Lead pollution under the Roman Empire would have lowered the IQ of Europeans

By analyzing ice cores collected in the Arctic, researchers found that exceptionally high levels of lead pollution during the height of the Roman Empire (27 BCE to 180 CE) likely led to to a drop in IQ of 2 to 3 points throughout Europe. These levels were up to 40 times higher than the historic peak of the 1970s and may even have contributed to the fall of the Empire.

The harmful effects of lead on human health and that of ecosystems are well documented. In adults, exposure to the metal is associated with many conditions, including infertility, anemia, cardiovascular disorders, brain disorders and certain cancers. In children, it is linked to a reduction in IQ and academic performance.

Following the adoption of Clean Air Act in the United States, banning the use of leaded gasoline, lead pollution has decreased significantly. However, although the US Centers for Disease Control and Prevention (CDC) considers a blood lead level of 3.5 µg/dL to be the threshold for medical intervention for children, no level higher than zero is risk-free. Even relatively limited exposure during early childhood could be associated with lower IQ and income decades later.

« As lead pollution has declined over the past 30 years, it has become increasingly clear to epidemiologists and medical experts how harmful lead is to human development. », Explains in a press release from the Desert Research Institute (DRI), Joseph McConnell, lead author of the new study.

Widespread exposure to lead over the past 150 years is primarily linked to the burning of coal and other fossil fuels. However, this harmful exposure began thousands of years ago, with evidence emerging in dental records from Roman times. As part of the new study, published in the journal PNASMcConnell and colleagues assessed the impacts of lead pollution on Europe and the Roman Empire. “ Lead is known to have many effects on human health, but we chose to focus on cognitive decline because it’s a phenomenon we can put a number on. “, explains Nathan Chellman of DRI, co-author of the study.

A decrease of 2.5 to 3 IQ points

To track levels of atmospheric lead pollution since Roman times, the DRI team analyzed three ice cores taken in the Arctic at depths of up to 3,400 meters. Quantification of pollutants, such as lead, can be used to infer the dynamics of mining and industrial activity over past periods.

The samples provided a precise timeline of atmospheric lead concentrations between 500 BCE and 600 CE, the period between the rise and fall of the Roman Empire. The team’s analyzes focused on the Pax Romanathe period (approximately 200 years) of the Empire’s height during which metal ore mining and processing operations were particularly intense. Computer modeling then made it possible to map the dynamics of atmospheric spread of lead across the whole of Europe and its impacts on the IQ of the population.

Analyzes revealed that atmospheric lead pollution began as early as the Iron Age and reached a peak at the end of the 2nd century BCE. It then declined suddenly during the 1st century BCE, during the crisis of the Roman Republic, before increasing again around the year 15 BCE, after the rise of the Roman Empire. It remained high until the period of the Antonine Plague between the year 165 and 180.

According to the team, nearly 500 kilotons of lead were released into the atmosphere during the Pax Romana40 times higher than during the peak in the 1970s. The population’s blood lead levels during this period would have been around 2.4 µg/dl, which may have reduced their IQ from 2.5 to 3 points. “ Although modest on an individual scale, a drop of 2 to 3 IQ points could have a significant impact when it concerns an entire population “, says Chellman. It is estimated that the Empire had a population of over 80 million at its peak, which would mean that around a quarter of the world’s population could have been exposed to lead pollution.

An “invisible” crisis that contributed to the fall of the Empire?

According to the researchers, doctors at the time were aware of the dangers of lead poisoning, but the metal was still widely used for pipes, cooking pots, toys, medicines and cosmetics. It was even ingested intentionally when the Romans discovered that it helped preserve and sweeten wine.

Furthermore, while ingestion through food primarily affected the upper classes, the entire population was exposed to air pollution from the Empire’s immense mining operations and smelter discharges. Lead was a byproduct of silver and gold refineries. The extraction of around thirty grams of silver released thousands of grams of lead, much of it being released into the atmosphere.

Experts suspect that the cognitive decline associated with this exposure may have contributed to the fall of the Empire, as this period was correlated with a major economic crisis in addition to the epidemic crisis. “ I leave it to epidemiologists, ancient historians, and archaeologists to determine whether the levels of lead air pollution and health effects we identified were enough to change history. », Tells McConnell to the Guardian. It was not until the Middle Ages that atmospheric lead levels began to rise again, notably due to the recovery of the economy, only to skyrocket during the 20th century with the industrial revolution and the use of fossil fuels.

Source : PNAS