Environmental Toxins

Fetuses, infants, and toddlers, as well as older children and teens are particularly susceptible to environmental insults due to their rapid rate of growth, development and reproduction of cells. This vulnerability makes children a specific focus of environmental health research on the effects of lead, chemical dump sites, pesticides, PCBs, benzene, environmental estrogens, and outdoor and indoor air contaminants. Scientists want to know which substances pose significant health risks, how to identify susceptible children, and how to intervene to prevent illness.

The following are some of the studies conducted or financed by the National Institute of Environmental Health Sciences, one of the National Institutes of Health, which focus on children:

Lead Poisoning
Lead (commonly found in old paint, household dust, soil, pipe solder and some ceramics) has long been known to cause severe health problems at high doses, including muscle and abdominal pain, mental impairment, paralysis, and even death. Recent studies supported by the NIEHS suggest that a young person's lead exposure is linked not only to lower IQs and lower high school graduation rates but to increased delinquency. Preliminary data from two other grantees' studies indicate that young girls exposed to lead store the metal in their bones. This lead can be released when they become pregnant years later, exposing their fetuses. Until recently, however, we have not appreciated the devastating effect of low exposures early in life.

Basic research financed by the NIEHS has shown the adverse effects of lead on children's IQ and physical development at levels previously considered safe. Based on these and other findings, public health officials declared lead the number one environmental hazard to American children and the federal Centers for Disease Control and Prevention lowered the acceptable blood lead level.

Research by the NIEHS grantees has helped identify sources of lead in the environment, design public health prevention efforts, and develop treatment to remove lead from exposed children, a process called chelation.

To help improve treatments, the NIEHS supported the study of dimercaptosuccinic acid (DMSA) as a chelating agent. Known generically as Succimer and the trade-name Chemet, DMSA binds with the lead, hastening its removal from the body and can be administered orally without hospitalization, an improvement over previous intravenous therapies.

Succimer is approved by the Food and Drug Administration to treat lead levels that exceed 45 micrograms per deciliter of blood. However, adverse effects of lead are evident at levels between 10 and 25 micrograms per deciliter, making treatment necessary for these lower concentrations.

In response to this need, the NIEHS conducted a clinical trial to test Succimer in 800 children with low lead concentrations in their blood to determine if oral chelation reduces or prevents lead-induced developmental delay.

The NIEHS continues to perform and support research on the effect of lead on children's health. This research has important implications for other environmental hazards as scientists believe that lead may be used as a model of how other substances can harm a fetus or developing child and adolescent, even at relatively low levels of exposure.

PCBs and Intelligence

PCBs, or polychlorinated biphenyls, were commonly used in electric transformers, paper recycling, and other commercial processes until they were shown to impair intelligence. Partly as the result of the NIEHS studies, PCBs have been banned from use; however, they are stable, persistent compounds that remain widespread in the environment. PCBs are fat soluble, meaning they concentrate in the fat of animals, and ultimately in the people who eat these animals.

NIEHS studies have shown that PCBs can cross the placenta and expose the developing fetus, and that nursing mothers can transfer PCBs to their infants through breast milk. Fetal exposure translates into lower IQ, poor reading comprehension, memory problems and difficulty in concentration. These dangers motivated the FDA and state advisories warn to against women of child-bearing age and children under 15 to avoid fish caught in contaminated waters.

Scientists with the NIEHS studied 117 children born to women poisoned by PCBs through food contamination in Taiwan in 1979. The PCBs were heat-degraded, causing them to partially convert to polychlorinated dibenzofurans (PCDFs). The exposed children presented carious (decaying) teeth, poor nail formation, and short stature, and were more likely to have behavioral problems, hyperactivity, and persistent developmental delays averaging 5 to 8 points on standard IQ scales. In addition, the developmental delays were as severe in children born up to six years after the initial exposure as those born in 1979, demonstrating the persistence of these chemicals in the body.

Childhood Asthma and Other Lung Problems
Asthma affects up to 20 million Americans and its prevalence and severity appear to be increasing among children, particularly Hispanics of Puerto Rican origin and African Americans.

Studies at Harvard University's Kresge Center for Environmental Health have shown a strong and consistent relationship between elevated indoor concentrations of oxides of nitrogen and lower respiratory tract symptoms; and suggest that exposure to sulfur dioxide, particulate matter, and acid aerosols in urban (outdoor) air is associated with bronchitis in children. A related study is assessing the degree of risk of minority and/or disadvantaged children from such things as air contaminants from kerosene heaters.

While recent emission controls on motor vehicles have decreased the release of toxic gases, the total number of cars and trucks on the road has increased, thereby negating the benefits of these controls. Tall stacks, used to disperse sulfur dioxide from large coal burning plants, have succeeded in reducing sulfur dioxide at ground level, but they have increased the proportion of sulfur dioxide converted into sulfuric acid in the air, so acid aerosol concentrations have diminished only slightly in recent years.

Estrogens in Pharmaceuticals and Pesticides
Studies by the NIEHS have shown the effects on children whose mothers took the potent synthetic estrogen DES during pregnancy to prevent miscarriage. The NIEHS and other research organizations have also performed many studies on pesticides, such as DDT and DDE, which contain chemicals that mimic estrogen or disrupt the body's hormones during pregnancy.

DDT, the first chlorinated hydrocarbon insecticide, was banned in the U.S. in 1972 after it was shown to cause an abnormality in calcium production, particularly in bird species whose eggs were laid with thin eggshells or without any shell at all. The Institute's scientists followed more than 700 North Carolina children exposed to DDE, the by-product produced when DDT begins to break down, in breast milk and found no related illness or lasting developmental abnormality. However, women with the highest levels of DDE in their milk breastfed their children less than 40 percent as long as women with lower levels. Another study in Mexico, where DDE levels in breast milk were often higher, showed a similar decrease in length of lactation, at least among second and later children. These decreased lactations may be related to the estrogenicity of DDE, since even very low doses of contraceptive estrogen can interfere with milk production.

Despite the research that has been done by the NIEHS and others, the prestigious National Academy of Sciences states in the recent report "Pesticides in the Diets of Infants and Children" that there remains a knowledge gap regarding the possible effects of pesticides on the development of the immune, nervous and reproductive systems from fetal, newborn and childhood exposures. The Academy recommended a significant research effort regarding these and other possible effects.

In response, the NIEHS is conducting a complex series of experiments with experts from the Environmental Protection Agency's Health Effects Research Laboratory evaluating the effects of common pesticides, including carbaryl, parathion, chlorpyrifos, atrazine and trichlorfon, in generations of rodents.

Childhood Leukemia
Clusters of childhood leukemia appear to occur around hazardous waste facilities, including Superfund sites. Studies supported by the Superfund Research Program, which the NIEHS administers, at the University of California-Berkeley are attempting to determine what environmental exposures, including tobacco smoke and poor diet, may lead to leukemia and how to identify those at risk.

Scientists are using a relatively new technique called fluorescence in situ hybridization, or FISH, which vividly paints chromosomes or portions of chromosomes with fluorescent molecules, helping to identify chromosomal abnormalities and gene mapping. FISH has shown for the first time that people working with benzene, a chemical in gasoline, develop chromosomal aberrations specifically related to leukemia. This work was conducted in China, but the same UC-Berkeley team has recently demonstrated that FISH can detect these risk markers for leukemia in children in low socioeconomic areas of the San Francisco Bay area.

Most Vulnerable Population Requirements
Faced with this research on PCBs, lead, and other substances, Congress now requires that the new Food Quality Protection Act and any reauthorization of other regulatory laws, such as the Clean Water Act, identify those populations "most vulnerable" to a substance and base risk assessment and regulations on those populations.

To emphasize the vulnerability of children to environmental hazards, the NIEHS Superfund Basic Research Program co-sponsored the national symposium Preventing Child Exposures to Environmental Hazards: Research and Policy Issues in Washington, D.C., in 1994, with the participation of the U.S. Surgeon General. In addition, the NIEHS' co-sponsored the First National Research Conference on Children's Environmental Health: Research, Practice, Prevention, Policy with the Children's Environmental Health Network at UC-Berkeley in 1997 in Washington D.C.

Based on the results of this conference and intramural and grantee accomplishments, the NIEHS will continue to use its available resources to pursue research to help children develop to their full potential as healthy and intelligent adults.

Reproduced from a publication from the National Institute of Environmental Health Sciences, National Institutes of Health.