The city of Flint, Michigan, is in the midst of a terrible and rightly shocking lead poisoning crisis. The number of kids testing positive for elevated lead levels in their bloodstreams has doubled in the past few years, after the city switched to a new, cheaper water source.

This is an extreme case, but the problem of lead exposure among children is not a local Flint story. If you look at public health data, you begin to realize two things. The first is that it's actually really hard to get good data on which kids do and don't experience lead exposure, and which parents should worry about the issue.

Second: The data that is available shows that lead exposure is a pervasive issue in the United States. In some places outside of Flint, more than half of children test positive for lead poisoning.

Houston County, Alabama, is, in a lot of ways, an unremarkable place. It has just over 100,000 residents and sits in the southeast corner of the state, bordering Florida and Georgia. Median household income there is about $40,000, slightly lower than average for the state.

But there is one way Houston County does stand out: In 2014, it reported the highest rate of lead poisoning in the nation of any counties that sent data to the Centers for Disease Control and Prevention.

Houston County tested 12 children for lead poisoning in 2014, which it defines as kids who have more than 5 micrograms of lead per deciliter of blood. Seven of those tests came back positive.

Nine counties nationwide told the CDC that 10 percent or more of their lead poisoning tests came back positive. Four of them are in Louisiana, two in Alabama, and the rest scattered across West Virginia, Kentucky, Indiana, and Oklahoma.

These are places that have told the federal government they actually have higher rates of lead poisoning than Flint, where officials say the number hovers around 4 percent. But these aren't places we talk about that much.

The map above uses CDC data to show lead poisoning rates across the country. The reason so many of the counties are light gray is that most counties simply don't report this information — nor are they required to.

Of the 3,143 counties in the United States, only 1,573 reported lead poisoning data in 2014. Forty-four percent of those counties reported no confirmed cases of lead in the bloodstream. But there are also the nine counties, largely in the south, where more than 10 percent of kids tested positive.

Twenty-one states do not regularly submit data to the CDC on lead surveillance programs in their states. Eleven of those 21 states do not submit any kind of lead surveillance data to the CDC — no state-level or county-level data. The other 10 states do submit data, but many haven't submitted anything in the past two years. For instance, North Carolina hasn't submitted its data since 2009. The states that don't submit any data include Alaska, Arkansas, Hawaii, Idaho, Montana, Nebraska, North Dakota, South Carolina, South Dakota, Utah, and Wyoming.

That means there are 1,570 counties we don't have any data on at all, because states are not mandated to submit their data to the CDC.

Lead poisoning is incredibly hazardous to children's health

Childhood lead exposure at nearly any level can lead to permanent learning disabilities, lower IQs, and even ADHD. Blood lead levels once believed to be safe — 30 μg/dL in the 1970s, then 25, then 15, then 10 — are now known to cause irreversible damage. The Environmental Protection Agency now says there is "no demonstrated safe concentration of lead in blood," and even levels as low as 2 μg/dL can reduce a child's IQ. CDC data estimates that almost 500,000 children in the US between the ages of 1 and 5 have a blood lead level above the 5 μg/dL standard.

The CDC recommends follow-up and intervention for kids with more than 10 micrograms of lead per deciliter of blood. But as my colleague, Matt Yglesias points out, 10 micrograms is a dubious threshold at best, as the underlying science suggests any amount of lead in the bloodstream is harmful. And if the state lead contamination tests are any indication, tons of kids are ingesting more lead than they should.

Black children are at twice the risk of lead exposure as white children

A 2013 study from the CDC found that lead exposure impacts black communities disproportionately to their white counterparts.

Although blood lead levels among all US children have dropped dramatically since the late 1990s, high blood lead levels among black children were still more than twice as prevalent as among their white counterparts.

It's possible there are other geographic trends to lead exposure, ones we might see if we had a more complete data set. But those are harder to know about when most counties don't provide information about — and possibly don't even test — the prevalence of lead poisoning among their children.