Slide 7 of 38
Children are not little adults. Children and adults have significantly different patterns of illness, injury, and death. Some of these differences may be explained by environmental factors. In particular, three factors may help explain the unique risks to children from environmental threats.
1. Their greater environmental exposures because of differences in childhood and adult consumption and activity patterns.
- Children eat, drink, and breathe proportionately more than adults
- They eat fewer types of foods than adults (milk products, juices, and fruits)
- Young children play and act differently than adults (playing outdoors, being on our near the floor, placing things in mouth)
2. Key physiological differences between children and adults that increase the dose of environmental contaminants that children receive.
- Children not only encounter greater exposure to environmental threats than adults encounter, their physiology also puts them at greater risk. Because children are still developing, contaminants more readily enter their bodies through direct contact with their skin and the digestive system. Thus, greater amounts of contaminants may enter children’s bodies than adults.
- Absorption through skin. Children’s skin is more permeable than the skin of adults. Therefore, children absorb larger amounts of toxicants. Children also have a greater surface area of skin relative to weight, so more skin is exposed that could absorb environmental agents. For example, studies of diaper dyes and hydrocortisone have demonstrated that a newborn’s skin absorbs three times more than an adult’s skin.
- Absorption in the digestive system. Young children also absorb more through their digestive systems because they retain ingested substances longer in the stomach and the intestines. For example, while an adult would absorb about 10 percent of ingested lead, a 1- to 2-year-old child would absorb 50 percent.
- Children's Brains and Organs May Receive Disproportionately More Contaminants
- Once environmental toxicants have been absorbed, they may reach the brain and other organs and tissues more easily in a child than in an adult. For example:
- Blood-brain barrier, blood volume, brain size. The blood-brain barrier impedes, but does not fully block chemicals from entering the brain and central nervous system. A newborn’s blood-brain barrier is more permeable than that of an adult. This leads to greater levels of toxicants in a child’s brain central nervous system.
- Brain size and blood volume. Children’s brains may be at greater risk of damage because the brain is a larger percentage of total body mass in a child than in an adult. Also, the brain and other organs receive a greater blood flow in proportion to weight in children.
- Blood and body composition. Body composition affects the likelihood that toxic chemicals will reach organs and tissues where damage may occur.
- The blood of infants and young children contains fewer components, such as serum proteins, that help prevent toxic chemicals from reaching susceptible sites in the body.
- On the other hand, newborns have more water and extracellular fluid in their bodies than adults do. Therefore, they may have a lower concentration of chemicals in affected tissues.
- Children's Bodies More Slowly Break Down and Eliminate Contaminants
- Metabolic and elimination processes are important influences on the environmental health of a developing fetus or child.
May be excreted more slowly; and
- Metabolic processes. After exposure occurs, these processes in adults generally lessen the harmful impact of absorbed toxicants. When the processes are absent or still developing in children, these toxicants:
May not be broken down to decrease their toxicity; and
May become more difficult to eliminate.
- Elimination processes. Following absorption and distribution, toxicants are eliminated through the kidneys and liver. These organs are not completely developed at birth. The kidney and liver systems do not reach full capacity until an infant is about 5 and 16 months old, respectively. As a result, absorbed chemicals in an infant:
- The windows of special vulnerability to environmental contaminants during periods of rapid development from before conception through adolescence.
3. Children's Developing Bodies are More Vulnerable to Contaminants
- Individual development proceeds from the formation of sperm and egg through fertilization, fetal development, infancy, childhood, adolescence, and adulthood. During each stage children may have windows of extra sensitivity to environmental agents. The resulting damage may become manifest immediately or not until later in life.
- Vulnerability can be broken into three broad periods of development:
Sperm and egg cell development in the parents through conception and the subsequent two weeks – enviornmental exposures before or soon after conception may alter genetic material passed onto the developing embryo or may disrupt developmental control mechanisms that monitor DNA synthesis, cell division, and cell growth
Pregnancy, which includes embryonic and fetal development – environmental agents can disrupt the embryonic and fetal development. For example, the interaction of genes with enviornmental conditions is believed to account for roughly one quarter to one half of all developmental defects.
Childhood, from infancy through adolescence – infancy and early childhood are critical times because many organ systems are growing and continuing to mature. For example, the brain and the immune system develop extensively during early childhood. Environmental exposures may harm these immature systems as well as behavioral, cognitive, and emotional development.
- For many environmental agents, the exact timing of harmful exposures and the mechanisms that are disrupted remain uncertain. For example, numerous childhood cancers are believed to be caused by parental or fetal exposures to environmental agents, but the period of time when the harmful exposure occurs may not be known.
- It should be noted that while this presentation is clearly focusing on environmental exposures, human development is clearly vulnerable to other exposures, such as ionizing radiation, tobacco smoke, alcohol, and other abused substances.