Although the environment has a profound effect on our health and well-being, it affects children differently and perhaps more intensely. Filthy air, dirty water, and contaminated foods, therefore, present specials risks to our children's health. "Environmental risks" refer to exposures to chemical, biological, and physical hazards in the air, water, soil, and food, at home, at school, and in the community. Some of these hazards are manmade, such as polychlorinated biphenyls and emissions from power plants, but others are natural, such as radon and mycotoxins.
Environmental hazards may pose different risks for children than for adults because children are not simply miniature adults. Pediatricians are especially aware of this, because they must tailor treatments and doses to the unique, complex, and changing needs of each developing child. This article provides information about why children's hazards are different, illustrated by some specific disease examples.
Developmental Staqes and Special Environmental Health Risks During Each Stage
WHY CHILDREN'S ENVIRONMENTAL HEALTH RISKS DIFFER
The Table shows eight stages of child development and gives examples of some stage-specific environmental hazards to the child during each stage. There are a number of reasons environmental risks are not the same for children as for adults. Children may be exposed differently, may absorb differently, and have a higher rate of metabolism. In addition, children have "windows of vulnerability" while they are growing and developing when their target organs may be more susceptible to environmental risks than the target organs of adults.
An important reason children's exposures are different from those of adults is their level of behavioral development. Children behave differently from adults, and their behavior changes as they develop. Most children actively explore their environments, and young children exhibit frequent hand-to-mouth and object-to- mouth behavior.1 Until they are able to walk, children cannot avoid hazardous environments. Until they have grown intellectually, children may not be able to recognize potential hazards. As they grow older, they begin to explore outdoor environments - old tires, empty lots, used drums, rivers, and streams.
Children spend their time in different physical locations than adults. Infants and young children spend lots of time on the floor. Because of their height, children inhale in a different breathing zone than adults. Young children breathe close to the floor, while the adult of average height is breathing 5 to 6 feet above the floor.2 In addition, children are exposed to preschool or school classroom environments and playgrounds, rather than to work and commuter environments. Schools may be built on relatively undesirable lands or may be old, poorly maintained, and poorly ventilated facilities. Even when adults and children are present in the same location and occupied in similar activities, children may be exposed internally to different concentrations of pollutants. Infants' respiratory rates for their body weights are much greater, perhaps twice as great as those of adults.3
Children's exposures also differ in the quantity and the type of food they consume.4 Because they are growing, children eat more food and drink more fluid per pound of body weight than do adults.5 For example, if an adult male consumed as much fluid as a child does, it would be equivalent to drinking 1.75 gallons of fluid per day. Furthermore, children's diets are different from those of adults. They contain more milk, fruits, and often more vegetables. Infants and young children also generally eat a less varied diet, a difference that can lead to even larger differences in exposure.
One example of how this exposure difference leads to different health effects in children has to do with exposure to different levels of mercury vapor.6 Mercury is heavier than air, so the highest concentrations of mercury vapor occur near the floor. Before 1991, many different brands of interior latex paint sold in the United States contained mercury, primarily for its preservative effects. Paint stores sold mercury paint additives that were used for control of mildew. During the first several months after paint was applied to a wall, mercury vapor was emitted into the indoor air, sometimes exposing children to high levels of mercury vapor and resulting in acrodynia, or mercury poisoning.7 In one case, a 4-year-old boy was poisoned after the entire interior of his fire-damaged home had been painted with 17 gallons of paint containing mercury.8 Remarkably, four other family members living in the same house under the same conditions remained unaffected, although urine tests documented that they were excreting elevated levels of mercury.
Scrotal cancer in chimney sweeps provides another example. In 1775, Sir Percivall Pott9 described an elevated incidence of cancer of the scrotum among boys who had assisted chimney sweeps by climbing into chimney flues in England. Chimney sweeping led to heavy soot exposure. Boys were selected for this work because it was easier for them to get into the chimneys than it was for most full-sized adults. Scrotal tumors occurred in these boys and young men who had worked as sweeps but were not common in adults with other occupations.
Another way that children differ from adults is in their absorption of chemicals from the environment. A major chemical exposure route during gestation for compounds, especially those of low molecular weight, is through the placenta. A newborn's skin is also more absorptive than an adult's skin.10 The skin surface area in infants and young children is 2 to 3 times greater in relation to body mass. Chemicals such as hexachlorophene were discovered to be neurotoxic for young infants due to their increased absorption."
The gastrointestinal tract of a child is less acid than that of an adult.2 This may affect children's health in several ways, including methemoglobinemia. This occurs among infants living in places where well water is contaminated by runoff from fertilizer and animal operations. Infants younger than 4 months are susceptible because the gastric pH of infants is higher than that of older children and adults, and infants lack or have minimally functional enzymes that may prevent the illness.3 The enhanced susceptibility to methemoglobinemia is of concern for infants fed formula mat was prepared with contaminated well water. When infants consume nitrates, the less acidic environment of their stomachs allows proliferation of normally present intestinal bacteria that reduce nitrates to nitrites.3 This can result in excess of nitrites that bind with hemoglobin to form methemoglobin, a form of hemoglobin with reduced oxygen-carrying capacity. Because infants have less functional enzymes generally, and specifically less functional methemoglobin reductase, they are not able to convert methemoglobin back to hemoglobin, as would adults and older children.3 The presence of methemoglobin and reduction in oxygen makes blood appear blue.
Newborns and young infants have immature metabolic systems that break down certain chemicals with difficulty, if at all. This can mean they have trouble eliminating and excreting a number of toxic chemicals. There are many examples well known to pediatricians, one being the antibiotic chloramphenicol, which, when administered to a newborn, can cause "gray baby syndrome." On the other hand, immature metabolic systems will not produce a more toxic breakdown product of a relatively innocuous chemical, which may be protective for infants. For example, if a pregnant woman takes an overdose of acetaminophen, she will likely have severe liver damage. However, the same overdose probably will not damage the liver of the infant, because the infant is unable to metabolize it, and the toxic metabolite does not cross the placenta.12-15
Children's metabolisms are often faster than that of adults. This can be protective in some instances, but in other cases it increases susceptibility. An example of this is the occurrence of carbon monoxide poisoning in young children. Children are more susceptible because organ systems with high metabolic rates and high oxygen demand are most severely affected by oxygen deprivation, the mechanism by which carbon monoxide affects health.3 There have been cases in which a snowbound automobile was found with unconscious but alive adults in the front seat, while the children in the back seat were dead from carbon monoxide poisoning.16
A fetus is also more vulnerable than an adult to carbon monoxide poisoning. Fetal hemoglobin has a higher affinity for carbon monoxide than adult blood, and the fetus eliminates carboxyhemoglobin more slowly than the adult. Thus, when women are exposed to carbon monoxide during pregnancy, less oxygen is available to the fetus. This is why it is especially important to avoid exposure to carbon monoxide during pregnancy.
Windows of Vulnerability
Children have windows of vulnerability when their organs are immature and may be more fragile than the organs of adults. For most chemicals, exposure at a young age appears to be more harmful to the developing organs than exposure at an older age. In fact, the younger the infant, the more likely the window of vulnerability for organ damage will be open. Exposure to environmental chemicals during the first trimester of pregnancy may be the most harmful to the developing organs, because this is when organogenesis occurs.17 The most famous example of this period of vulnerability is thalidomide, the drug used in some countries for morning sickness as a sedative during the 1950s and early 1960s. Thalidomide became well-known when its use during pregnancy was associated with birth defects in infants such as limb reductions, or phocomelia.18
This drug was never licensed for use in the United States, but more than 10,000 children were born in other countries with deformed arms and legs; 8,000 children were born in Germany alone. The exact gestational age at which thalidomide was most harmful was between days 22 and 36 of pregnancy.19 It is thought that thalidomide acts by interfering with angiogenesis, especially in relation to the limb buds, during this window of vulnerability. This interferes with normal development, which results in truncation of the limb.
Because of this window of vulnerability during the first trimester, physicians must not prescribe thalidomide during pregnancy. However, thalidomide is finding important therapeutic uses in adults with certain cancers, inflammatory diseases, skin diseases, Hansen's disease (leprosy), and HIV infection.20"23 This demonstrates that because a substance is determined to be harmful during one period of development, it does not necessarily mean that it is harmful during subsequent periods. In the case of thalidomide, it is this same interference with angiogenesis that causes the therapeutic effects in adults.
The developing brain is also vulnerable to injury from radiation during pregnancy. Mental retardation and microcephaly occurred in children born to women who were pregnant when the atomic bomb was dropped on Hiroshima, Japan, in 1945.23 The very first finding was small head size, which can occur at 0.1 Sievert (Sv) if the fetus is exposed at 4 to 17 weeks of gestational age. Mental retardation can occur at 0.6 Sv if the fetus is exposed between the 8th and 15th weeks of pregnancy.23 Mental retardation is probably because of interruption in the proliferation and migration of neurons from near the cerebral ventricles to the cortex.
The increased vulnerability of the infant respiratory tract is due largely to the prolonged period of development of infant lungs. The lungs are growing rapidly during the first year of Ufe and develop more alveoli up until the fourth year of life.24 Exposure to environmental tobacco smoke has harmful effects on the developing lungs of young infants25'26 and has been associated with ear infections in young children.27 Because the adult lung and respiratory tract is mature, environmental tobacco smoke does not have these same effects. Similarly, outdoor pollutants such as airborne particulates and nitrogen dioxide have been linked to significant deficits in growth of lung function in fourth graders, but less significant lung growth deficits were noted in seventh and tenth graders in the same polluted area.28
Acute lung bleeding has occurred among infants in Cleveland, OH, exposed to toxigenic molds.29 The infants in these homes had severe, lifethreatening pulmonary hemorrhage, but older children and adults living in the same moldy environments had no visible effects.30 Other investigators have reported an association between pulmonary hemorrhage and toxigenic molds in infants and children in Kansas, Missouri, Texas, Delaware, and North Carolina.31-35 One hypothesis is that potent toxins present on the surface of certain toxigenic molds are protein synthesis inhibitors, which may cause focal areas of capillary fragility in an infant's lungs and induce bleeding. Adults are not similarly affected because their lungs have stopped growing. Additional work is ongoing to further explore this hypothesis.
The vulnerability of some target organs extends into childhood. A good example of an organ with prolonged target organ susceptibility is the thyroid gland. After the nuclear disaster in Chernobyl in 1986, 3 to 4 million Ukrainian people were exposed to radiation including about 1.26 million children. Beginning in 1990, there was an epidemic of thyroid cancer in children, largely because the thyroid glands of children are unusually sensitive to exposure to radioactive iodine. The thyroid in children is approximately 2 to 10 times as rathosensitive to induction of neoplasms as that of the adult.36
Not only does too much of a chemical have a harmful effect on developing organs, but too little of certain nutrients may also increase a child's risk of developing certain diseases. One example is Keshan's disease, a cardiomyopathy that was first described among children in Keshan Province in China.37 It occurs in the most selenium-deficient areas of China and is linked not only to selenium deficiency but also to infection with Coxsackie virus. In this example, a genetic change in the virus occurs when it passes through a selenium-deficient child, which results in increased virulence of the virus and the development of severe cardiomyopathy that may lead to death.38-40 Adults are not similarly adversely affected. Future research will be needed to determine whether other nutrient changes can change viruses or make them more virulent and, likewise, whether chemicals can change viruses and make them more virulent.
Differences between children and adults with respect to exposure rates, absorption of chemicals, metabolism, and organ development make children uniquely vulnerable to environmental hazards. Moreover, biology does not exist outside of the social life of the child. At least two important implications flow from these findings. First, pediatricians must pay closer attention to the conditions of childhood and the specific details of a child's life in attempting a complete understanding of childhood diseases, particularly their sources in environmental and social conditions. At a minimum, pediatricians must inquire more carefully about environmental exposures and children's complaints in order to make accurate diagnoses. Perhaps equally critical, pediatricians must engage in more aggressive prevention efforts. While they often try to prevent exposure by educating parents about keeping household chemicals away from young children, they might likewise consider that the information they possess about children's special vulnerability to environmental risks could usefully inform political and social decisions to protect children from those risks. This includes, for example, supporting air pollution standards that are protective of children and advocating for stricter controls for certain chemicals to reduce health risks for this vulnerable group.
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Developmental Staqes and Special Environmental Health Risks During Each Stage