The reports - to be published almost simultaneously in two leading research journals - are the first in an expected wave of new studies this year examining how lead exposure influences learning disabilities in children, violent behavior in teens and mental dysfunction in the elderly.
Researchers who have reviewed the two studies caution that they are both preliminary and do not establish a firm causal link between lead exposure and aberrant behavior.
But they say the results go to the heart of scientific research on the subject.
In one of the first experiments of its kind, Baltimore's Kennedy Krieger Institute for children found in a two-year study that relatively low levels of lead fed to a colony of nursing mother rats in their drinking water caused brain abnormalities in their offspring that stunted their sensory perception.
Experts say the research, which appears today in the Proceedings of the National Academy of Sciences, may have implications for human brain development, since lead tends to have similar effects in animals and young children.
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"And that suggests there may be physical changes in actual brain structure - which is exactly what our research found."
In the second study, a private consulting group working under contract for the U.S. Department of Housing and Urban Development used computers to track lead consumption in paint and gasoline over the past century and uncovered a striking coincidence.
As the amount of lead released into the environment in paint and auto exhaust rose and fell through the decades, so did a broad range of reported violent crimes - including rape, robbery, assault and murder - a researcher at ICF Consulting in Fairfax, Va., found.
"But no one ever looked at the possible effects of lead exposure. When you put that data up on the chart, the consistency is quite astonishing."
Lead has been shown to increase aggressive behavior in humans in repeated studies since at least 1943, when doctors at Boston's Children's Hospital first noted a tendency toward "cruel impulsive behavior" and "irritability" in children exposed to lead.
But scientists have yet to establish precisely how lead fuels anger, except that it causes humans and animals alike to have difficulty learning and adapting to changes in the environment.
The quest for the exact mechanism - and the lowest dosage at which the toxin begins to cause mental problems - has been among the most controversial in the field over the past two decades.
"Both of these reports are striking in their general observations about the possible gross effects of lead exposure," said Dr. Ellen K. Silbergeld, a professor of toxicology at the University of Maryland and an authority on lead poisoning. "And they both fit rather nicely into the existing research.
"But they raise more questions than they answer. That said, it doesn't make the findings any less disturbing, especially for a city like Baltimore."
More than 7,000 children a year are exposed to highly toxic dust from disintegrating lead paint in Baltimore, and at least 1,200 are poisoned.
Mostly poor and disproportionately African-American, they live primarily in the dilapidated rental housing enclaves of Park Heights, Sandtown and Middle East.
These neighborhoods have been associated with high rates of childhood lead poisoning since at least the 1930s, according to historical health records at the .
In impoverished rural communities and cities such as Chicago; Milwaukee; Providence, R.I.; Newark, N.J.; and Philadelphia, lead paint exposure constitutes a con-tinuing public health crisis affecting more that a million children annually, according the U.S. Centers for Disease Control and Prevention.
In Baltimore, the Kennedy Krieger Institute on Broadway is the primary treatment facility for the hardest-hit kids, an oasis amid the city's worst slums.
"Their new research is totally consistent with a growing body of literature that strongly suggests that the relatively low levels of lead exposure now considered safe - or at least not seriously damaging - may not be safe at all," said Dr. Deborah Cory-Slechta of the University of Rochester in New York.
A leading expert on animal research into the hazards of lead, Cory-Slechta has overseen multiple studies in rat colonies demonstrating that low levels of the toxin can disrupt key brain chemistry that controls inhibition, learning and impulsiveness.
"We see it in rats, we see it in monkeys, and we see something very much like it in children," Cory-Slechta said.
"There is a persistent tendency among lead-dosed organisms to have problems of control, adaptability and discernment, even at very low doses."
Until 1960, Baltimore averaged about 10 lead poisoning deaths a year for decades, historical Health Department records show. But improvements in early detection and treatment - and the proximity of some of the world's most renowned medical centers - now prevent most of lead's worst ravages.
Yet pediatricians and school officials note that subtle behavioral effects persist long after poisoned children are treated.
That's because even low doses of lead can cause a broad range of functional problems, research has shown, including a loss of self-control, shortened attention span and a host of learning disorders that often cause lead-exposed children to perform poorly in school and ultimately to drop out.
Among suspected causes are that lead inhibits the bodies of growing children from absorbing iron, zinc and calcium, minerals essential to proper brain and nerve development. It disrupts the normal release of dopamine, a powerful neuro chemical that controls an array of brain functions. And it becomes lodged in bones and teeth, leaching into the bloodstream for years.
"The fact that Kennedy Krieger has now seen actual physical changes in the brain structure of rats is fairly novel, but not surprising," Cory-Slechta said. "How that might affect behavior or cognitive function is harder to say, except that it's probably not good.
"It's also not much of a leap, based on what we know about the common effects of lead exposure across species, to theorize that it probably alters human brain structures as well. How - and how much - is still the question."
Under microscopic examination, the area of the brain known as the somatosensory cortex had shrunk in the lead-exposed rat pups. This part of the rat's brain receives and translates information from the animal's highly sensitive whiskers, their primary sensory organ.
"We saw average reductions of 11 percent, and up to 16 percent at higher levels of exposure," said Dr. Michael V. Johnston, a pediatric neurologist involved in the study. "In layman's terms, the rat's ability to comprehend his world has been blunted.
"We theorize that this would cause more confusion in the mind of the rat - like static in the brain - with a whole range of possible behavioral dysfunctions."
Dr. Gary W. Goldstein, a co-author of the study, noted that the most severe stunting of growth seen in the rats' brains occurred at exposure levels below the threshold at which medical treatment is typically given to children.
"We're seeing some fairly pronounced effects at a level of 30 micrograms of lead per deciliter of blood, which isn't even considered a treatment level right now," Goldstein said. "That's comparable to what we see in approximately 5 percent of the kids we treat, and that's cause for real concern."
"The disorganizing effect that they've shown in rat whiskers could just as easily be appearing in other sensory functions in other species," she said. "It is not hard to imagine that eyesight, touch, hearing, smell or all the above could be affected in other animals through a similar mechanism."
Perhaps even more troubling, the hearing loss in the University of Wisconsin monkeys occurred more than 10 years after they were exposed to lead in their infancy - placing them doubly in jeopardy among their normal peers.
"We've been testing this group throughout their lives," she said. "And they've always tended to be less successful within the social structure. They play less, they grow up having problems of social adaption, standing in the group.
"They tend not to be as successful in terms of their access to food, their access to breeding opportunities ... at fighting, predation and tool use. Obviously, these are all key behaviors for animals operating in a hierarchical social structure.
"Basically, they have trouble competing."
These persistent long-term deficits are not unlike those seen in children, researchers agree.
"While the findings of this research have tended to be controversial and have not been totally consistent ... when the results from a number of studies are combined there is evidence of a highly significant association between lead levels and child intelligence," researchers at New Zealand's Christchurch Hospital wrote in 1993.
In what is regarded as the most exhaustive of these studies, the Christchurch doctors began with 1,265 children and subjected them to a series of tests through their teen years.
That research revealed that "as lead levels increased there was a tendency for tests scores to decline, for teacher ratings to become less positive and for reports of inattentive/restless behavior to increase."
Australian researchers looking at a group of 375 children in the town of Port Pirie beginning in 1979 reported 13 years later that IQ scores for the group declined by three points for every 10-20 micrograms of lead exposure and persisted into later childhood "even though blood lead concentrations ... had declined substantially."
These observations have been central for U.S. researchers trying to estimate the benefits of expensive lead control laws.
Three years ago, the U.S. Department of Housing and Urban Development commissioned such an "econometric" study by ICF Consulting of Fairfax, Va., as a prelude to strict new regulations requiring the removal or repair of lead paint in old federally funded or subsidized housing.
Set to begin in September, the program carries an estimated price of $253 million in the first year alone.
But the ICF study predicted that the regulations would save as much as $1.14 billion nationwide by cutting IQ deficits among children exposed to lead paint in federal housing, improving their ability to find and keep good-paying jobs in the future.
"We know that lower IQ is associated with an increased tendency toward criminality, not to mention teen pregnancy," said economist Rick Nevin.
"So it's reasonable to theorize that a reduction in lead exposure would improve IQ and thereby bring reductions in both these behaviors and further cost savings," he said.
Using historical data from the U.S. Bureau of Mines, Nevin entered lead mining statistics into a computer model. Then, he added crime and teen birth rate data from the FBI and the National Center For Health Statistics.
To be sure that economic factors didn't skew his results, he also compiled unemployment figures from the U.S. Department of Labor.
"In precisely the age groups affected, at almost every point on the chart, we saw a startlingly consistent correlation between lead consumption, the birth rate at the time, and the rates of violent crime and teen pregnancy as these kids aged into adulthood," said Nevin, who holds multiple degrees in economics, math and management.
"I am sensitive enough to how astonishing this is that I've devoted substantial amounts of time trying to explain the results by other means, without much success."
Dr. Herbert L. Needleman of the University of Pittsburgh Medical School, a pioneer in studies of the link between early childhood lead poisoning and later aggressive behavior, cautioned that such global studies are fraught with potential for error.
"The biggest caveat you have to bear in mind with this kind of broad historical study is that a lot of other things were in play at the same time - an infinite number of major global events - that could dramatically affect the outcome," Needleman said.
"So studies of this kind can't possibly establish cause, definitively.
"What they can do rather well, when they're done right, is to suggest avenues for further research. And I'd have to say this one is very suggestive."