Two thousand years ago, the Roman Empire hummed with activity. Roads stretched like arteries, connecting distant provinces. Mines yielded treasures buried deep in the earth, and smelters worked tirelessly to transform raw silver ore into the coins that fueled an empire. But hidden in this industrious age of antiquity was a poison that quietly shaped the fate of Rome: lead.

Lead air pollution, researchers now suggest, was so pervasive during the height of the Roman Empire that it may have lowered the average IQ of Europeans by 2.5 to 3 points. This startling finding was made after scientists uncovered evidence of lead’s toll by analyzing Greenland ice cores—ancient time capsules of Earth’s atmosphere.

“You built up this layer cake year after year of environmental history,” explained Joe McConnell, a climate and environmental scientist at the Desert Research Institute in Nevada and lead author of the study.

His team drilled deep into the Greenland ice sheet, extracting long cylinders of ice that revealed lead pollution peaks corresponding to Rome’s economic milestones.

A Heavy Toll on Mind and Body

To mint their coins, Romans melted lead-rich minerals like galena, releasing significant quantities of lead into the air. For every ounce of silver extracted, thousands of ounces of lead were emitted. This pollution, carried by atmospheric currents, left its mark even on the icy expanse of Greenland. “Virtually nobody escaped,” McConnell said.

While those living near mines in Iberia (modern Spain) bore the brunt, the study reveals that the effects of lead were felt across the empire. According to the research, more than 500 kilotons of lead were released into the atmosphere during the nearly 200-year height of the Roman Empire.

The researchers used modern data on lead exposure to estimate its impacts on Roman health. They found that Roman-era atmospheric lead levels were roughly twice what children in the U.S. are exposed to today—and one-third of the levels recorded in the late 1970s, before the phase-out of leaded gasoline.

Lead is a potent neurotoxin with no safe level of exposure. Even low concentrations are associated with cognitive decline, reproductive issues, and mental health problems.

Once ingested or inhaled, this toxic metal masquerades as calcium or other essential minerals, slipping past biological defenses and embedding itself in the brain. There, it begins its slow work of sabotage. For children, whose brains are in the midst of rapid growth and reorganization, the effects can be profound. Lead interferes with the formation of synapses, the tiny connections that allow neurons to communicate. It disrupts myelination, the process by which nerve fibers are insulated to speed up signals. It even tampers with neurotransmitters, the chemicals that carry messages from one neuron to another. The result is a brain that struggles to process information efficiently.

Behavioral changes emerge as well, compounding the problem. Children with higher lead exposure often struggle with attention deficits and emotional regulation, issues that further impede learning. These effects ripple outward, influencing academic performance and long-term success.

“An IQ reduction of 2 to 3 points doesn’t sound like much,” said study co-author Nathan Chellman, “but when you apply that to essentially the entire European population, it’s kind of a big deal.”

Lead wasn’t just in the air. The Romans extensively utilized lead in their daily lives, from plumbing systems—hence the term “plumbing,” derived from the Latin plumbum for lead—to cookware and wine sweeteners.

Historians have long debated the role of lead in Rome’s decline. The empire was already grappling with plagues, political instability, and economic struggles, but this study adds weight to the theory that lead pollution compounded its troubles. “I’m quite convinced lead was one of the factors that contributed to the decline of the Roman Empire,” said Dr. Bruce Lanphear, a professor of health sciences at Simon Fraser University, told NBC News. “But it was only one factor. It’s never just one thing.”

Lessons from History’s Lead-Laden Air

Rome didn’t invent ore mining nor did the practice ever really go away after the empire’s downfall, so one might expect that lead pollution (and its adverse effects) has been a constant threat for thousands of years. The advent of the Industrial Revolution marked a dramatic increase in lead emissions. Industrial activities, notably metal smelting and the burning of coal released substantial amounts of lead into the atmosphere compared to Rome’s heyday.

During the 20th century, leaded gasoline created an unprecedented spike in atmospheric lead, with pollution levels 40 times higher than those during the Roman Empire. Lead from automotive emissions settled into soil and water bodies, leading to elevated blood lead levels in populations worldwide. The Clean Air Act of 1970 significantly cut down on this exposure, but lead pollution is still a major problem across the world.

A 2021 study found that about half of children in developing countries are exposed to very high levels of lead. At least 1.5 million people die annually from cardiovascular diseases (like heart disease) caused by lead poisoning, imposing a global cost of about $6 trillion a year.

A study in New Zealand found that children with high blood lead levels had IQs 5.8 points lower than those with low blood lead levels.

“As lead pollution has declined during the last 30 years, it has become more and more apparent to epidemiologists and medical experts just how bad lead is for human development,” McConnell says.

The Roman case, however, underscores how industrial activity has long been intertwined with human health. “This is the earliest unambiguous example of human impacts on the environment,” McConnell added.

Rome’s lead-laden air may have dimmed the intellect of its citizens and contributed to its fall. Today, as we confront modern environmental challenges, history offers a sobering perspective—and a call to action.

The findings appeared in the Proceedings of the National Academy of Sciences.