dendrites to link nerve cells (neuronal differentiation and pathfinding)
• Developing synapses or points of communication with other cells (synaptogenesis)
• Refining the synapses (naturation and pruning), and, finally,
• Forming the supportive tissue that surrounds nerve cells and makes for efficient communication among them (gliagenesis or myelination).

Sensitivities continue through childhood. An increasing number of studies are showing effects to the reproductive system and the brain following early-life exposures to an array of chemicals. This is well-proven for childhood exposures to lead, and is shown in laboratory animals for, among other things, high-volume chemicals like dibutyl phthalate and bis(2-ethylhexyl) phthalate.

2. Pound for pound, children get higher doses of chemicals than adults

For example, one part per billion of benzene in drinking water, air, or food translates into a greater exposure for a small child than an adult, because:

• Children in the first six months of life drink seven times as much water per pound as does the average American adult.
• Children one through five years of age eat three to four times more food per pound than the average adult.
• Resting infants breathe twice as much air, pound for pound, as resting adults (Landrigan et al 1998).
• Children from birth to two years of age have more than twice the body surface area of an adult - an important factor for the many chemicals that can absorb through skin (EPA 1997).
• Just doing what children normally do can increase doses of chemicals. For instance, children put their hands in their mouths more often than adults, and play closer to the floor where some chemicals are more likely to be concentrated (Landrigan et al 1998).

References

Landrigan PJ, JE Carlson, CF Bearer, JS Cranmer, RD Bullard, RA Etzel, J Groopken, JA McLachlan, FP Perera, JR Reigart, L Robison, L Schell, WA Suk. Children's health and the environment: A new agenda for prevention research. Environmental Health Perspectives 106, Supplement 3, June 1998.

U.S. Environmental Protection Agency (EPA). 1997. Exposure Factors Handbook. Volume 1 - General Factors. EPA/600/P-95/002Fa. August 1997.

Klassen, Curtis D. Casarett & Doull's Toxicology. The Basic Science of Poisons. Fifth Edition. The McGraw-Hill Companies, Inc. 1996.