Vandals sacking the city of Rome, effectively ending the power of the Roman Empire, 455 AD.
 (North Wind Picture Archives via AP Images)

How bad is lead for human health? The answer is not encouraging: The Centers for Disease Control and Prevention states flatly that no lead level is safe for children. As we've been reminded by the ongoing water crisis in Flint, Mich., lead can irreversibly harm brain development in children, causing learning disabilities, behavioral issues and other problems. At high levels, it can lead to kidney damage, seizures and even death.

But could lead poisoning bring down an entire empire? Some researchers have questioned whether it contributed to the fall of Rome.

Back in 1983, a Canadian research scientist, Jerome Nriagu, examined evidence of the diets of 30 Roman emperors and "usurpers" who reigned between 30 B.C. and 220 A.D. Nriagu concluded that 19 "had a predilection to the lead-tainted" food and wine popular then and probably suffered from lead poisoning, as well as a form of gout.

We're not talking about small amounts of lead. To sweeten their wines and other foods, the Romans  would boil down grapes into a variety of syrups, all of which had one thing in common, according to Nriagu's article in the New England Journal of Medicine: They were simmered slowly in lead pots or lead-lined copper kettles.

When the recipes were tested in modern days, they produced syrups with lead concentrations of 240 to 1000 milligrams per liter. "One teaspoon (5ml) of such syrup would have been more than enough to cause chronic lead poisoning," Nriagu wrote.

Given the gluttonous habits of Roman aristocrats, it would be no surprise if they showed the impact of lead in their diets, Nriagu believed. Here's how he described "the dull-witted and absent-minded Claudius," whom he considered most likely to have suffered lead poisoning: "He had disturbed speech, weak limbs, an ungainly gait, tremor, fits of excessive and inappropriate laughter and unseemly anger, and he often slobbered." However, the researcher admitted that the cause of these maladies was "a matter of longstanding debate."

Indeed, his own lead-poisoning theory was quickly, even vehemently disputed. In 1984, for example, classicist John Scarborough authored "The Myth of Lead Poisoning Among the Romans: An Essay Review" and tore Nriagu's argument apart.

Scarborough wrote that Nriagu's basic premise couldn't be trusted because of sloppy work. He also concluded that the Romans were aware of the harm lead could cause, that lead poisoning wasn't endemic in their society and that Rome did not fall because of it.

In an interview Wednesday, Nriagu stood by his work. The professor emeritus of environmental health sciences at the University of Michigan said that "Scarborough knows nothing, absolutely nothing, about lead poisoning. Absolutely zero."

Still others followed up on the question. Three decades after Nriagu's paper, a team of archaeologists and scientists examined how lead pipes contaminated ancient Roman "tap water." By measuring lead isotopes in the sediment of the Tiber River and Trajanic Harbor, they estimated that the piped water probably contained 100 times as much lead as local spring water.

Yet French researcher Hugo Delile and his team, reporting in the Proceedings of the National Academy of Sciences in 2014, concluded that such concentrations were "unlikely to have been truly harmful." The group also claimed that enough criticism of Nriagu's theory had been amassed over three decades to largely debunk it.

"Lead is no longer seen as the prime culprit of Rome’s demise," Delile wrote.

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