Caroline Cassels Medscape Medical News 2006. © 2006 Medscape
November 8, 2006 — An online review article published November 8 in the Lancet says environmental exposure to toxic chemicals in utero and in the early stages of life may be creating a "silent pandemic" of neurodevelopmental disorders.
In their paper, Philippe Grandjean, MD, from the University of Southern Denmark in Odense, Denmark, and Philip Landrigan, MD, from the Mount Sinai School of Medicine, in New York, call for new, stricter approaches to chemical testing and controls that recognize the "unique vulnerability of the developing brain.
"In conducting their review, the authors used the US National Library of Medicine hazardous substances data bank, supplemented by fact sheets from the US Agency for Toxic Substances and Disease Registry, and the integrated risk information system of the US Environmental Protection Agency to identify industrial chemicals that have proven neurotoxic effects in humans.
"The combined evidence suggests that neurodevelopmental disorders caused by industrial chemicals have created a silent pandemic in modern society," they write.
Significant Impact
The article cites 5 industrial chemicals, including lead, methylmercury, and polychlorinated biphenyls (PCBs), solvents, and pesticides, that are recognized causes of neurodevelopmental disorders. Exposure to these chemicals during early fetal development can cause brain injury at doses much lower than those affecting adults. Recognition of these risks has given rise to evidence-based programs of prevention, such as the elimination of lead additives in gasoline and house paint.
While such initiatives have been effective, most have been initiated only after substantial delays, they point out. Recent research into lead neurotoxicity has shown that even very low exposures cause large functional decrements in children. Similarly, low prenatal exposure to methylmercury has been shown to have a significant impact, with 1 New Zealand study demonstrating a 3-point decrement in IQ and changes in affect in babies born to women with mercury concentrations in hair of greater than 6 µg/g, they write.
Tip of the Toxic Iceberg
But these "proven" brain-damaging chemicals may just be the tip of a potentially huge neurotoxic iceberg, Drs. Grandjean and Landrigan write. According to the authors, there are an additional 200 chemicals that are known to cause clinical neurotoxic effects in adults.
In addition, despite an absence of systematic testing, many other chemicals have been shown to have neurotoxic effects in animals. In 1981, 100,000 chemicals in the European Union were registered for commercial use. In the United States, 80,000 are currently registered, yet fewer than half of these substances have been subjected "to even token laboratory testing," they write."Nearly 3000 of these substances are produced in quantities of almost 500,000 kg every year, but for nearly half of these high-volume chemicals, no basic toxicity data are publicly available, and 80% have no information about developmental or pediatric toxicity," they write.
An expert committee from the US National Research Council concluded that 3% of developmental disabilities are the direct result of environmental exposure to such substances and that another 25% arise through interactions between environmental factors and individual genetic susceptibility. However, these estimates, the authors note, were based on scarce information about neurotoxicity and therefore likely underestimate the true prevalence of chemically induced abnormalities. Lowering the BarThe absence of testing and the high level of proof required for chemical-control legislation are the main impediments to the prevention of developmental disorders caused by exposure to chemical pollutants, they write.
As a result, Drs. Grandjean and Landrigan are calling for new approaches to control chemical exposures to protect the most vulnerable. The bar on exposure limits for chemicals should be set at values that recognize the unique sensitivity of the developing fetus and young children and aim at protecting brain development, they assert.This precautionary approach, which is now beginning to be used in the European Union, would mean that any early indication of a potential for a serious toxic effect, such as developmental neurotoxicity, should lead to strict regulation.
Restrictions could then be relaxed if it is subsequently proven that the substance is less harmful than initially thought. In the meantime, the authors say that practicing clinicians should counsel their patients, particularly pregnant women, about avoiding exposure to chemicals of unknown and untested neurotoxic potential.
Lancet. Published online November 8, 2006.
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