Declining IQ: The Impact of Lead Pollution on Cognitive Decline in Europe During the Roman Empire

Proceedings of the National Academy of Sciences, this research is based on ice cores taken in the Arctic to reconstruct the diffusion of this toxic metal. The results show that this pollution would have affected the cognitive abilities of millions of individuals in Europe, reducing the average intelligence quotient of the population by 2 to 3 points. These findings shed light on the lasting consequences of ancient industrialization, well before the modern era.

The omnipresence of lead: a toxic industrial by-product

Between the 1st century BC. BC and the 3rd century AD. BC, the Roman Empire experienced unprecedented economic and territorial expansion, marked by the large-scale industrialization of mining. At the heart of this activity was galena ore. This abundant resource rich in lead was used to extract silver. This process required it to be melted at high temperatures, releasing large quantities of lead into the air in particulate form. According to the study, this mining activity would have generated around 500,000 tonnes of atmospheric lead during the Pax Romana. This period of relative political stability spanned two centuries.

Ice cores taken in the Arctic bear witness to this ancient pollution. They reveal significant peaks of lead coinciding with periods of strong Roman industrial activity. The researchers cut the ice cores into millimeter sections corresponding to precise periods. By melting them, they released the particles they contained. Using mass spectrometers, they then analyzed metal concentrations and isotopes. Gas bubbles trapped in ice offer a glimpse into the atmosphere of past eras. Pollutants like lead can be used to interpret mining and industrial activity. The team then creates precise timelines using records of well-dated volcanic eruptions, which mark the ice records like postcards from the past.

Lead in society

This toxic by-product was obviously ignored by the health standards of the time. Especially since this pollution was invisible to the naked eye. It then spread over considerable distances. Lead particles affected all strata of Roman society because lead remained omnipresent in the daily lives of the Romans.

The elites, living in urban centers close to mines, were particularly exposed to particles. But also via lead pipes or kitchen utensils. It was also found in cosmetics, and even as an additive in wine to improve its preservation and taste. However, rural populations were not spared. Winds and precipitation dispersed these toxic particles, contaminating agricultural soils, water sources, and the air everyone breathed.

Consequences on cognitive health and collective intelligence

Lead is a powerful neurotoxicant whose deleterious effects on the nervous system are now well documented. In adults, high levels of lead exposure are linked to infertility, anemia, memory loss, cardiovascular disease, cancer, and reduced immune response. But even at low concentrations, lead can hinder cognitive development, especially in children. They are particularly vulnerable to it. The recent study, combining data from ice cores and modern epidemiological models, estimated that Roman children had an average blood lead level of 3.4 µg/dl. 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, they state that no level exposure to lead is not without risk.

By applying epidemiological models, researchers quantified the impact of this widespread exposure on the collective intelligence of Roman populations. They estimate that such a high blood lead level would have caused an average decrease of 2.5 to 3 points in intelligence quotient (IQ) across the entire population of the Empire. While this reduction may seem modest individually, it becomes significant on the scale of an entire society. It affects one's overall ability to solve complex problems, to innovate, and to maintain advanced social organization. Furthermore, working-class children living near the foundries could have suffered even greater cognitive deficits.

Corroborating historical and archaeological evidence

Strong archaeological evidence supports these conclusions. Isotopic analyzes carried out on Roman skeletons reveal lead levels well above modern levels considered safe. These levels often exceed 10 µg/dl in certain cases. A concentration associated today with severe effects on the nervous system and general health. Teeth, in particular, are an excellent marker of lead exposure. They retain traces of the metal accumulated during childhood, a critical period for cognitive development. Samples from archaeological sites in Italy, France and Spain show the globalization of this intoxication.

Andrew Wilson points out that events like epidemics (notably the Antonine plague, which killed around 10% of the population between 165 and 180 AD) and wars would have exacerbated exposure to lead. The disruption of water supply systems and public infrastructure could have increased dependence on contaminated resources. However, the magnitude of the cognitive effects remains debated. Caleb Finch, a specialist in the biology of aging at the University of Southern California, notes in an article in Science that the Roman Empire, despite these environmental challenges, maintained exceptional productivity and major technological advances. This suggests that cognitive impacts may have been concentrated in certain regions. Or they were diluted by factors like social resilience or differential access to polluted resources. Current debates show the complexity of accurately assessing the impact of such widespread intoxication in such a broad socio-economic context.

Environmental heritage and contemporary lessons

This research illustrates a crucial fact: industrial pollution, often perceived as a modern problem, dates back thousands of years. The Roman Empire generated levels of lead pollution so high that they were not equaled until the 20th century. These ancient emissions bear witness to the lasting impact of human activities on the environment, well before the industrial revolution. Lead particles released in Roman times, carried by winds thousands of kilometers, were deposited in Arctic ice, creating a lingering toxic footprint. Today, this historical data provides valuable insight into how human societies shape, sometimes irreversibly, their environments.

The parallel with the 20th century is striking. At the height of the use of leaded fuels in the 1970s, atmospheric lead concentrations were up to 40 times higher than during the Pax Romana. However, strict environmental policies, such as the gradual ban on leaded gasoline, reduced them. Blood lead levels in children fell from 15 µg/dl in 1976 to less than 1 µg/dl today in the United States. This decline illustrates the effectiveness of regulatory interventions. It also highlights the importance of environmental vigilance to prevent lasting health impacts. For Dr. Joseph McConnell, this study goes beyond the simple historical framework: “ It demonstrates that the impact of human activity on health is not a new thing, but a constant, highlighting the need to learn lessons from the past to shape a more sustainable future. “.

Source: JR McConnell, et al., “Pan-European atmospheric lead pollution, enhanced blood lead levels, and cognitive decline from Roman-era mining and smelting”, Proc. Natl. Acad. Sci. USA, 122 (3) e2419630121