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Chapter 2

CHILDREN'S SPECIAL VULNERABILITY TO ENVIRONMENTAL RISKS

The central theme of a pivotal report by the National Academy of Sciences in 1993 was that children are not "little adults" -- a fact already known to legions of parents, teachers, and others worldwide.[1] Infants and children are different from adults because they are growing and developing. Environmental hazards affect children differently than adults. Their bodies are not fully mature, and therefore may not be capable of detoxifying certain harmful compounds. The very nature of children's behavior -- highly curious and physical -- can also put them at increased risk. That children are uniquely vulnerable to environmental hazards is well established in the scientific literature. In fact, the World Health Organization recommended more than a decade ago that, "when health risks from chemicals are evaluated, the special characteristics of infants and children must be recognized."[2]

GREATER EXPOSURE

Pound for pound, children breathe more air, drink more water, and consume more food than adults. This higher rate of intake means that children will receive higher doses of whatever contaminants are present in the air, water, or food. In addition, infants have a relatively greater surface area of skin than adults, thereby increasing their potential dermal absorption of certain compounds.

Differential Intake

  • Children ages one through five eat three to four times more per unit of body weight than the average adult American.[3] The average one-year-old drinks twenty-one times more apple juice and eleven times more grape juice, and eats two to seven and a half times more grapes, bananas, apples, pears, carrots and broccoli than the average adult.[4]
  • Infants and children drink more than two and a half times as much water daily as adults do as a percentage of body weight.[5] (An infant living solely on formula consumes about one-seventh of his or her own weight of water each day, which corresponds to approximately three gallons, or thirty-five cans of soda, for a 155-pound adult man.[6])
  • The air intake of a resting infant is twice that of an adult under the same conditions.[7]
  • A typical newborn weighs one-twentieth of the weight of an adult male, but the infant's surface area is one-eighth as great. Therefore, the total area of skin that could be exposed to a chemical (by swimming or bathing in polluted water or rolling in dirt) is two and a half times as great per unit of body weight in the infant as in the adult.[8]

Childhood Behavior

The typical nature of children's behavior also increases their exposure to environmental toxicants. An infant frequently explores objects by placing them in his or her mouth. This common hand-to-mouth behavior increases an infant's ingestion of substances in soil, household dust, floors and carpets, and on the objects themselves. In recognition of this, the U.S. Environmental Protection Agency (EPA) recently proposed assuming that children aged three to five years old put their hands to their mouths an average of one and a half times per hour.[9] As children grow, their endless curiosity and lack of fear can further increase their exposure to environmental hazards. With considerable physical energy, children can explore locations without regard for the consequences of their actions. At rest, children's breathing rates are faster than those of adults, and children's greater levels of physical activity can increase their breathing rates even further.

Children often play at ground level. In contrast, an adult's common breathing zone is four to six feet above the floor. Children will receive greater inhalation and dermal exposure to chemicals present on floors, carpet, grass, or dirt. Also, heavier chemicals such as lead and particulates will settle and accumulate in the air at ground level.

INCREASED SUSCEPTIBILITY

Human infants and children differ from adults not only in their size but also in the relative immaturity of their biochemical and physiological functions. Childhood is characterized by rapid physical and mental growth. Accordingly, certain organs may not be fully developed and may be more vulnerable to injury. Children absorb, metabolize, and excrete compounds differently than adults.

Rapid Growth and Development

The fetus is particularly sensitive to environmental toxicants.[10] Chemicals can affect the children born to women exposed during pregnancy, while the women remain unaffected. For example, the children of women from Michigan who ate two to three meals of fish contaminated with PCBs per month for six years before pregnancy had lower birth weights, memory deficits at seven months and four years of age, and cognitive deficits persisted at eleven years of age.[11] In Iraq, children born to women who during pregnancy inadvertently ate seed grain treated with mercury to prevent fungus had severe developmental and mental deficits.[12]

An infant gains weight more rapidly during the first four to six months after birth than at any other time during his or her life.[13] Typical newborns double their weight during the first five to six months and by their first birthday will weigh three times their birthweight.[14]

The growth of integral parts of the central nervous system (brain) and the immune system (thymus) proceeds most rapidly in the first six years of life. At age six, a child's weight is only about 30 percent of an adult's, but the child's thymus is approximately the size of the adult's, and the brain is about 80 percent of adult size.[15]

Many organs are not fully developed at birth and continue developing for years. The nervous system, lungs, immune system, and reproductive organs undergo extensive growth and development in utero and throughout infancy and early childhood. For example, sex organ development is not complete until puberty; myelination, the insulating of the nerve fibers, of the brain is not complete until adolescence; and the alveoli, or terminal air sacs in the lung where oxygen from the air enters the blood, continue to increase in number until adolescence.[16]

Differential Absorption, Metabolism, and Excretion

Infants' and children's pathways of absorption, metabolism, and excretion of compounds are different from those of adults.[17] In some instances, children may be more susceptible than adults due to their increased rates of absorption or decreased rates of elimination of foreign compounds. In other cases, the opposite may be true. Children will absorb about 50 percent of lead ingested, whereas adults will absorb only about 10 to 15 percent.[18] Kidneys are the principal pathway for elimination of most chemicals from the body. At birth an infant's kidney's filtration rate is a fraction of adult values, and by age one the rate is at adult levels.[19]

Recent molecular epidemiological data indicate that infants and children retain greater amounts of certain environmental toxicants. In a study of Polish newborns and their mothers, biomarkers, levels of polycyclic aromatic hydrocarbon (PAH)-induced DNA damage, were measured. Among newborns exposed to PAHs in utero, the level of DNA damage was comparable to the level in their mothers, even though the estimated dose to the fetus was one-tenth of that to the mother.[20] Similarly, in young children (under two years of age), levels of an indicator of exposures to PAHs (1-hydroxypyrene glucuronide) in urine were higher than in their mothers.[21] Another study investigated PCB levels in individuals residing on a Mohawk reservation downstream from pollution sources on the St. Lawrence River. PCBs were found in the breast milk and urine of women who ate fish caught in the river. The PCB concentrations in the urine of breast-fed infants were ten times higher than in the urine of their mothers.[22]

Longer Lifetimes

Children have more years of future life than most adults. Therefore, they have more time to develop any chronic diseases that might be triggered by early environmental exposures. Many diseases initiated by chemical hazards require decades to develop. Early childhood exposure to certain carcinogens or toxicants may be more likely to lead to disease than the same exposures experienced later in life.

Scientific Research Needs

Though children's unique vulnerability to environmental risk is now an accepted scientific axiom, much remains to be done to safeguard the next generation. In particular, much is still unknown about the effects of environmental exposures on children and infants: Are there critical periods of vulnerability during childhood? What are the differential susceptibilities for children? We need to better understand which differences between children and adults apply to all environmental hazards (e.g., differential air, water, or food intake rates) and which differences relate to specific categories of toxicants (e.g., neurotoxins' potentially greater impact on the developing central nervous system). Research identifying new types of toxic effects that have potentially significant impacts on children, such as endocrine disruption or immunotoxicity, is critical. Further study is important for determining how certain doses and health endpoints may be of primary concern in adults while different doses and endpoints may be more relevant to children. During childhood, for example, potential exposure to a neurotoxin will likely be of greater concern than exposure to a substance that elevates the likelihood of high blood pressure or heart disease; far later in life, the risk of cardiovascular disease will likely be of paramount significance. More scientific research is imperative. In the meantime, sufficient evidence exists to warrant increased protection for children from environmental hazards.


Notes

1. National Research Council, Pesticides in the Diets of Infants and Children, Washington D.C.: National Academy Press,1993.

2. International Programme on Chemical Safety, Principles for Evaluating Health Risks From Chemicals During Infancy and Early Childhood: The Need for a Special Approach, Environmental Health Criteria 59, World Health Organization, 1986.

3. Wiles, R. and C. Campbell, Pesticides in Children's Food, Environmental Working Group, 1993.

4. Wiles and Campbell, Pesticides in Children's Food.

5. Plunkett, L. et al., "Differences Between Adults and Children Affecting Exposure Assessment," Similarities and Differences Between Children and Adults: Implications for Risk Assessment, International Life Sciences Institute, 1992, pp. 79-94.

6. Principles for Evaluating Health Risks From Chemicals During Infancy and Early Childhood.

7. Principles for Evaluating Health Risks From Chemicals During Infancy and Early Childhood.

8. Principles for Evaluating Health Risks From Chemicals During Infancy and Early Childhood.

9. U.S. EPA, Residential Exposure Assessment Work Group, Draft - Standard Operating Procedures for Residential Exposure Assessments, July 18, 1997.

10. Birnbaum, L.S., "Endocrine Effects of Prenatal Exposures to PCBs, Dioxins, and Other Xenobiotics: Implications for Policy and Future Research," Environmental Health Perspectives, vol. 102, no. 8, 1994, pp.676-679. Y.L. Guo et al., "Growth Abnormalities in the Population Exposed in Utero and Early Postnatally to Polychlorinated Biphenyls and Dibenzrofurans," Environmental Health Perspectives, vol. 105, suppl. 6, September 1995, pp.117-122.

11. Jacobson, J.L. et al., "The Transfer of Polychlorinated Biphenyls (PCBs) and Polybrominated Biphenyls (PBBs) across the Human Placenta and into Maternal Milk," American Journal of Public Health, vol. 74, 1984, pp.378-9. J. Jacobson et al., "Effects of In Utero Exposure to Polychlorinated Biphenyls and Related Contaminants on Cognitive Functioning in Young Children," Pediatrics, vol. 116, 1990, pp.38-45. S.W. Jacobson et al., "The Effect of Intrauterine PCB Exposure on Visual Recognition Memory," Child Dev, vol. 56,1985, pp.853-60. J.L. Jacobson et al., "Effects of Exposure to PCBs and Related Compounds on Growth and Activity in Children," Neurotoxicol. Teratol., vol.12, 1990, pp. 319-26.

12. Gilbert, S. G. and K. Grant-Webster, "Neurobehavioral Effects of Developmental Methyl-Mercury Exposure," Environmental Health Perspectives, vol. 103, supp. 6, September 1995, pp. 135-142.

13. Principles for Evaluating Health Risks From Chemicals During Infancy and Early Childhood: The Need for a Special Approach.

14. Wiles and Campbell, Pesticides in Children's Food.

15. Sonawane, B. and R. Beliles, "The Susceptibility of Children to Immunotoxic and Neurotoxic Agents," Poster Abstract, lst National Research Conference on Children's Environmental Health, Children's Environmental Health Network, February 21-23, 1997, Washington, D.C.

16. Bearer, C., "How Are Children Different from Adults?" Environmental Health Perspectives, vol. 103, supp. 6, September 1995, pp. 7-12.

17. Pesticides in the Diets of Infants and Children, 1993.

18. Royce, S. and H. Needleman, Case Studies in Environmental Medicine: Lead Toxicity, Agency for Toxic Substances and Disease Registry, 1995.

19. Bearer, "How Are Children Different from Adults?"

20. Perera, F., "Molecular Epidemiology: Insights Into Cancer Susceptibility, Risk Assessment and Prevention," JNCI, vol 88, April 17, 1996, pp.496-509.

21. Perera, "Molecular Epidemiology."

22. U.S. Department of Health and Human Services, ATSDR, and Bureau of Environmental and Occupational Epidemiology, New York State Department of Health, Exposure to PCBs from Hazardous Waste Among Mohawk Women and Infants at Akwesasne

last revised 11/25/1997

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