In one city after another, the tests showed startling numbers of children with unsafe blood lead levels: Poughkeepsie and Syracuse and Buffalo. Erie and Reading. Cleveland and Cincinnati.

In those cities and others around the country, 14 percent of kids — and in some cases more — have troubling amounts of the toxic metal in their blood, according to new research published Wednesday. The findings underscore how despite long-running public health efforts to reduce lead exposure, many U.S. children still live in environments where they're likely to encounter a substance that can lead to lasting behavioral, mental and physical problems.

"We've been making progress for decades, but we have a ways to go," said Harvey Kaufman, senior medical director at Quest Diagnostics and a co-author of the study, which was published in the Journal of Pediatrics. "With blood [lead] levels in kids, there is no safe level."

Kaufman and two colleagues at Quest, the nation's largest lab testing provider, examined more than 5.2 million blood tests for infants and children under age 6 that were taken between 2009 and 2015. The results spanned every state and the District of Columbia.

The researchers found that while blood lead levels declined nationally overall during that period, roughly 3 percent of children across the country had levels that exceed five micrograms per deciliter — the threshold that the Centers for Disease Control and Prevention considers cause for concern. But in some places and among particular demographics, those figures are much higher.

In certain regions of the country, including parts of New York, Pennsylvania and Ohio, more than 1 in 7 children tested for elevated levels of lead in their blood. Minnesota had the highest overall rate of young children with disturbing blood lead levels, at 10.3 percent. That was followed by Pennsylvania (7.8 percent), Kentucky (7.1 percent), Ohio (7 percent) and Connecticut (6.7 percent).

"It’s a tragedy that anywhere in the United States of America, we have 14 percent of children with lead levels above the CDC threshold," said Leonardo Trasande, associate professor of pediatrics at New York University School of Medicine. "Lead is the most obvious tip of the proverbial iceberg when it comes to environmental health threats. Often where there is childhood lead exposure, there are other environmental hazards of great concern."

So why are kids in certain places more at risk of lead poisoning?

Living in an area with a high proportion of pre-1950s construction — an area when lead-based paints were widely used and lead pipes were still the norm in some places — increases potential exposures for children. So older housing stock equals more risk.

But being poor also plays a role. Wednesday's study showed that children living in Zip codes with higher poverty rates had a greater proportion of elevated blood lead levels, while children in more affluent Zip codes were "much less likely" to suffer that fate.

The news wasn't entirely bad. States such as California and Florida had the lowest rates of elevated blood lead levels in children, at 1.4 percent and 1.1. percent, respectively. And over the six years included in the study, New Hampshire saw the largest absolute decline in high blood lead levels, from 9.7 percent to 2.6 percent.

Quest's study does acknowledge some limitations. The researchers were confined to tests that were actually ordered by physicians, and only those conducted by the company rather than its competitors. It's also possible that certain populations or certain places deemed at higher risk are being tested more frequently. But overall, Kaufman said, the data include a huge number of samples across every state, and repeat specimens were omitted, all of which help to eliminate aberrations in the findings.

A federal advisory committee in 2012 estimated that there are nearly a half million children in the United States with blood lead levels that exceed the CDC threshold. While the most substantial threats are still lead-based paint and lead contamination in soil, the recent water crisis in Flint, Mich., has highlighted the many pathways of exposure. There, an estimated 9,000 children under age 6 have been exposed to lead in drinking water, and officials have identified hundreds with elevated blood lead levels.

Public health officials have long warned that lead is unsafe at any level in the blood. Even small amounts can contribute to a range of problems, including lower IQs, shortened attention spans, antisocial behavior and health issues such as hypertension, anemia and damage to the kidneys and reproductive organs.

A recent Reuters investigation found that in much of the country, lead testing isn't required for young children and infants. And even when it is, those blood tests often aren't done. In addition, Trasande said many pediatricians "have taken their eye off the ball" when it comes to testing, because elevated lead in blood isn't as widespread as it once was.

"It’s easy to lose sight of this ongoing and insidious battle," he said. "It's not as if we can write a prescription for this problem. The prescription is prevention."

That battle, of course, is not a new one. In 2000, the federal government released a multifaceted plan "to eliminate childhood lead poisoning in the United States as a major public health problem by the year 2010," largely by abating lead hazards in homes and expanding blood lead screening and follow-up care for at-risk children. But in part because of a shortage of congressional funding for abatement programs, lead risks remain a reality for millions of U.S. children.

A more recent government initiative has a new goal — eliminating elevated blood lead levels in children by 2020. But that's an unlikely target in the absence of even more aggressive measures.

"We really need to keep the pressure on policymakers and others to keep driving lead out of our homes and our environment," said Kaufman, "so that kids and pregnant women are safe."

For those interested in more history on the nation's lead poisoning problem, HBO's John Oliver dove deep into the issue in a recent episode of "Last Week Tonight," complete with a musical number alongside the cast of Sesame Street:

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