Lead Pollution Caused Widespread IQ Declines in Ancient Rome

DRI scientists used Arctic ice core records to reconstruct historic atmospheric lead pollution in Ancient Rome and link exposure to cognitive declines

Peer-Reviewed Publication

Desert Research Institute

 Press Release: January 6, 2025

Lead exposure is responsible for a range of human health impacts, with even relatively low levels impacting the cognitive development of children. DRI scientists have previously used atmospheric pollution records preserved in Arctic ice cores to identify periods of lead pollution throughout the Roman Empire, and now new research expands on this finding to identify how this pollution may have affected the European population.

The study, published Jan. 6th in Proceedings of the National Academy of Sciences (PNAS), examined three ice core records to identify lead pollution levels in the Arctic between 500 BCE through 600 CE. This era spans the rise of the Roman Republic through the fall of the Roman Empire, with the study focusing on the approximately 200-year height of the Empire called the Pax Romana. Lead isotopes allowed the research team to identify mining and smelting operations throughout Europe as the likely source of pollution during this period. Advanced computer modeling of atmospheric movement then produced maps of atmospheric lead pollution levels across Europe. Combined with research linking lead exposure to cognitive decline, the research team also identified likely reductions in IQ levels of at least 2 to 3 points among the European population.

“This is the first study to take a pollution record from an ice core and invert it to get atmospheric concentrations of pollution and then assess human impacts,” says Joe McConnell, research professor of hydrology at DRI and lead author of the study. “The idea that we can do this for 2,000 years ago is pretty novel and exciting.”

Records of the Past Preserved in Ice

McConnell’s Ice Core Laboratory at DRI has spent decades examining ice cores from places like Greenland and Antarctica, where sheets of ice have built up over millennia. Using enormous drills, they painstakingly extract columns of ice as much as 11,000 feet (3,400 meters) long, reaching more distant depths of Earth’s history with each inch. McConnell’s team creates precise timelines using records of well-dated volcanic eruptions, which stamp the ice record like postcards from the past. Gas bubbles trapped in the ice offer insight into the atmosphere of past eras, while pollutants like lead can be used to interpret mining and industrial activity.

McConnell began developing methods to create very detailed lead records in ice more than twenty years ago, when he applied them to more recent history. When archeologists and historians learned of this work, they approached him hoping to apply these new techniques to the Roman period, seeking answers to lingering historical questions. “The resulting research changed our understanding of the era by finding precise linkages between the lead pollution records and historical events such as population declines associated with periodic plagues and pandemics,” adds coauthor and ancient historian Andrew Wilson of Oxford University.

A Growing Understanding of the Harms of Lead Pollution

Ancient lead pollution stemmed largely from silver mining, whereby the lead-rich mineral galena was melted down to extract silver. For every ounce of silver obtained, this process produced thousands of ounces of lead—much of which was released to the atmosphere.

In the 20th century, lead pollution predominantly came from the emissions of vehicles burning leaded gasoline. Following the passing of the Clean Air Act in the U.S. in 1970, which restricted the use of leaded gasoline, researchers have tracked the sharp decline of lead in human blood. However, the nationwide exposure, particularly for children born between 1950 and 1985, allowed scientists to track lead’s impact on health and cognitive development.

“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.

In adults, high levels of lead exposure are linked to infertility, anemia, memory loss, cardiovascular disease, cancer, and reduced immune response, among other impacts. In children, even low levels of exposure have been connected to reduced IQ, concentration challenges, and reduced academic success. While the U.S. Centers for Disease Control and Prevention (CDC) considers a blood lead level of 3.5 µg/dl the point for medical intervention for children, they have stated that there is no level of lead exposure without risk.

“Lead is known to have a wide range of human health impacts, but we chose to focus on cognitive decline because it’s something we can put a number on,” says study coauthor Nathan Chellman, assistant research professor of snow and ice hydrology at DRI. “An IQ reduction of 2 to 3 points doesn’t sound like much, but when you apply that to essentially the entire European population, it’s kind of a big deal.”

The study found that atmospheric lead pollution began during the Iron Age and reached a peak during the late 2nd century BCE at the height of the Roman Republic. It then declined sharply during the 1st century BCE, during the crisis of the Roman Republic, before increasing around 15 BCE following the rise of the Roman Empire. Lead pollution remained high until the Antonine Plague from 165 to the 180s CE, which severely affected the Roman Empire. It wasn’t until the High Middle Ages in the early 2nd millennium CE that lead pollution in the Arctic exceeded the sustained high levels of the Roman Empire. According to the research, more than 500 kilotons of lead were released to the atmosphere during the nearly 200-year height of the Roman Empire.

Although ice core records show that Arctic lead pollution was up to 40-fold higher during the highest historical peak in the early 1970s, the insight gained from this study demonstrates how “humans have been impacting their health for thousands of years through industrial activity,” McConnell says.

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More information: The full study, Pan-European atmospheric lead pollution, enhanced blood lead levels, and cognitive decline from Roman-era mining and smelting, is available from Proceedings of the National Academy of Sciences (PNAS)

Study authors include: Joseph McConnell (DRI), Nathan Chellman (DRI), Andreas Plach (U. of Vienna), Sophia Wensman (DRI), Gill Plunkett (Queen’s University Belfast), Andreas Stohl (U. of Vienna), Nicole-Kristine Smith (DRI, Yale), Bo Møllesøe Vinther (U. of Copenhagen), Dorthe Dahl-Jensen (U. of Copenhagen, U. of Manitoba), Jørgen Peder Steffenson (U. of Copenhagen), Diedrich Fritzsche (Alfred Wegener Institute for Polar and Marine Research), Sandra Camara-Brugger (DRI, U. of Basel), Brandon McDonald (DRI, Tufts University), and Andrew Wilson (U. of Oxford).

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