When I was a kid, my dad half-joked that the Roman Empire fell because the Romans used lead to make their pipes, thus poisoning their drinking water supply. Even at age 7, I found that one hard to believe. But maybe, in some crucial ways, my dad was on to something. Strong evidence shows that lead is a uniquely toxic substance that can wreak permanent and devastating changes on both the human brain and a human life. Lead is also still ubiquitous in the U.S.

Until just a few years ago, plumbing fixtures containing as much as 8% lead could be sold as “lead-free.” When those pipes become corroded, as they often do, people start consuming lead. And don’t be misled by the photos of brownish-yellow water coming out of pipes in the famously lead-contaminated city of Flint, Michigan — lead in drinking water is generally colorless, tasteless and odorless.

People are breathing and eating lead too. Many houses built before 1978 have lead paint in them, sometimes buried under more recent coats of paint.

When kids ingest lead, it gets stored in their bones, kidneys, liver and brains. It’s the last of these that is particularly insidious. Scientists continue to find ways that lead damages brain function, but that doesn’t explain exactly how the brain damage will contribute to altered behavior and consciousness. The only way to determine that is to examine the behavior children who were exposed to lead (since children are more susceptible to the metal’s effects). Kids with high levels of lead in their blood do worse on tests, have trouble paying attention and do poorly in a number of other visible ways.

In the past, only children with blood-lead concentrations of 10 micrograms per deciliter were considered exposed to lead. But a number of studies from around the world have shown that even lower levels can have major deleterious effects on brain function. So even though official levels of lead poisoning have gone down substantially in rich countries — especially after the banning of leaded gasoline — there are still probably a very large number of American children whose personalities and cognitive capacities are being altered.

Perhaps the worst consequence of this silent brain damage is crime. Mother Jones writer Kevin Drum has been tireless in collating the vast pile of evidence regarding the link. Much of the research focuses on making sure that it’s not poverty or other social factors causing the correlation between lead and crime.

For example, a 2018 paper by Anna Aizer and Janet Currie use the banning of leaded gasoline, together with families’ proximity to roads, to estimate the effect of lead on school suspensions and juvenile detention in Rhode Island, while looking at differences between siblings to control for family disadvantage. They find that when blood lead falls by one microgram per decileter, the probability of a boy being sent to juvenile detention falls by a jaw-dropping 27% to 74%. The biggest effects were for poor African-American boys. Most of these students had blood lead levels well below the official cutoff for lead poisoning, showing the destructive effects of even small amounts of the metal.

In a 2019 paper economists Stephen Billings and Kevin Schnepel identify lead-poisoned children who were eligible for interventions, including blood lead reduction, in Charlotte, North Carolina, and compared them to children who just missed the cutoff for intervention. Tracking the kids throughout their lives, they find much lower crime rates among those who had received the interventions.

These are only two particularly well-done studies amid a growing mountain of evidence on the link between lead and crime. This evidence suggests that cleaning up lead paint, replacing lead pipes, and removing lead from soil could have major beneficial effects for those cities still wracked by high levels of violence.

So far, lead cleanup hasn’t been a central issue for most of the Democratic presidential candidates. An exception is Texas’ Julian Castro, who has released a comprehensive and detailed plan for lead remediation, detection and treatment throughout the U.S.

Lead poisoning may not send the U.S. down the path of ancient Rome. But just in case, we should clean up all the lead.

Noah Smith is a Bloomberg Opinion columnist.