Scientists have long known that children with high levels of toxic lead in their bloodstream are more likely than others to behave impulsively, have shorter attention spans and lower IQs and do poorly in school.

Research out Tuesday finds that even children with just moderate levels of lead in their first three years of life are nearly three times as likely to be suspended from school by the time they’re 9 or 10 as those whose blood-lead levels were below recent treatment thresholds.

The study, appearing in the journal Environmental Research, analyzed medical and school discipline records of 3,763 children in Milwaukee Public Schools.

The federal Centers for Disease Control and Prevention last year told physicians that there’s essentially no safe level of lead, but the agency previously said that children with 10 to 20 micrograms of lead per deciliter of blood had enough of the toxin in their system to present a “level of concern.” Children with 5 or fewer micrograms of lead were considered safe, more or less because the margin of error in measuring blood-lead levels put them essentially at zero.

Researchers led by Michael Amato, of the psychology department of the University of Wisconsin-Madison, studied children whose blood-lead levels were between 10 and 20 micrograms and compared them with children whose levels were below 5. Children at the “acute end of poisoning,” with more than 20 micrograms, had long been studied, he said. But researchers knew less about those in the moderate range – a large group who often find themselves in a “policy loophole,” he said.

Nearly 1 in 3 students exposed to lead had been suspended, compared with just over one in 10 of those not exposed.

Lead exposure, the researchers calculated, explained 23 percent of the gap in suspension rates between black and white Milwaukee students.

Hans Steiner, a professor of psychiatry at Stanford University and an attending physician at Stanford’s Lucile Packard Children’s Hospital, said the findings could help schools struggling with behavior problems, including those in Silicon Valley, for whom he has consulted for years.

“They’re fairly sophisticated and well-funded,” Steiner said of schools in California’s high-tech corridor, “and this would not even occur to them.”

In Milwaukee, long-standing programs to get rid of lead in kids’ homes have cleaned up the most heavily contaminated sites, but Amato said the city faces a bigger challenge cleaning up places that aren’t so obviously affected.

“However you measure it, there has been quite a lot of improvement in Milwaukee over the past 10 years,” he said. “But there is a long way to go.”

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