The story of Flint, Michigan's children being poisoned by lead-contaminated drinking water has, rightly, shocked and scandalized the nation. But while the situation in Flint is certainly an extreme case, the problem is much more widespread than many realize: Children in essentially every city in America are being exposed to hazardous levels of toxic lead, and very little is being done about it.

At the most severe levels, according to the World Health Organization, "lead attacks the brain and central nervous system to cause coma, convulsions, and even death." Thankfully, very little lead poisoning that severe is happening in the United States. But lead's impact on the brain — particularly the developing brains of children and fetuses — is severe and systematic, "resulting in reduced [IQ], behavioral changes such as shortening of attention span and increased antisocial behavior, and reduced educational attainment." At least mild versions of these impacts are felt at even low levels of exposure "that cause no obvious symptoms and that previously were considered safe."

The CDC recommends follow-up and intervention for kids who have more than 10 micrograms of lead per deciliter of blood. But this is basically just a nice round number that leads to the happy conclusion that most kids' brains aren't being poisoned by lead. The underlying science offers the much less reassuring conclusion that any amount of lead is harmful and tons of kids are ingesting more lead than they should:

Neurological research is demonstrating that lead's effects are even more appalling, more permanent, and appear at far lower levels than we ever thought. For starters, it turns out that childhood lead exposure at nearly any level can seriously and permanently reduce IQ. Blood lead levels are measured in micrograms per deciliter, and levels once believed safe—65 μg/dL, then 25, then 15, then 10—are now known to cause serious damage. The EPA now says flatly that there is "no demonstrated safe concentration of lead in blood," and it turns out that even levels under 10 μg/dL can reduce IQ by as much as seven points. An estimated 2.5 percent of children nationwide have lead levels above 5 μg/dL.

So maybe 5 is the new 10? No such luck.

Braun et al. find that within the range of 2 and 5 micrograms of lead per deciliter, more blood lead is associated with higher levels of ADHD.

Then Nigg et al. studied a population with blood levels "slightly below United States and Western Europe population exposure averages, with a mean of 0.73 and a maximum of 2.2 μg/dL," and found that even at this range, more lead means more ADHD.

Our scientific understanding of this issue is limited by the fact that it's hard to chemically detect very low levels of lead in the blood. But to the extent that scientists have been able to study low levels of lead exposure, they have found that there is no safe point. More lead is always worse, and the level of blood lead enjoyed by the typical American child is at least somewhat hazardous.

Urban soil lead contamination is woefully understudied

The main thing we know about non-catastrophic lead in the United States is that the biggest problem is inner-city soil contaminated by decades-old gasoline. Gas went unleaded in the mid-1970s, but all the old lead burned in the past was dumped into the air and then fell back to earth. The tiny lead particles don't biodegrade. They mix in with the soil, get tracked into houses, and, most of all, end up on the hands and toys of little kids, who have a marked tendency to stick anything and everything into their mouths, leading to the ingestion of lead.

This lead is everywhere, but it's most heavily concentrated in places that were close to a lot of vehicle traffic during the leaded gasoline days — in other words, the centers of big cities.

But there's very little systematic research on the lead situation in most cities. An exception is New Orleans, which happens to benefit from proximity to one of America's leading lead researchers, Tulane's Howard Mielke. Here's what he found:

But most cities are not blessed to have local researchers and comprehensive studies. If you're lucky, what's happened is that someone noticed the recent urban farming trend and decided to look into how much heavy metal contamination there is in the local food supply. Here's a look at DC's community gardens:

Only the Stadium-Armory level of lead contamination is at a truly alarming level. But all the central city locations studied — including Capitol Hill, Dupont Circle, and LeDroit Park — exceed the Norwegian government standard of roughly 100 parts per million of lead in soil.

And here's a look at New York City:

Brooklyn, it turns out, is basically a gigantic toxic wasteland. And the problem in these cities probably isn't limited to community gardens — there's all kinds of old dirt in parks, backyards, and other places. And though certain kinds of plants grown in metal-contaminated soils can be particularly damaging, lead-contaminated soil really shouldn't be seen as primarily an agricultural concern. The dirt and dust on their own manage to work their way into kids' mouths.

Nobody is really doing much about this

The good news about Flint, such as it is, is that the presence of excessive quantities of lead in municipal drinking water is treated as a genuine scandal and an emergency. The presence of excessive quantities of lead in urban soil, by contrast, is something we are essentially shrugging at as a society.

The District of Columbia's Department of Energy and Environment, for example, offers the official advice to parents of young children that "it’s best to cover any bare soil" in your backyard because children "get lead-contaminated dirt under their fingernails, and dogs can roll around and bring the lead-contaminated dirt into the home."

But this is rather quiet and obscure advice, and there's certainly no citywide program to cover up backyards everywhere with impermeable surfaces. The Department of Parks and Recreation has thoughtfully covered up the soil in the city's playgrounds so kids don't play in contaminated dirt. But many of the city's parks, like the one in Logan Circle a few blocks from my house, are among the many urban parks in the city operated by the National Park Service, which neither covers its soil nor tests it for lead.

Having urban parks that are unsafe for children to play in seems generally ill-advised, but there's also no publicity or signage to indicate that the seemingly wholesome activity of kids rolling around in the dirt might be harmful.