When Democratic presidential hopefuls Hillary Clinton and Sen. Bernie Sanders faced off Sunday in a debate in Flint, Mich., the lead poisoning of Flint’s drinking water was naturally a topic the candidates addressed. Clinton vowed to remain at the side of Flint residents, no matter the outcome of the political campaign.

Despite the spotlight provided by the presidential debate, lead poisoning is a decades-long issue and still mostly an unseen crisis.

Clinton noted that lead poisoning is not unique to Flint, pointing to a New York Times article reporting that Cleveland has higher lead poisoning levels than Flint. The Times article said the effort to eliminate lead in drinking water and from paint inside homes “is a tragic reminder that one of the great public health crusades of the 20th century remains unfinished.”

Across the United States, 37 million homes and apartments are believfed to have lead paint on walls or woodwork. When that paint chips or is sanded, lead can be eaten by small children or inhaled with dust. It’s believed 4 million of the homes with lead paint contain small children, the most vulnerable population for the neurotoxic effects of lead.

The Centers for Disease Control and Prevention reports that 23 percent of children in Philadelphia, Allentown, Pa., and Atlantic City have excessive levels of lead in their blood.

Lead issues are concentrated in older cities with older homes, mostly in the Northeast and Midwest. When tested in 2013, Pittsburgh’s drinking water lead level was 14.7 parts per billion, just below the 15 parts per billion that the Environmental Protection Agency sets as an “action level.”

It’s worth noting that the EPA’s 15-ppm level is arbitrary. The World Health Organization sets its lead limit at 10 ppm. And the CDC says there is no safe level of lead. None.

Beyond various “safe” limits, the testing itself is a problem. Testing does not alwasy focus on the high-risk areas, meaning low-income populations in city centers. It’s also likely that not enough homes are being tested to reveal the true scope of the problem.

When more homes have been tested, particuarly in older cities such as Cleveland or Pittsburgh, the lead levels moved higher.

Test methodology is another problem. The EPA says water should sit in pipes overnight before samples are drawn for testing. This is recommended because lead from old pipes leaches into water that sits in pipes in service lines coming from water mains, and in household pipes.

Under water testing in Philadelphia and Detroit, people were told to let the water run a few minutes before drawing a sample for testing. This pre-flushing masks the level of lead in the water, by drawing in water from main lines. The pre-flushing protocol could help municipal water systems minimize the lead dangers and the costly repairs that higher levels might trigger.

Lead in gasoline was phased out in the mid 1970s, the same period that lead paint was banned. But lead was a health danger before the 1970s and has been for the decades since.

The tragic contamination of drinking water in Flint has given the issue more visibility. But it’s troubling that lead contamination of the same or higher levels have been found in Cleveland and other older American cities.

Expressions of outrage from presidential candidates might be comforting, but it’s not enough. Eliminating lead-caused health dangers will mean federal, state and local efforts to improve and expand testing, including a consistent policy on pre-flushing water lines. Any renewed effort to eliminate lead from drinking water and for keeping small children from ingesting it through paint chips or dust will require money, lots of money.

It will cost billions of dollars to get the lead out of drinking water and homes, and that’s only a small part of the trillions of dollars needed to upgrade America’s infrastructure, from underground water lines and gas lines to highways, bridges, rail lines, airports and sea ports.

Flint is a reminder of the massive job of updating America’s infrastructure. Yet politicians in Washington do nothing.