Introduction
Toxins in the environment—most notably, organic compounds used in herbicides, fungicides, and pesticides—have been found to affect the motor ability, cognition, and aggression levels of children. One pioneering study of Yaqui children in northwestern Mexico found profound behavioral differences when children who had been exposed were compared with a control group of similar ethnic background in a nearby location whose environment did not contain high levels of the same chemicals. Other substances, such as lead (in paint or soil) at levels of fewer than five hundred parts per million, also can cause neurological problems when ingested by young children, even as airborne dust.
The Yaquis and Pesticide Contamination
A landmark study of the effect of toxic chemicals on the mental health of children involved two groups of Yaquis, an indigenous farming people who live and work in and near the Yaqui Valley in Sonora, Mexico. After World War II, a lack of water and financing forced the Yaquis off their farms in the valley. They leased their lands to outsiders, mainly corporate farmers, who were heavy users of pesticides, herbicides, and fungicides. These chemicals were applied by aerial spraying, by tractor, and by hand, bringing widespread contamination of land, water, and people in the Yaqui Valley.
About the same time, valley farm operations became mechanized, and irrigation and transport systems were established. Farming became big business. Yaqui families living in the foothills moved into the valley for employment, and some valley residents moved into the foothills to maintain family farms under local ownership.
Typically two crops per year were planted in the valley, and pesticides were applied as many as forty-five times per crop. Between 1959 and 1990, thirty-three different compounds were used for the control of cotton pests alone. Multiple organophosphate and organochlorine mixtures as well as pyrethroids were used. Specific compounds include dichloro-diphenyl-trichloroethane (DDT), dieldrin, endosulfan, endrin, heptachlor, and parathion-methyl. Organochlorines, which accumulate most often in body fat, are especially dangerous because of biomagnification (also known as "bioaccumulation"). In biomagnification, the effects of organochlorines increase in nearly exponential fashion up the food chain. As recently as 1986, 163 different pesticide formulations were sold in the southern region of Sonora. Substances banned in the United States, such as lindane and endrin, are readily available to farmers living in the Mexican part of the valley.
Contamination of the human population in the Yaqui Valley has been documented, with women’s breast milk, after one month of lactation, found to contain levels of lindane, heptachlor, benzene hexachloride, aldrin, and endrin that were all above limits set by the Food and Agricultural Organization of the United Nations. In 1990, high levels of several pesticides were found in the cord blood of newborns and in the breast milk of Yaqui Valley residents.
In the valley, pesticide use was widespread and continued throughout the year, with little governmental control. Household insect sprays were applied daily in the lowland homes. By contrast, in the foothills, residents usually did not use synthetic pesticides or herbicides on their small farms. They maintained traditional methods of pest control in their gardens and usually killed insects in their homes by swatting them. Most of these people were exposed to pesticides only when the government sprayed DDT each spring to control malaria.
Specific Effects of Organic Pesticides
In addition to producing physiological symptoms (burns on skin, loss of fingernails, rashes, and death in some cases), the chemicals used in the Yaqui Valley affected the motor skills, growth, and development of children living there. During the 1990s, Elizabeth Guillette, an anthropologist and research scientist at the University of Arizona, studied the affect of pesticide exposure on Yaqui children. Guillette’s studies confirmed local observations that exposure to pesticides had a serious affect on the children’s health, including their physical and mental development. Hers was the first study to examine neurobehavioral affects, including cognition, memory, and motor ability, in children exposed to pesticides.
In Guillette’s study, the behavior of children living in the valley who were regularly exposed to many synthetic chemicals was compared with that of children living in the foothills, where pesticide use was largely avoided. The study selected two groups of four- and five-year-old Yaqui children who resided in the Yaqui Valley of northwestern Mexico. These children shared similar genetic backgrounds, diets, water-mineral contents, cultural patterns, and social behaviors. Guillette adapted a series of motor and cognitive tests into simple games the children could play, including hopping, ball catching, and picture drawing.
Before her study, Guillette had assumed that differences between the two groups would be subtle. Instead, she was shocked. According to an account by Jon R. Luoma, writing in Mother Jones, the lowland children had much greater difficulty catching a ball or dropping a raisin into a bottle cap—both tests of hand-eye coordination. They also showed less physical stamina. The most striking differences were revealed in the children’s drawings of people. Pictures created by the foothill children were recognizable as people. The lowland children’s drawings were infantile scribbles. The lowland youngsters were still exhibiting some motor problems, particularly with balance. Guillette found that the exposed children also demonstrated decreases in stamina, gross and fine eye-hand coordination, and thirty-minute memory.
Children who had experienced prolonged exposure to pesticides also became more violent. Guillette said she noticed that exposed Yaqui children often would walk by somebody else and strike the person without apparent provocation. The children who had been routinely exposed to chemicals became easily upset or angry after their parents made minor corrective comments. These behaviors were not noted in the foothills children. Valley children also appeared less creative in their play. They roamed an area aimlessly or swam in irrigation canals with minimal group interaction. Often, they sat in groups and did nothing. Foothill children, by contrast, were always busy and interacting with each other when engaged in group play.
Guillette concluded that toxins in the environment had placed the children of the agricultural area of the Yaqui Valley at a disadvantage for participating in normal childhood activities. In a follow up two years after her initial study, Guillette observed improvements in the children's drawings, although those of the lowland children were far less detailed, and lowland children exhibited ongoing difficulties with balance. Whether these effects would remain as the children grew into adulthood was not known.
According to psychology researchers Sue Koger and Deborah Winter, pesticide exposure may also be a risk factor for autism, conduct disorder, schizophrenia, and Parkinson's disease.
Neurological Effects of Toxic Metals
Lead exposure is most dangerous to children under the age of six because their bodies are developing rapidly. It can damage brain function and the nervous system and also contribute to physiological problems, such as hearing loss and stunted growth. The most common pathway for lead to enter children’s bodies is not ingestion of paint chips (very few children eat paint) but through routine hand-to-mouth contact with lead dust during painting or other remodeling of home interiors. In 1978, the Consumer Product Safety Commission banned paint containing more than six hundred parts per million of lead. However, lead is still used in roadway paint and by the military as a paint additive to accelerate drying, increase durability, maintain appearance, and repel destructive moisture. It can also be found in plumbing and in some dishes and glassware.
Toxicity due to lead exposure can affect any organ system in the human body. No standard for entirely safe exposure to lead exists, so any known exposure should be avoided.
Some studies indicate that lead poisoning can influence behavior, especially in children, contributing to juvenile delinquency and violent crime, although questions have been raised regarding whether the lead itself actually causes such behavior. Exposure to lead, especially at high levels, can produce coma, convulsions, hyperirritability, and stupor, sometimes leading to death. Lead exposure, even at low levels if maintained for months or years, can impede intelligence. Other symptoms of prolonged exposure include poor attention span, muscular tremor, and loss of memory. The younger a child, the greater the danger. Because the brains of younger children are still developing, they are afflicted with neurological problems from lead at lower levels than older children and adults.
If exposed to lead late in life, adults may experience many of the same symptoms as children, although they can tolerate a higher level of exposure without evident ill effects. Lead exposure in adults can cause decreased libido, mood changes, headache, lower cognitive performance, impeded hand dexterity, and delayed reaction time, decreased visual motor performance, dizziness, fatigue, and forgetfulness. Attention-deficit hyperactivity disorder (ADHD) may be caused by lead exposure, especially in childhood and adolescence. Some of lead’s neurological effects, including ADHD, may persist into adulthood, even after the source of exposure has been removed.
Because of lead’s toxicity, cities that contain closed smelters have been declared Environmental Protection Agency Superfund sites. In one such city, Omaha, Nebraska, lead toxicity has been detected in the soil at least five miles from a former smelter site on the Missouri River at the city’s eastern border.
Mercury has long been known to cause neurological damage. In the Minamata Bay mercury poisoning, a Japanese manufacturing company dumped mercury-containing waste from the late 1950s to early 1970s into the nearby bay, which accumulated the seafood consumed by the local population. Those with so-called Minamata disease suffered brain lesions, paralysis, loss of speech, sight, or sound, tremors, and cognitive impairment, which had the most marked affects on patients who were fetuses and young children at the time. Studies conducted in the early 2000s found that pre- or postnatal mercury exposure was associated with learning disabilities, attention disorders, and reduced IQ. Mercury bioaccumulation in seafood continues to be a problem in Japan and elsewhere. Exposure can also come from broken thermometers, barometers, thermostats, or fluorescent light bulbs containing mercury, as well as cosmetic products.
Aluminum may be linked to dementia in older adults. Other metals with known toxicities include arsenic, cadmium, copper, iron, lithium, manganese, and thallium.
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