Study Can't Pinpoint Extent of Lead Exposure
Saturday, January 31, 2009
Scientists say they might never know how many children were harmed when lead levels in the District's water spiked early this decade. The number could be as few as 700 or as many as tens of thousands.
The ultimate effects depend not only on how much of the toxic metal the children ingested but also how vulnerable they were as a consequence of genetics, their home environment and their experience in school.
The effects could include the loss of two to three IQ points and a higher risk for behavioral problems, even in children whose bloodstream lead stayed below the threshold of concern set by federal health officials.
But at the exposure levels experienced by the District's children, the public health consequences are likely to be very slight, even under the most pessimistic assumptions, according to several experts as well as published studies.
A new study published this week has raised questions about the health effects on children resulting from the District's drinking water crisis of 2001 through 2004.
Unlike some toxic substances, there is no known "safe" level for lead exposure. The official threshold is 10 micrograms per deciliter of blood. But even at more modest increments above zero, researchers have found small effects on intelligence as measured by IQ.
"At these levels, the effects are subtle," said Howard Frumkin, director of the National Center for Environmental Health at the Centers for Disease Control and Prevention. "They are detectable in population studies but generally not in individual children."
Studies have shown that lead exposure explains less than 4 percent of the variation in intelligence among individuals. In contrast, societal and parenting factors account for more than 40 percent.
Just how lead damages the brain is not completely understood. Damage might arise from lead's ability to substitute for calcium, which is involved in the storage and release of neurotransmitters in the brain, among many other actions.
But although the exposure's health effects might be small in magnitude, they could be wide in scope.
"We suspect that there are thousands, and possibly tens of thousands, of children who have experienced harm as a result of increased lead exposure" in Washington, said Bruce P. Lanphear, a doctor and lead-poisoning expert at Simon Fraser University in Vancouver, British Columbia.
One of the authors of the new study said she is advising parents that in the worst-case scenario a child might have lost three to four IQ points.