Tens of thousands of aging pipes that bring water into Minnesota homes also carry the threat of an invisible neurotoxin: lead.

Prompted by the lead poisoning crisis in Flint, Mich., the state of Minnesota examined the scope of the problem here and what it would cost to fix it. On Thursday, the Minnesota Department of Health provided its estimate: up to $4 billion over two decades. But the benefits, in better health, higher IQs and productivity, could be double that amount, it says.

"We see this report on lead in drinking water as an important next step in that decadeslong public health effort to remove this very significant health threat," Health Commissioner Jan Malcolm said Thursday.

Lead can cause cognitive damage, particularly in children under 6 years old, and harm the nervous system, red blood cells and kidneys. Health officials have continually tightened lead safety standards, according to the department's report, and in 2012 the Centers for Disease Control and Prevention determined that there is no level of safe exposure.

Cities and utilities often treat public water supplies with chemicals, commonly a type of phosphate, to prevent corrosion by creating a protective film inside the pipes. But the report emphasized the need to replace the lead plumbing altogether.

An estimated 100,000 old lead service lines remain across Minnesota. The report says a rough count indicates the majority, 60,000, are in Minneapolis, with another 28,000 in St. Paul and 5,000 in Duluth.

Lead service lines and plumbing fixtures inside the home are the most significant source of lead in public drinking water. A huge challenge in removing them is that the water supplier owns part of the pipe, and the property owner owns the stretch running into the building. The dividing line on ownership is typically somewhere around the curb.

No one knows how many of the estimated 100,000 lead service lines belong to cities vs. private property owners, said Tannie Eshenaur, the Health Department's planning director for drinking water protection. That's why a statewide inventory is such a crucial first step.

While the report doesn't delve into disparities, it notes that minority and low-income residents are more likely to be exposed to lead in other ways, such as through peeling or flaking lead-based paint in older homes, and that "reducing further harm is a priority." Getting landlords to replace lead pipes in their buildings, for example, is important to ensuring that all communities see improvements.

Replacing lead service lines can cost a homeowner anywhere from $2,500 to more than $8,000 per line, money that they're often reluctant to pay, according to the report.

The report noted that in St. Paul, the local water utility encourages property owners to replace their section of the water pipe when city crews are out replacing water lines to the street.

"Unfortunately, fewer than half of residents choose to pay the $3,000-$4,000 required to address their half of the work," the report said.

The Health Department is gearing up to advise cities on locating lead service pipes in their area for a statewide inventory. Some cities have a good handle on it already and the audit won't be a huge task, said Sandeep Burman, manager of the Health Department's drinking water protection section.

"Other cities may have to go to the extreme, which is to knock on doors," Burman said. "Over the next one to two years we hope to see a lot of communities in Minnesota complete these inventories."

Health officials welcomed a proposed increase in water service connection fees in Gov. Tim Walz's budget. "Having more robust support will make it easier to do the inventory," Eshenaur said.

To help property owners pay for removing their lead pipes, officials are eyeing the Drinking Water State Revolving Fund. The fund, a mix of federal dollars, state money and loan repayments, is jointly run by the Health Department and the state Public Facilities Authority. The two agencies are discussing how the funding might be transferred to cities and utilities that would devise ways to provide it customers, via grants or low interest loans, for example.

"There's a reluctance to spend public dollars on private property," Eshenaur said.

Some cities and utilities already offer residents a hand. Customers of St. Paul Regional Water Services, for example, can get a low-interest loan to finance lead pipe removal, and then tack loan repayments onto their property taxes.

"You can go all the way up to 20 years if you want," said production manager Jim Bode.

But only about 160 homeowners have done it in the past few years, Bode said.