Study Can't Pinpoint Extent of Lead Exposure
Several Factors Determine Any Harm to Children's IQ; Experts View Public Health Impact as Slight
Saturday, January 31, 2009; Page B01
IQ 損失多少分等說法是假科學
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.
"That is all I can say," said Dana Best, a pediatrician and epidemiologist at Children's National Medical Center. "With enrichment, with a good school environment, it is likely that the loss will not affect your child in a significant way."There are three reasons it is hard to gauge the health effects of the lead in the District's drinking water, which rose in the second half of 2001 and returned to normal by 2005.
First, it is impossible to know how many children were exposed and for how long. Lead levels in water fluctuate by neighborhood, by day and even by the time of day. Water consumption also varies, with some families drinking only tap water and others using bottled water all or part of the time.
Parenting practices are also important factors. Babies given formula mixed with tap water get far more lead than breast-fed infants.
Second, research has shown that on a microgram-by-microgram exposure, lead has a greater effect on IQ at low concentrations than at high concentrations. In other words, the brain is more sensitive to small doses than to large ones, even though large ones ultimately do more damage.
The consequence of this counterintuitive fact is that a child whose blood lead rises from two micrograms to five micrograms for several years might not only be affected, but that the effect might also be more exaggerated than one would expect.
The third reason it is impossible to know the effect of a population-wide exposure like the "lead plume" in Washington's drinking water is that vulnerability varies from person to person.
Three genes are thought to play a role in heightening or dampening a person's risk of suffering lead toxicity. Probably more important is the capacity of attentive parents, interesting activities and good schools to counteract the damaging effects.
In the new study, published in the journal Environmental Science & Technology, researchers examined more than 28,000 blood-lead tests done at Children's from 1999 to 2007. They used the upward trend of tests in the "elevated" range to deduce what might have been happening in the city as a whole during the years that water lead levels rose and then fell back.
The fraction of tests in children less than 16 months of age that were above 10 micrograms -- the official threshold of concern -- rose from 0.5 percent to 4.8 percent in the second half of 2001, about six months after an additive was put in Washington's water that unintentionally leached lead out of pipes. By 2004, the fraction of Children's lead tests that were elevated was back down to 1.8 percent.
By one estimate, about 46,000 District children younger than age 6 were exposed to elevated lead through drinking water in 2004.
Using a mathematical model, researchers estimated that 517 children younger than age 2 1/2 had elevated blood lead levels in 2003 "due to high water lead levels," and that 342 had elevated levels in 2004 from the same exposure.
The two sets of results are close enough that they suggest a ballpark estimate for the number of children whose elevated lead was attributable to the water problem.
"But the sad reality is that because there is so little blood lead data for children age 0 to 1 years of age, we can never know the exact numbers," one of the authors, Marc Edwards of Virginia Tech, said in an e-mail. "I know that this answer is extremely frustrating for us . . . and DC consumers."
So what might the consequences of that exposure be?
A study published in the New England Journal of Medicine in 2003 attempted to answer that.
Researchers there examined the experiences of 172 children in Rochester, N.Y. They found that when a child's childhood-long blood-lead level was 10 micrograms, his or her IQ was on average 7.4 points lower than a child with a lifelong zero reading.
The decline in IQ in children with blood-lead readings of 30 micrograms compared with those with 10-microgram levels was not as steep: about three IQ points. However, children at that level had suffered the effect of the zero-to-10-microgram rise, so the overall damage was greater.
An analysis of seven studies from four countries (the United States, Mexico, Australia and Yugoslavia) found a similar effect, although not as dramatic.
Those studies found that when blood lead rose from 2.4 to 10 micrograms, the IQ loss was 3.9 points. When it rose from 10 to 20, there was a loss of another 1.9 IQ points.
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