The Lancet has just published (NOvember 8, 2006, online publication) a major review of the scientific evidence suggesting developmental disorders in children traceable to chemicals in the environment is significant and largely overlooked. Authored by two internationally recognized scientists, Philippe Grandjean (Harvard School of Public Health and University of Southern Denmark) and Philip Landrigan (Mt. Sinai School of Medicine), the paper identifies 201 industrial chemicals with the capacity to cause a neurodevelopmental defect (NDD) such as autism, attention deficit disorder and mental retardation:
A developing brain is much more susceptible to the toxic effects of chemicals than an adult brain. During development, the brain undergoes a highly complex series of processes at different stages. An interference–for example, from toxic substances–that disrupts those processes, can have permanent consequences. That vulnerability lasts from fetal development through infancy and childhood to adolescence. Research has shown that environmental toxicants, such as lead or mercury, at low levels of exposure can have subclinical effects – not clinically visible, but still important adverse effects, such as decreases in intelligence or changes in behavior. (HealthOrbit)
“The human brain is a precious and vulnerable organ. And because optimal brain function depends on the integrity of the organ, even limited damage may have serious consequences,” Grandjean pointed out. Developmental defects, usually involving the nervous system, are extremely common, estimated to afflict one in six children. But relating NDDs to environmental chemicals is technically difficult. When Grandjean and Landrigan examined the published literature on just five of the chemical — the well-known neurotoxins lead, methylmercury, arsenic PCBs and toluene — they found a characteristic pattern. Adult toxicity was recognized first, usually in an occupational setting. Then childhood poisonings were reported. Finally, there is a steadily growing body of epidemiological evidence showing neurobehaviioral effects in children at lower and lower levels. Grandjean and Landrigan are among the world’s authorities on the effects of these compounds.
These well-documented chemicals are only five on the list of 202 chemicals that also have neurodevelopmental effects:
“Even if substantial documentation on their toxicity is available, most chemicals are not regulated to protect the developing brain,” says 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 Landrigan conclude that industrial chemicals are responsible for what they call a silent pandemic that has caused impaired brain development in millions of children worldwide. It 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 petrol, which may have reduced IQ scores above 130 (considered superior intelligence) by more than half and increased the number of scores less than 70.
“Other harmful consequences from lead exposure include shortened attention spans, slowed motor coordination and heightened aggressiveness, which can lead to problems in school and diminished economic productivity as an adult. And the consequences of childhood neurotoxicant exposure later in life may include increased risk of Parkinson”s disease and other neurogenerative diseases,” says Landrigan.
A word of explanation of the IQ score statements. If you think of IQ scores as dstiributed in a bell shaped curve, the two “tails,” the upper (above 130) and lower (below 70) parts of the curve are small. If you take the whole curve and shift it slightly to the left, even a few IQ points, you can substantially reduce the area below the curve above 130 relative to its previous amount and increase the area of the curve below 70, relative to its previous amount. Thus even a rather small effect on IQ, say a point or two, not easily noticeable in any particular individual, can have fairly large effects on a population basis in terms of and increase in the proportion of the population considered mentally retarded (IQ below 70) or a decrease in those considered of high IQ (above 130).
Grandjean and Perez, in a supplement, note that 21 of the chemicals are among the top fifty in a list commonly found in hazardous waste sites in the US.
The majority of the 201 compounds are therefore undoubtedly present in the environment, in food, or in consumer goods.
The number of neurotoxic chemicals is likely to be much larger, as indicated by toxicology tests. Twenty years ago, about 750 chemicals had shown neurotoxic effects in laboratory animals9. The number is thought to exceed 1,000 today, although no authoritative estimate of the true number of neurotoxicants is available.
The incomplete information and the associated uncertainties can easily lead to underestimation of the pandemic of developmental neurotoxicity. Because of the vast societal importance of optimizing human brain development, we propose immediate action to protect the brains of future generations:
1. Documentation of chemicals that have caused toxic effects on the nervous system in humans to facilitate targeted preventive action against releases of these chemicals;
2. Documentation of human exposures to neurotoxic chemicals and identification of subgroups at risk due to residence, occupation, diet, and other factors;
3. Research on the consequences of developmental exposures to neurotoxic chemicals to expand our understanding of the long-term consequences of such exposures; and
4. Screening for neurotoxicity of commonly used chemicals to identify those that may present a hazard to brain development. (Supplement ot Lancet paper)
This is a sobering account by two highly respected scientists (full disclosure: the Reveres know both of them well). It is not a cause for panic about the health of our children. It is a cogent argument for investing in prudent measures to identify and limit developmental neurotoxicants in our environment for the sake of coming generations.