About 2,000 years ago, the Roman Empire was booming. But something ominous was in the air. Literally.

Widespread lead pollution in the air took its toll on health and intelligence, researchers reported Monday in the journal Proceedings of the National Academy of Sciences.

During approximately two centuries starting from 27 BC. AD, a period of relative stability and prosperity known as the Pax Romana, the empire spread across Europe, the Middle East and North Africa. Its economy relied on the minting of silver coins, which required large-scale mining operations.

But mining silver from the Earth creates a lot of lead, said Joseph McConnell, an environmental scientist at the Desert Research Institute, a nonprofit group based in Nevada, and lead author of the new study. “If you produce an ounce of silver, you would produce about 10,000 ounces of lead.”

And lead has a number of negative effects on the human body. “There is no such thing as a 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 discovered lead in layers of ice collected in Russia and Greenland dating back to the time of the Roman Empire. The 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 hypothesized.

Lead levels measured by Dr. McConnell and his colleagues were remarkably low, about one lead-containing molecule per trillion water molecules. But the ice samples were collected thousands of miles from southern Europe, and lead concentrations would be highly dispersed after such a long journey.

To estimate the amount of lead originally emitted by Roman mining operations, the researchers worked backwards: 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 the second case, they assumed that silver mining was equally distributed over dozens of locations.

The team calculated that between 3,300 and 4,600 tons of lead were emitted into the atmosphere each year from Roman silver mining operations. The researchers then estimated how all that lead would be dispersed throughout the Roman Empire.

“We ran the model in a forward direction to see how these emissions would be distributed,” said Dr. McConnell.

With these atmospheric lead concentrations in hand, the researchers then used contemporary data to estimate how much lead would have entered the bloodstream of people in ancient Rome.

dr. McConnell and his colleagues focused on infants and children. Young people are especially sensitive to taking in lead from their environment by swallowing and inhaling it, said Dr. Bruce Lanphear, a public medicine physician at Simon Fraser University in British Columbia who was not involved in the research. “Pound by pound, children, especially infants, eat more and breathe more.”

In recent decades, blood lead levels in children have been linked to a number of indicators of physical and mental health, including IQ, said Dr. Cory-Slecht. “We have actual data on IQ scores in children with different blood lead concentrations.”

Using these contemporary relationships, dr. McConnell and his team estimated that children in most of the Roman Empire would have about 2 to 5 extra micrograms of lead per deciliter of blood. Such levels correspond to a drop in IQ of approximately 2 or 3 points.

By comparison, American children in the 1970s had an average increase in blood lead levels of about 15 micrograms more lead per deciliter of blood before leaded gasoline and lead paint were phased out. Their corresponding average drop in IQ was about 9 points.

But exposure to lead would have other negative effects on the Romans. Higher blood lead levels are also associated with higher rates of preterm birth and reduced cognitive functioning in old age. “It follows you all your life,” said dr. Lanphear.

Some scholars have hypothesized that lead poisoning played an important role in the fall 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 found that although the pipes used to distribute water in Rome had increased lead levels, it was unlikely that the water was actually harmful.

These new discoveries make sense, said Hugo Delile, a geoarchaeologist at France’s National Center 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. To McConnell, the research also gives dubious credit to Roman mining. “As far as I know, it’s the earliest example of widespread industrial pollution,” he said.