Lead poisoning is not exactly a new problem. Humans have mined lead — and been poisoned by it — for thousands of years. But it's easy to assume that after decades of research and the implementation of laws banning lead in paint, gasoline, pipes, and beyond, it's essentially a thing of the past. 

It's not.

In 2014, more than nine percent of Pennsylvania children under age seven who were tested had levels of lead in their blood above the threshold set by the Centers for Disease Control and Prevention. And researchers say even children with levels below that threshold are at risk, because small amounts of lead in the blood can have permanent health effects.

Also, most children in the state are never tested for lead at all.

That might change soon, as the Governor's office reviews legislation that would mandate lead testing for all children under a certain age.

Why do we only talk about lead poisoning in children? What about adults?

Both adults and children can be poisoned by lead. Exposure to very high levels can cause vomiting, seizures, coma, and even death.

For adults, lower level lead exposure has been linked to immune system suppression, motor nerve dysfunction, renal failure, gout, and hypertension.

But researchers say young children who have been exposed to lead also suffer drops in IQ, behavioral problems, and learning disabilities.

That's because young children, particularly between the ages of one and two, are more vulnerable to the dangers of lead than older children and adults.

When young children consume lead, they absorb more of it through their intestines than an adult would. And in young kids, the "blood-brain barrier" — the barrier that keeps toxins in the blood from getting into the brain — is still developing, making it easier for lead to pass through.

"Older kids whose brains are more developed are not as susceptible to insult or injury," said Dr. Denise Salerno, professor of clinical pediatrics at the Lewis Katz School of Medicine at Temple University.

Children also generally have higher levels of lead in their blood between the ages of one and two because they tend to put things in their mouths, including lead paint chips. It doesn't help that lead tastes sweet, so kids keep going back for more. 

Where would a child be exposed to lead? I thought it was banned from nearly everything.

In 1924, a physician named Dr. John C. Ruddock wrote in a medical journal that, "a child lives in a lead world." There was lead everywhere in children's homes and communities, including in the paint on their walls and toys, in the car exhaust pumped into the air, and yes, in the pipes that carried water into their homes.

Decades later, researchers had established what a devastating effect lead has on children.

In 1973, the Environmental Protection Agency created a regulation that began to phase out leaded gasoline, eliminating it entirely with another rule in the mid-1990s. The U.S. Consumer Product Safety Commission banned lead from residential paint in 1978, and Congress banned lead from pipes used for drinking water in 1986.

But there are still a lot of places kids come into contact with lead.

About 70 percent of Pennsylvania's homes were built before 1978, when lead paint was banned, according to data from the 2014 American Community Survey. But the paint remains in many homes. That lead can end up in the air when a home is renovated or in a child's mouth when the paint starts to chip.

And many homes are still connected to the city's water system by lead service pipes, typically owned by residents. Some cities, including Madison, Wisconsin, have mandated that homeowners replace their pipes, with some financial help from the city. For the most part, cities treat their water with chemicals so it won't absorb lead from old pipes. But the water treatment doesn't always work.

Kids can also come into contact with lead through soil, still contaminated by the chipping of exterior lead-based paint and by years of lead released into the air from car exhaust, as well as through homemade pottery and lead-painted toys made in other countries.

How much lead does a child have to be exposed to before getting sick?

Researchers say any lead in a child's blood is concerning, because they haven't found a blood lead level at which they don't see ill effects. What's worse, some of those effects appear to be permanent.

"Every time you look for the cognitive health effects of lead at lower and lower levels, you find them," said Dr. Mary Hovinga, associate professor of epidemiology at the Dornsife School of Public Health at Drexel University.

That's why, in 2012, the CDC lowered its threshold for what's considered an "elevated blood lead level" from 10 micrograms per deciliter (μg/dL) to 5 μg/dL. The new threshold is based on the 97.5th percentile of U.S. children, between ages one and five, tested for lead. 

Of U.S. children who are tested, 450,000 have blood lead levels higher than the new threshold.

That threshold will be updated every four years based on the latest data.

The Pennsylvania Department of Health continues to use the threshold of 10 μg/dL in its programs and in reporting some of its data.

What are the blood lead levels for children in Pennsylvania?

In 2014, about 140,000 Pennsylvania children under seven were tested for lead. More than nine percent had blood lead levels above the CDC threshold.

That's a decrease from previous years. In 2007, for instance, that number was 23 percent.

But it's still a high percentage — way higher than the four percent of children who have elevated blood lead levels in Flint.

The problem is worse in some Pennsylvania cities. In Allentown, 23 percent of children have elevated blood lead levels. In Altoona, it's about 20 percent. In Scranton, about 19 percent. And the levels are higher than 10 percent in Philadelphia, Johnstown, and several other cities.

But these test results may not actually reflect children's lead levels on the whole.

That's because in Pennsylvania, only 14 percent of children under age seven are tested for lead at all.

Only 26 percent of children between ages one and two are tested.

Why are so few children tested?

In many states, including Pennsylvania, pediatricians are required to test all children whose families receive Medicaid benefits between the ages of one and two. Some states have passed legislation requiring that all children between those ages be tested.

Pennsylvania has no such law.

In the absence of a mandate, the testing is at the discretion of doctors.

The doctors have a couple guidelines they could follow. The American Academy of Pediatrics and the CDC both recommend some version of targeted screening — figuring out which children are at greatest risk for lead poisoning and screening them. 

There are reasons pediatricians might choose not to test their patients for lead. In one survey of pediatricians in Vermont, doctors who didn't screen many of their patients for lead cited a few: low risk of exposure, parental opposition, and difficulty obtaining a blood sample.

Whatever the reason, in Pennsylvania, the percentage of children tested for lead might soon increase dramatically.

More testing being considered

After looking at the state's latest data and watching the news out of Flint, the Pennsylvania Department of Health has shifted its policy to support testing all children between ages one and two, said Dr. Loren Robinson, deputy secretary for health promotion and disease prevention at the department.

Universal testing may be more expensive, Robinson says, but it makes sense in Pennsylvania, which has a lot of old homes.

She says testing every child in this age range will give the state a better understanding of the problem. "If every single child is tested, we'll know exactly where the lead is," Robinson said. "Right now...we don't really have a great idea."

Governor Tom Wolf's Office of Policy and Planning is currently reviewing draft legislation that would mandate lead testing for every child at one and two years, spokesman Jeff Sheridan wrote in an email. The proposal would need to be passed by the state legislature.

State Representative Angel Cruz (D—Philadelphia) has also said he's planning to introduce legislation that would require all children under six to be tested for lead.

Testing isn't enough

Even if the state legislature passes a law requiring universal testing, there's something else to keep in mind: public health experts say testing children for lead, on its own, isn't enough.

Have you ever heard the phrase "a canary in a coal mine?" Up until the 20th century, workers used to take canaries down to the mines with them as an early warning sign of danger. Because canaries are smaller and more susceptible to dangerous gases like methane and carbon monoxide, they will show signs of being sickened way before a human will. When that happened, it was a sign to workers to evacuate the mine. 

Researchers say that because children are more susceptible to damage from lead, and some of that damage appears to be permanent, waiting until they get sick or have elevated blood lead levels is like using them as the proverbial "canary in a coal mine."

Or, as the CDC puts it, "screening children for elevated BLLs and dealing with their housing only when their BLL is already elevated should no longer be acceptable practice."

Instead, CDC supports primary prevention, "a strategy that emphasizes the prevention of lead exposure, rather than a response to exposure after it has taken place," according to the 2012 guidelines. The approach requires that we do something to ensure that children don't live in homes with lead hazards. 

Making that happen will require a change in state and city policies, and it could require a lot of money.

PhillyMag.com writes about some of the ways this could be accomplished: including increasing federal funding, investing more money in educating residents about lead poisoning, and even changing the way the Department of Environmental Protection tests the water for lead. Cities could also tighten their lead laws, requiring landlords to do more to certify that their properties are lead-free.

Background knowledge thanks to "Lead Wars," a book about the clash between scientists and industry over lead policy.

Correction 2/26/16: A previous version of this story incorrectly stated that the proposed universal testing legislation would not need to be approved by the state legislature.