Brett Carlsen/Getty Images Michigan Gov. Rick Snyder (R) doesn't know how many lead pipes there are in Flint.

Just how many lead pipes are there in Flint, Michigan, where the water has been undrinkable because of high lead levels? Nobody knows.

"A lot of work is being done to even understand where the lead services lines fully are, so I would say any numbers you're hearing at this point are still speculation," Michigan Gov. Rick Snyder (R) said Wednesday.

It's a problem that's much bigger than Flint: there are millions of lead pipes all across America, putting children at risk of stunted growth, brain damage and a lifetime of diminished potential. Just this week, residents of Sebring, a town of 8,000 in rural Ohio, were told not to touch their tap water out of lead fears similar to Flint's.

"This is a situation that has the potential to occur in however many places around the country there are lead pipes," Jerry Paulson, emeritus professor of pediatrics and environmental health at George Washington University, said in an interview. "Unless and until those pipes are removed, those communities are at some degree of risk."

Roughly 10 million American homes and buildings receive water from service lines that are at least partially lead, according to the Environmental Protection Agency. Service lines are the pipes connecting water mains to people's houses. Lead ones are mostly found in the Midwest and Northeast.

Despite the life-altering consequences of lead poisoning, there is no national plan to get rid of those pipes. A top reason for continuing to use lead service lines instead of immediately digging them up is that utilities can treat water so it forms a coating on the interior of the pipes -- a corrosion barrier that helps prevent lead particles from dislodging and traveling to your faucet. But if the water chemistry changes, the corrosion controls can fail.

That's what happened in Flint after the Michigan Department of Environmental Quality told the Flint Water Treatment Plant not to maintain corrosion controls that had previously been in place before the city switched water sources in 2014.

There is nothing a water utility can do to completely prevent lead leaching from a lead service line. Yanna Lambrinidou, water safety expert

The federal Safe Drinking Water Act requires water systems and state regulators to monitor lead levels coming out of people's faucets, and if 10 percent of samples have more than 15 parts per billion of lead, then the state has to evaluate its corrosion controls. And if that doesn't reduce the lead levels, then the law requires public water systems to begin replacing 7 percent of their lead service lines every year.

Even replacing the lines can be trouble, however, as the law only requires replacing the lines on public property -- replacing the portion of a lead service line on private property is up to the owner -- and it turns out that replacing just the public portion of a lead service line can cause lead levels to spike in a homeowner's water. That's because the work involved in replacing just part of a lead service line can jostle free lead in the the remaining part of the pipe.

(The Safe Drinking Water Act originally called for utilities to replace the entirety of a lead service line, but lobbying and a lawsuit by the American Water Works Association watered down the rule.)

"There is nothing a water utility can do to completely prevent lead leaching from a lead service line," said Yanna Lambrinidou, a water safety expert who teaches at Virginia Tech.

The EPA is currently considering changes to the Safe Drinking Water Act's lead rules, and an advisory panel has proposed a more proactive approach to replacing lead pipes. Instead of just waiting for higher levels of a deadly neurotoxin to show up in people's tap water, the proposal would encourage public water systems to go ahead and replace the pipes.

Lambrinidou was a member of the working group that crafted the recommendation, but she wound up dissenting because she believed it didn't do enough to force utilities to get lead pipes out of the ground.

"The rule needs to be such that it enforces actual lead service line replacement," Lambrinidou said.

What happened in Flint seems to show a weakness of the rule, since state regulators and the EPA agreed there was some ambiguity about whether Flint should have been required to implement corrosion controls. Snyder eventually admitted that his government had made a terrible mistake, and multiple state officials resigned, as did the EPA's regional administrator.

Flint reconnected to its original water source in October, and officials said yesterday there had been progress in the process of re-coating the interior of the city's aging pipes.

"Longer term, though, I think everyone understands we'd like to see those pipes replaced," Snyder said.

This story has been updated with a more recent EPA estimate of the number of lead service lines in use.