Patricia Robinson recalls a time when she fondly watched her son, Michael, then a toddler, sit in the windowsill of her Englewood home, completely engrossed. Matchbox car in hand, he would run the toy back and forth over the brown painted surface, making little vrooms and beep-beeps as he played.

A recent study out of the University of Illinois at Chicago examined the blood lead levels of third graders between 2003 and 2006—students now likely to be roaming the halls at CPS high schools. It turns out that at three-quarters of Chicago’s 464 elementary schools, the students’ average blood lead level was high enough to be considered poisoned, according to standards set by the Centers for Disease Control and Prevention. And although lead poisoning is rarely mentioned in the debate on how to improve schools, the UIC research shows just how much it may be damaging kids’ ability to succeed. According to the study, lead-poisoned students in Chicago Public Schools are more likely to fail the third grade and score notably lower on their yearly standardized tests.

How do kids become lead poisoned? It’s not usually from eating paint chips. Instead, lead is typically ingested as dust—dust that’s created when old windows and doors are opened and closed, scattering a fine layer of the invisible stuff on a home’s floors and walls. As is presumed to be the case with Michael, children get this dust on their hands, then put their hands in their mouths. It doesn’t take much: a sugar packet’s worth of lead dust scattered over an area the size of a football field is enough to poison a child.

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Evens felt that to attract the money and attention necessary to rid Chicago of lead hazards, someone needed to quantify how much damage was being done. So she left the health department and got her PhD in environmental health. Her dissertation project? The largest study ever done on how lead poisoning affects schoolchildren.

“Throughout the country, low-income children have more lead exposure than middle-class children,” Rothstein says. “All children are less exposed to lead than they used to be, but the biggest declines have been for middle-class children.”

In addition to being the largest study of its kind, Evens’s research points out that even very low levels of lead can have a negative impact on a child’s ability to learn.

Here’s what to know if you think your home is at risk.