Roughly 2,000 years ago, the Roman Empire was flourishing. But something sinister was in the air. Literally.
Widespread pollution in the form of airborne lead was taking a toll on health and intelligence, researchers reported on Monday in the journal Proceedings of the National Academy of Sciences.
During the roughly two centuries starting in 27 B.C., a period of relative stability and prosperity known as the Pax Romana, the empire extended throughout Europe, the Middle East and North Africa. Its economy relied on silver coinage, which required huge mining operations.
But extracting silver from the Earth creates a whole lot of lead, said Joseph McConnell, an environmental scientist at the Desert Research Institute, a nonprofit group based in Nevada, and the lead author of the new research. “If you produce an ounce of silver, you’d have produced something like 10,000 ounces of lead.”
And lead has a host of negative effects on the human body. “There is no such thing as any safe level of lead exposure,” said Deborah Cory-Slechta, a neurotoxicologist at the University of Rochester Medical Center who was not involved in the research.
Dr. McConnell and his colleagues have now detected lead in layers of ice collected in Russia and Greenland that date to the time of the Roman Empire. Lead entered the atmosphere from Roman mining operations, hitched a ride on air currents and eventually fell out of the atmosphere as snow in the Arctic, the team surmised.
The levels of lead that Dr. McConnell and his collaborators measured were extremely low, roughly one lead-containing molecule per trillion molecules of water. But the ice samples were collected thousands of miles from southern Europe, and lead concentrations would have been highly dispersed after such a long journey.
In order to estimate the amount of lead originally emitted by Roman mining operations, the researchers worked backward: Using powerful computer models of the planet’s atmosphere and making assumptions about the location of the mining sites, the team varied the amount of lead emitted to match the concentrations they measured in the ice. In one case, they assumed that all silver production took place at a historically important mining site in southwestern Spain known as Rio Tinto. In another case, they presumed that silver mining was equally spread out across dozens of sites.
The team calculated that anywhere from 3,300 to 4,600 tons of lead were being emitted into the atmosphere each year by Roman silver-mining operations. The researchers then estimated how all that lead would be scattered across the Roman Empire.
“We ran the model in the forward direction to see how those emissions would be distributed,” Dr. McConnell said.
With those atmospheric-lead concentrations in hand, the researchers next used modern-day data to estimate how much lead would have entered the bloodstreams of people in ancient Rome.
Dr. McConnell and his colleagues focused on infants and children. Young people are particularly susceptible to taking up lead from their environment via ingestion and inhalation, said Dr. Bruce Lanphear, a public heath physician at Simon Fraser University in British Columbia who was not involved in the research. “Pound for pound, children, particularly infants, eat more and breathe more.”
In recent decades, lead levels in children’s blood have been correlated with a slew of physical and mental health metrics, including I.Q., Dr. Cory-Slechta said. “We have actual data on I.Q. scores in kids with different blood-lead concentrations.”
Using those modern-day relationships, Dr. McConnell and his team estimated that children across much of the Roman Empire would have had around 2 to 5 additional micrograms of lead, per deciliter of blood. Such levels correspond to I.Q. declines of roughly 2 or 3 points.
For comparison, American children in the 1970s had average blood-lead-level enhancements of around 15 micrograms more lead per deciliter of blood before the phasing out of leaded gasoline and leaded paints. Their corresponding average I.Q. decline was about 9 points.
But lead exposure would have had other negative effects on Romans as well. Higher levels of lead in the blood have also been linked to higher incidences of preterm births and reduced cognitive functioning in old age. “It follows you throughout life,” Dr. Lanphear said.
Some scholars have hypothesized that lead poisoning played an important role in the decline of the Roman Empire. But that idea has been called into question, at least when it comes to water contaminated by lead pipes. A 2014 study showed that, while the pipes used to distribute water in Rome increased lead levels, the water was unlikely to be truly harmful.
These new findings make sense, said Hugo Delile, a geoarchaeologist at the French National Centre for Scientific Research, who was not involved in the research. “They confirm the extent of lead pollution resulting from Roman mining and metallurgical activities.”
According to Dr. McConnell, the research also confers a dubious honor on Roman mining. “To my knowledge, it’s the earliest example of widespread industrial pollution,” he said.