A silent pandemic: industrial chemicals are impairing the brain development of children worldwide.

Fetal and early childhood exposures to industrial chemicals in the environment can damage the developing brain and can lead to neurodevelopmental disorders such as autism, attention deficit disorder (ADHD), and mental retardation. Insufficient research has been done, however, to identify the individual chemicals that can cause injury to the developing brains of children.

In a new review study, whose results were published online in The Lancet on November 8, 2006, researchers from the Harvard School of Public Health and the Mount Sinai School of Medicine systematically examined publicly available data on chemical toxicity in order to identify the industrial chemicals that are the most likely to damage the developing brain.

They found that 202 industrial chemicals have the capacity to damage the human brain, and they conclude that chemical pollution may have harmed the brains of millions of children worldwide. The authors conclude further that the toxic effects of industrial chemicals on children have generally been overlooked.

To protect children, the researchers urge a precautionary approach with respect to chemical testing and control. This approach, which is beginning to be applied in the European Union, puts in place strong regulations that can later be relaxed if the hazard turns out to be less than anticipated, rather than requiring, as current regulations do, a high level of proof of hazard. At present in the United States, requirements for toxicity testing of chemicals are minimal.

"Because optimal brain function depends on the integrity of the organ, even limited damage may have serious consequences," said Philippe Grandjean, adjunct professor at the Harvard School of Public Health and the lead author of the study.

One out of every six children has a developmental disability, usually involving the nervous system. Treating neurodevelopmental disorders is difficult and costly to both families and society.

During development, the brain undergoes a highly complex series of processes at different stages. Any interference--for example, from toxic substances--that disrupts those processes can have permanent consequences. This vulnerability lasts from fetal development through infancy and childhood to adolescence. Research has shown that at low levels of exposure, environmental toxicants such as lead and mercury can cause subclinical but still important adverse effects, such as decreases in intelligence or changes in behavior.

"Even if substantial documentation on toxicity is available, most chemicals are not regulated to protect the developing brain," said Grandjean. "Only a few substances, such as lead and mercury, are controlled with the purpose of protecting children. The 200 other chemicals that are known to be toxic to the human brain are not regulated to prevent adverse effects on the fetus or a small child."

Grandjean and co-author Philip Landrigan, professor at the Mount Sinai School of Medicine, conclude that industrial chemicals are responsible for what they call a silent pandemic that has caused impaired brain development in millions of children worldwide. The pandemic is silent because the subclinical effects of individual toxic chemicals are not apparent in available health statistics. To point out the subclinical risk to large populations, the authors note that virtually all children born in industrialized countries between 1960 and 1980 were exposed to lead from gasoline, which may have reduced the number of IQ scores above 130 (considered superior intelligence) by more than half and may also have increased the number of scores of less than 70. Today, it's estimated that the economic cost of lead poisoning in U.S. children is S43 billion annually; for methylmercury toxicity, the estimated cost is $8.7 billion each year.

"Other harmful consequences from lead exposure include shortened attention spans, slowed motor coordination, and heightened aggressiveness," said Landrigan. "And the consequences of childhood neurotoxicant exposure later in life may include increased risk of Parkinson's disease and other neurodegenerative diseases."

The researchers believe that the total impact of the pandemic is much greater than currently recognized. In supplementary documentation (see below for a link), about half of the 202 chemicals known to be toxic to the brain are among the chemicals most commonly used.

Testing chemicals for toxicity is a highly efficient public health measure. Yet fewer than half of the thousands of chemicals currently used in commerce have been tested for acute toxicity, and, although new chemicals undergo more thorough testing, access to the data may be restricted because companies fear exposing proprietary information. Also, current toxicity testing rarely includes neurobehavioral functions.

Support for the research was provided by the Danish Medical Research Council, the (U.S.) National Institute of Environmental Health Sciences, and the U.S. Environmental Protection Agency. To view supplementary documentation on industrial chemicals and the risks of toxic effects on brain development, visit www.hsph.harvard.edu/neurotoxicant/appendix.doc.