In some of America's oldest and largest urban areas, places like New York and Chicago, one in five census tracts — geographic areas that usually have between 1,200 and 8,000 people — have very high risks of lead exposure. (We use a scale of one to 10 we use in the map above; these areas rank as a 10.)

In younger metro areas, like Los Angeles, risks are smaller but still present; 12 percent of census tracts there have risk scores of 10. And it's important to remember that public health experts say no level of lead exposure is safe for children.

How we created this map

Our map uses a methodology that Washington State’s Department of Health pioneered earlier this year to estimate kids’ risk of lead exposure in different neighborhoods. Their mission was to determine how to focus scarce public health dollars on the kids most at risk of being poisoned by lead. We worked with one of the chief epidemiologists who created the map, Rad Cunningham, to replicate the state’s methodology nationally and apply it to all 72,241 census tracts in the United States.

The idea wasn't just to treat lead poisoning — the effects of lead exposure are often irreversible — but rather to use mapping tools to prevent exposure from happening in the first place.

To do this, researchers in Washington combed through the literature on lead poisoning to understand what factors best correlate with risk. They found that there were two — the age of the houses (which predicts the likelihood of lead paint) and poverty — that the literature consistently finds to be correlated with more kids coming into contact with lead.

"It's an extremely valuable tool," says Surili Patel, who is the American Public Health Association's senior program manager for environmental health. She has worked with Washington state, but not on this map. "I can see this helping the medical community and communities at risk understand where they are and why to pay attention."

Rob Walker, a public health official in Iowa who chairs the Center for Disease Control’s working group on Environmental Health Tracking said that Washington’s map uses “the best data available and a sound methodology” to create the risk scores.

Why it took so long to come up with a map like this

It can be frustratingly difficult — sometimes impossible — for a parent or even a public health researcher to find good data on where the risks of lead exposure are highest.

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

Cities and states aren't required to report data on how many kids have lead poisoning, or to even test all children for exposure. Flint's crisis earlier this year 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.

In lieu of consistent reporting standards, the country gets left with a hodgepodge of data. Different states have different standards; for example, because Washington and neighboring Oregon or Idaho have different standards for which kids they screen, it's difficult to compare lead exposure risk levels.

"You get to compare side by side, an apples-to-apples understanding," Patel of the APHA says of the national map. "It goes to the bigger picture of getting uniform data."