Update: With Bernie Sanders and Hillary Clinton debating in Flint, Mich., on Sunday night, we are re-upping this post from January.

Lead tastes sweet. Hundreds of years ago, some forms of lead were actually used instead of sugar, with predictably negative results. That taste means that a kid in a house with peeling paint that contained lead -- as many house paints did a century ago -- might pick up a chip and pop it in his mouth.

In the late 1990s, I worked with a team of people that was dispatched by the Health Department of Columbus, Ohio, to houses in poor neighborhoods to do lead abatement. From a practical standpoint, that meant sanding down patches of peeling paint and cleaning the dust out of window sills, where paint collected after it rubbed off as the window was opened and closed. Kids didn't just eat paint chips. These were old houses with young kids, and our goal was to limit the likelihood of lead exposure by children as they crawled around in lead dust. In other words, it was to build as much of a barrier between the kid and the lead as possible, well after the point at which lead surrounded them.

The crisis that's still unfolding in Flint, Mich., centers on the extremely bad decision to use water from the Flint River as a cost-saving measure for the city as it built a new supply line from Lake Huron. That transition resulted in vastly elevated levels of lead in the water flowing into people's homes, causing a spike in lead levels in local children. What's happening in Flint is also a reminder of the pervasiveness of lead in our environment -- and the damage that the metal can do.

The General Motors Flint Assembly in Flint, Mich. seen in 2011. (Carlos Osorio/AP)

David Rosner, professor of public health and history at Columbia University and the author of "Lead Wars," a history of the battle over lead in the environment, explained the special significance of Flint in the history of American industry's embrace of lead when I spoke with him by phone  Tuesday.

"Lead was introduced into gasoline as tetraethyl lead by General Motors," Rosner said, "the people that brought you Flint, Michigan." Meaning the city: Flint was home to GM at its inception. When the company created a new type of fuel that burned lead, it had a new advantage over its competitors at Ford. It was only the newest way in which the industry relied on lead in manufacturing its product.

"In the 1930s, 1940s, 1950s, the industry basically quenched concerns about low-level exposures by saying that lead was a 'gift of God,' essential to modern industry, essential to modern production. It had to go into everything and we couldn't live without it," Rosner said. "Gift of God" is not Rosner's phrasing; it was an expression used by a representative of GM. Lead, Rosner says, "became the basis for the new car industry" -- even as concerns about it were increasing.

General Motors' success "led to the creation of a whole set of ancillary industries all along the Flint River, going up to Saginaw," Rosner said. "Battery plants, paint manufacturers, the soldering materials. The modern car is a giant industrial product that contains large amounts of all sorts of toxins. Not only oils and lubricants, but lead. Basically, the Flint River was the outlet for all this stuff." (In October  2014, General Motors stopped using Flint River water at its facility because it was worried about corrosion.)

Flint, in other words, was an epicenter of America's lead problem. But the problem predates Flint.

There's a reason people used to paint their houses with lead paints. Lead paint offered a number of advantages, including durability. As recently as the 1990s, the paint used on roads contained lead. Lead was used to transport water in pipes (including in Flint). It was used in toys. It was used in industries besides automobiles. A hundred years ago, people saw the effects of acute lead poisoning -- comas and convulsions -- from workers at facilities producing lead products. Over time, it became clear that chronic exposure to lead created a different, more subtle set of problems.

Research in the 1970s, conducted in part by analyzing the levels of lead in childrens' baby teeth, revealed that higher levels of lead in the body were linked to "lower IQs, less verbal competence, worse speech processing, and worse attention," in the words of the American Psychological Association. (When ingested, lead replaces minerals in bones -- like teeth -- and seeps into the blood over time.) There were behavioral problems, too. Children with higher levels of lead in their bodies are more hyperactive and can act out.

There's some speculation that the transition away from the use of lead, thanks to this research, may have contributed to the drop in the crime rate after peaking in the 1990s, though that's not proven.

While it was once thought that low levels of lead in the blood were acceptable, that's now come into question. The CDC used to track levels of blood poisoning by looking for children with 10 micrograms of lead per deciliter in their blood. Over the past few years, they've lowered that to 5 micrograms. In their data from 2014, the number of children tested with 10 micrograms or more was steady at about 0.5 percent. The number with 5 micrograms or more was much higher, at 4.2 percent.

"It's down to a point where we no longer use any number as a line between healthy and diseased," Rosner said. "In other words, we don't have any level below which  lead is safe."

A child suffering from  acute lead poisoning goes through a treatment called "chelation," in which an agent like EDTA is introduced into the bloodstream to bind to the lead and eliminate it from the body. Children are more susceptible to lead poisoning in part because they are still growing and in part because they are more likely to put their hands in their mouths after crawling through dust or dirt that was exposed to years of lead-filled car exhaust.

This is particularly true for children from lower-income homes, which overlaps with black and Latino children. Older homes that haven't been maintained are more likely to still have lead paint on the walls; the process of removing that paint is often cost-prohibitive. Homes closer to areas with a lot of traffic had more exposure to lead burned in cars.

Rosner's book deals with the political fight that surrounds lead's ubiquity. The extent to which lead exists around us and the extent to which it's a danger to children is a "real conundrum," he says. "Public health people often say it's a housing problem -- get rid of all the lead in the housing, in the pipes and the wall, and therefore it's a housing problem for [Housing and Urban Development] to deal with. HUD turns around and says it's a health problem; we can't replace all the housing in the country." Without spending a massive amount of money, we're stuck.

The problem in Flint is far worse and far more immediate, but also one that epitomizes the path we took to get to this point. The Flint River, Rosner says, was "the sewer for these industries." He added: "We're in some sense suffering from a century of blowback -- these children are suffering from a century of blowback from this industrial history that this community represented."

That thrust of that statement could be applied to children -- particularly poor children -- everywhere else in the country.