Learning disabilities. Cerebral palsy. Mental retardation. A “silent pandemic” of these and other neurodevelopmental disorders is under way owing to industrial chemicals in the environment that impair brain development in fetuses and young children. That’s the conclusion of a data analysis by researchers at the Harvard School of Public Health (HSPH) and the Mount Sinai School of Medicine, who point to 201 chemicals – most of them common – known to inflict lasting neurological damage in humans. Information on possible neurotoxic effects exists, however, for only a small fraction of the thousands of chemicals in use around the world.
The findings, published online in The Lancet (Nov. 8, 2006) and soon to be in print, stem from a careful review of publicly available data by lead author Philippe Grandjean, an adjunct professor in HSPH’s Department of Environmental Health, and Philip Landrigan, a professor of pediatrics and chair of Community and Preventive Medicine at Mount Sinai. Their research was funded by the Danish Medical Research Council and, in the United States, by the National Institute of Environmental Health Sciences and the Environmental Protection Agency.
In their report, the researchers urge countries to adopt the “precautionary” approach for chemical testing and control recently embraced by the European Union (EU). The EU has put into place strong regulations that can later be relaxed if a potential hazard proves less dangerous than anticipated, instead of requiring a high level of proof of toxicity at the outset. By contrast, U.S. requirements for the testing of chemicals for brain toxicity are minimal, the authors say.
“The human brain is a precious and vulnerable organ, and a developing brain is far more susceptible to the toxic effects of chemicals than an adult brain,” Grandjean says. “Even limited damage may have serious consequences.”
This vulnerability, Grandjean explains, lasts from conception through infancy, childhood, and adolescence. Even at very low levels of exposure, toxicants in the environment such as lead or mercury can have subclinical effects – adverse outcomes that, while not obvious, are important and measurable, such as reductions in intelligence or slowed motor coordination.
One out of six American children has a developmental disability, usually involving the nervous system, according to the U.S. Centers for Disease Control and Prevention. A growing body of evidence links industrial chemicals to neurodevelopmental disorders; treatments for such disorders are difficult and costly to families and society. Lead, for example, became the first substance identified as having toxic effects on early brain development only about 100 years ago, even though its neurotoxicity in adults had been known for centuries.
Tip of the iceberg
In compiling their list of 201 chemicals that are toxic to the human brain, Grandjean and Landrigan drew on the Hazardous Substances Data Bank of the National Library of Medicine and other sources. The list is far from comprehensive, the authors say: More than 1,000 substances are known to cause neurotoxicity in laboratory animals.
Only for five substances – lead, methylmercury, arsenic, PCBs, and toluene – did sufficient documentation exist to permit the authors to explore how society first recognized their toxicity and much later took steps to control human exposures. In all five cases, the researchers found, the recognition of toxic effects in adults and episodes of poisoning in children was followed by an accumulation of epidemiological evidence linking low-level exposures in children to neurobehavioral deficits.
While existing health statistics do not reveal the subclinical effects of individual chemicals, studies of large populations suggest they are widespread and serious. For example, virtually all children born in industrialized countries between 1960 and 1980 were exposed to lead from gasoline. Research suggests that, for these children, the number of IQ scores above 130 (considered superior) fell by more than half, while the number of scores under 70 increased (100 is average). Today, the economic costs of lead poisoning in U.S. children are estimated at $43 billion annually; for methylmercury toxicity, they total $8.7 billion a year, according to Landrigan and his collaborators.
“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 in adulthood,” says Landrigan. “And the consequences of childhood neurotoxicant exposure later in life may include an increased risk of Parkinson’s disease and other neurodegenerative diseases.”
“The brains of our children are a vital resource, and we have yet to recognize how vulnerable they are,” Grandjean says. “We must make protection of the young brain a paramount public health goal.”