Brain Storm

Are We Threatening Our Intelligence with Chemical Pollution?

The 101st Congress declared the 1990s the “Decade of the Brain” to focus attention on the most vulnerable of human organs. But after 10 years of intensive research focusing on chemical threats to mental development, have we learned anything?

Yes. Although average IQ scores have continued to increase about three points a decade, certain populations are at high risk of impaired mental development through exposure to potentially dangerous neurotoxins, a phenomenon Dr. Christopher Williams, of the University of London’s Institute of Education, calls Environmentally-Mediated Intellectual Decline (EMID).

Scientists say relatively low levels of pollutants that seem to have no immediate impact on adults may have devastating effects on children or fetuses, often disrupting the thyroid—a key organ in mental development. “A baby’s intelligence depends as much on the levels of thyroid hormone reaching the brain during critical periods of development as on inheriting smart genes,” write the authors of Our Stolen Future, a groundbreaking book that made the connection between pollution and reproductive abnormalities. Unfortunately, only 10 percent of the 70,000 chemicals used commercially have been tested for their neurological effects.


According to Dr. Theo Colborn, an endocrine disruptor specialist with the World Wildlife Fund and a coauthor of Our Stolen Future, five percent of American babies have had toxic exposure to polychlorinated biphenyls (PCBs) in breast milk high enough to affect intelligence. A 1996 study by Sandra and Joseph Jacobson of Wayne State University found that children exposed to low levels of PCBs in the womb grew up with a lowered Intelligence Quotient (IQ), poor reading comprehension, short- and long-term memory problems and difficulty paying attention.

PCBs, commercialized by Monsanto in 1929, have been found in nearly every human on Earth. The Jacobsons discovered that women eating just two to three meals per month of Great Lakes fish had children suffering from measurable mental decline. Their follow-up study, testing children at age 11, found that those with the highest prenatal PCB exposure had the lowest overall IQ and verbal scores; 11 percent measured an IQ drop of 6.2 points.

Domestic PCB use was prohibited in the 1970s, but production continued in many other parts of the world until recently, says Walter Rogan, an epidemiologist with the National Institute of Environmental Health Sciences (NIEHS). But Rogan says halting production doesn’t stop the threat, as PCBs resist the natural processes that break down chemicals, and can last “for decades.” He adds that the primary source of exposure is through diet—particularly fish and meat.


Of the hundreds of known or suspected neurotoxic pollutants, lead is the most studied, affecting virtually every organ system at high doses. It’s the low, chronic doses, however, that produce lower IQ scores, learning disabilities, reduced attention spans and behavioral problems. According to The Third National Health and Nutrition Examination Survey, nearly one million American children under six now have blood-lead levels higher than the Center for Disease Control’s “level of concern.” It’s also estimated that 17 percent of American children have lead levels elevated enough to affect their intelligence.

In the longest follow-up study of lead to date, the University of Pittsburgh’s Dr. Herbert Needleman found that first- and second-grade children with high lead levels had difficulties with speech, Attention Deficit Disorder (ADD) and poor academic performance. Eleven years later, studies show that the same students had more learning disabilities, greater absenteeism, lower class ranking and were less likely to graduate from high school.

Most American homes built before 1960 still have lead paint on their interior walls—a major source of exposure. And although lead was phased out of gasoline in the 1980s, Dr. Howard Mielke, an environmental toxicologist at Xavier University in New Orleans, warns that it still resides in soil, especially concentrated in inner-city areas of high auto traffic. “People living there are often low-income, so they have the horrible combination of not being able to move out of the place or control their environment,” he adds.


Mercury is another ubiquitous metal with a malignant history: its use to process felt in Victorian times gave rise to the term “mad hatter.” Not so well-recognized, however, are the 1.6 million women and children the Environmental Protection Agency (EPA) warns are at risk now. Severe fetal exposure to mercury can result in “abnormal placement of brain structures and gross impairment of motor and mental development…and poor to non-existent language development,” note the authors of Toxics A to Z.

While regulations exist for medical and municipal waste incinerators, no regulations currently exist for coal-fired power plants, the largest single source of background mercury pollution. This elemental mercury can cause irreversible memory, motor and speech problems, but a greater danger is posed after it falls back to Earth as methyl mercury, an even more toxic form which climbs up the aquatic food chain in seafood. Children in the Faroe Islands, who ate fish with mercury levels comparable to the U.S., suffered neurological damage, including the inability to concentrate, comprehend and learn. The EPA reports that people consuming more than 30 pounds of fish per year are in the high-risk group for such possible effects.


Considered one of the most toxic chemicals ever created, dioxin is an unwanted byproduct of many industrial processes, including waste incineration, paper bleaching and pesticide manufacturing. The good news is that dioxin levels have dropped in the environment by 50 percent in the last decade. Unfortunately, the dioxin that still persists is detrimental to IQ.

The authors of a Dutch study analyzing pregnant women near a Rotterdam hazardous waste incinerator found that even low levels—background levels deemed as normal exposure—resulted in abnormal mental development in offspring because of thyroid disruption. According to the EPA, exposure shortly before or after birth is more likely to impair intellectual development.


A 1994 study published in the Journal of the American Medical Association cited childhood leukemia patients treated with radiation as being “more likely than their sibling controls to enter a special education or learning disabled program.” Another study reported a 13-point drop in IQ with high-dose radiotherapy. In 1995, the Russian Academy of Sciences found that 95 percent of children in the eastern mining town of Baley suffered from intellectual decline due to radiation from uranium and thorium mining activities. The rate of Down’s Syndrome in Baley was four times higher than average. Other recent studies link radiation at Chernobyl and weapons testing sites with increased Down’s Syndrome rates.


Although pesticides have long been identified as potent carcinogens, they are part of an upcoming wave of r

esearch examining impaired mental function. One recent study led by Elizabeth Guillette, a researcher at the University of Arizona, found that pesticide-exposed Yaqui children demonstrated striking differences from unexposed children in memory, eye-hand coordination and drawing skills. And adult Nicaraguan workers overexposed to organophosphate pesticides have been found to have persistent problems with memory, attention span and concentration, notes a study in the medical journal Lancet.

Dr. Gina Solomon of the Natural Resources Defense Council thinks people are finally waking up to the full toxic potential of pesticides. “But the chemicals are already out there, they’re in widespread use, and they haven’t been tested for these subtle functional effects.”

Additional threats to IQ include nutrient deficiencies (mainly of iodine, calcium and iron) which heighten chemical exposures; volatile organic compounds (VOCs); furans; cadmium; zinc; aluminum; and antimony. Scientists are also discovering that neurological effects can skip generations. According to Williams, “If your child is not affected, it doesn’t mean that your grandchild won’t be.”