We know, without a doubt, that there is a lead poisoning crisis in Flint, Michigan — the headlines and the data make that fact abundantly, horrifyingly clear.

But what's equally terrifying is that we don't know how many other Flints there are — we don't know how many cities in the United States have similar lead poisoning crises.

Cities and states aren't required to report this information, or to even test all children for exposure. Flint's crisis was unusually visual because of the unmistakable rust coloring the water, but lead normally isn't the type of toxin you can smell, see, or taste. It's different from secondhand smoke or air pollution in that it is invisible.

"We don’t really know where the problem is; we don’t know where to target our resources to protect kids," says Bruce Lanphear, a professor at Simon Fraser University in Vancouver who studies the effects of low-level lead exposure in children.

It turns out that in America, it can be startlingly easy to live in an area where there is an alarmingly high rate of lead poisoning among children — and not even know about it.

Every state measures lead exposure differently

If you want to understand the scope of America's lead exposure data problem, it's best to start with this map my colleague Sarah Frostenson made. It shows that only 26 states report lead poisoning cases to the federal government's central database, maintained by the Centers for Disease Control and Prevention.

The problem with this map is that it's incomplete: There are 21 states that don't send in any information. So if you live in California or Texas, America's two most populous states, this data isn't much good for understanding what lead poisoning looks like where you live.

It gets worse. Each state in this map that does report data uses a different methodology to calculate the percentage of children with high rates of lead exposure.

In 2014, Genevieve Sykes, a researcher at the University of Alaska, looked at state guidelines for who is and isn't screened for lead exposure. She found that no two states were the same — some required all children to be screened, while others homed in on very specific populations. Only one state, Delaware, actually followed CDC guidelines regarding which kids should get tested. Everyone else set their own baselines — which means each state in the map above includes different populations of children in their samples. For most cities in America, we don't actually know the overall rate of lead poisoning.

Lots of states only require reporting of especially high lead levels. That's a huge problem.

There are also huge gaps in terms of how test results get reported to the state — if they get reported at all.

The CDC established in 2009 that there is no safe level of lead exposure for children. But there are nine states that only require reporting blood lead levels over a certain threshold. Lanphear's research suggests this is a huge oversight. Lanphear has shown that the effects are steepest at the lowest levels and then begin to taper off. Higher levels of lead are damaging, but the evidence suggests the bulk of the damage comes with relatively low levels of exposure.

"The evidence is quite definitive there is no safe level of lead — we've had a few dozen studies confirm that fact at this point," Lanphear says.

But in the nine states that only require doctors to report blood lead tests if the result is above a certain threshold, lower-dose cases never make their way into government statistics. Additionally, there are two states (Nevada and South Dakota) that don't require reporting of cases at all — it all depends on whether the doctor decides to provide that information.

"Having access to all test results, regardless of negative or positive, is critical for producing accurate screening recommendations," University of Alaska's Sykes writes in her review paper. But right now that doesn't happen.

It's possible for a lead poisoning epidemic to go completely unnoticed. Just look at Allentown, Pennsylvania.

Lead poisoning can wreak havoc on the body. Studies have linked exposure to lead to delayed cognitive development, increased rates of ADHD, and even to higher rates of juvenile delinquency and the development of kidney disease among older adults.

Still, it's possible for high rates of lead exposure to go unnoticed — just look at Allentown, Pennsylvania.

In 2014, the Pennsylvania Department of Health quietly released a report that found Allentown had the highest rate of lead poisoning in the state, with nearly a quarter of kids in the city affected — way higher than Flint.

But the report wasn't widely circulated — the 57-page document didn't become a national scandal after its release. "Nobody really knew about the report," says Ken Heffentrager, vice president of the Allentown Tenant Association.

Heffentrager says that he and others knew the city had really old houses, which are a key risk for lead exposure, as those houses are more likely to have lead-based paints. But it wasn't like things seemed especially out of order in Allentown.

"We have come across several homes that said they had a lead issue, but they were places where it was dealt with pretty quickly and then put back on the market," says Heffentrager.

After Vox's Frostenson wrote about the report last month, Allentown became very aware of the numbers. The city is now beginning to deal with its lead problem more fully. There's a city council meeting scheduled on the issue this coming Thursday.

But for Allentown's kids, the help comes too late. As Aaron Carroll has written at the Upshot, it's incredibly hard to reverse the long-term effects of lead poisoning. And what happened in Allentown points to one of the biggest challenges America has when tackling its lead problem. Lead is poisonous but also really difficult to detect. It's not like cigarette smoke or other toxins that you can see and smell. That should make the case for keeping excellent data; we're fighting an enemy that can hide easily. But we haven't decided to put resources toward that work — and kids suffer as a result.