Small Traces of Pollutants Can Affect Brain Development, Survey Finds
Even tiny traces of common man-made chemicals found in the environment can have subtle but disruptive effects on the human brain as it grows in the womb and develops throughout the years of childhood, six prominent scientists said Thursday.
Presenting the results from an international meeting of government and academic researchers, they said the potential damage can range from almost imperceptible learning disabilities and behavioral problems to clinical attention deficit disorders and severe mental retardation.
The chemicals include dioxins, PCBs, many pesticides and common industrial chemicals. The scientists said these substances mimic the actions of important naturally occurring hormones so closely that they can interfere with the body’s endocrine system as it helps build and maintain the organ that governs all human behavior and consciousness.
In doses too small to be of any consequence to an adult, the chemicals could cause “profound and irreversible†changes in the structure and function of an infant’s developing memory and learning abilities, they said.
The chemicals are so ubiquitous that they are found in virtually every pregnant woman in the world and in every nursing mother’s breast milk.
At a news conference in Washington, the scientists presented a formal consensus paper signed by 18 researchers from the U.S. Centers for Disease Control and Prevention, the Department of Veterans Affairs and the Environmental Protection Agency, as well as hospitals and universities in Europe, Canada and the United States.
The scientists said the chemicals that mimic human and animal hormones “can undermine neurological and behavioral development and subsequent potential of individuals exposed in the womb,†or, for various wildlife forms, exposed in the egg.
“Widespread loss of this nature can change the character of human societies or destabilize wildlife populations,†the scientists said in their statement.
They acknowledged that their concerns are controversial and that a definitive link is still far from proved. Precisely how such substances interfere with the molecular chemistry of the brain is unknown, they said.
They drew their conclusions from a review of a growing body of experimental studies and systematic wildlife surveys that strongly suggest that the chemicals can block or alter the hormones that regulate early embyronic development in animals and human beings.
No single study makes the case against the chemicals, they said. Instead, the scientists relied on a mosaic of unsettling scientific findings in several otherwise unrelated fields of research.
Sexual, neurological and immune dysfunctions have been reported in wildlife. Several provocative but limited studies of children born to women eating toxin-laden fish from the Great Lakes showed that they do not perform as well as other children on tests of memory, intelligence and verbal ability.
To address the uncertainties, the scientists called Thursday for a concerted international research effort to investigate the effects these synthetic chemicals may have on the brain. Their concern grows out of a rising understanding of the molecular biology and chemistry of the brain.
In recent years, scientists have identified thousands of genes involved in shaping the brain and have made dramatic strides in understanding its intricate neural circuitry. Their work reveals the brain to be particularly vulnerable to subtle chemical and sensory variations in the environment.
Studies suggest that the brain is extremely sensitive to even minute traces of synthetic chemicals, especially those that affect the thyroid, a gland that produces hormones to regulate a variety of metabolic functions.
Starting about 10 weeks after conception, regular pulses of thyroid hormones are critical for every phase of brain development. Developing brain circuits can be permanently damaged by too much or too little of the thyroid hormones. The amounts involved can be less than one part in a trillion.
Dr. Stuart Stein, chief of neurology at Children’s Hospital of Orange County, who signed the statement, said: “Anything that affects thyroid hormones during fetal life is important. It is becoming clear that certain compounds to which we are exposed may alter thyroid hormone levels in the womb and thus brain development.
“The linkage between thyroid hormone and brain development, particularly in the fetus, is pretty strong. The linkage between synthetic chemicals and brain development is not as strong. This really has to be nailed more tightly,†he said.
Health concerns stemming from commercial chemicals have drawn the attention of industry and environmental groups in recent years. The concerns were initially spurred by fears that the compounds may be implicated in a variety of human cancers and reproductive problems.
More recent, other environmental researchers have raised the fear that the same chemicals may also impair the body’s immune system and undercut its ability to resist a variety of diseases.
Researchers have identified at least 51 pesticides, byproducts and classes of industrial chemicals that disrupt the hormonal balance of humans and animals. Dubbed “endocrine disrupting chemicals†(EDCs), they include large chemical families such as the 209 polychlorinated biphenyls, the 75 dioxins and the 135 furans. Included in the list of endocrine disruptors is atrazine, the most heavily used pesticide in U.S. agriculture.
In response to the growing concern, the Chemical Industry Institute of Toxicology has undertaken a multimillion-dollar effort to better understand how natural and synthetic chemicals affect the human hormone system. The National Toxicology Program is conducting a $10 million research effort to address fears that the chemicals may be implicated in reproductive health problems.
“One of the problems is that we deliberately manufacture chemicals that have biological activity,†said John Leatherland, a Canadian researcher who is an authority on the links between synthetic hormones in the environment and their effect on wildlife.
“It should not surprise us that some of those biologically active chemicals do things we did not predict when we started manufacturing them,†he said.
