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Lead pollution likely led to widespread declines in IQ during the ancient Roman era, a study has found.
The negative health effects of lead exposure in modernity have been widely recognized. Over the past 150 years or so, atmospheric lead pollution has largely resulted from burning fossil fuels, particularly the ubiquitous use of leaded gasoline starting in the 1920s—a practice that has now been phased out.
But while this might seem like a distinctly modern problem, historical and archaeological evidence—such as ancient texts and skeletal remains—indicate that people living across Roman territory thousands of years ago were subjected to significant levels of atmospheric lead exposure that potentially impacted human health.
Although there were a number of lead exposure routes in antiquity, including the use of glazed tableware, paints, cosmetics and intentional ingestion, the most significant source for the nonelite, rural majority of the population may have been through background air pollution arising from the mining and smelting of silver and lead ores.
These activities, which took place on a large scale, were a significant part of the ancient Roman economy.
"The Roman period, both the republic and the empire, was among the most important in the history of Western civilization," study lead author Joseph McConnell, with the Desert Research Institute in Reno, Nevada, told Newsweek. "Lead pollution from Roman-era silver mining and smelting arguably is the earliest unambiguous example of large-scale (continental to hemispheric) human impacts on the environment."
In the study published in Proceedings of the National Academy of Sciences, researchers used Arctic ice core records and atmospheric aerosol modeling to estimate the concentrations and potential health impact of lead pollution during the Roman era.
Ice core records are long cylinders of ice drilled from the thick ice sheets in polar regions. They essentially function as time capsules, preserving detailed information about Earth's climate and environment over hundreds of thousands of years.
The study authors analyzed three ice cores from the Arctic spanning the period 500 B.C. to A.D. 600. This revealed that European atmospheric lead concentrations increased sharply around 15 B. C.—following the rise of the Roman Empire—and remained relatively high until the decline of the Pax Romana, starting around A.D. 165.
The Pax Romana was a roughly 200-year period of relative peace, stability and prosperity across the Roman Empire, beginning with the rule of the first emperor Augustus in 27 B.C. and lasting until around A.D. 180.
The atmospheric lead pollution levels seen after 15 B.C. were not exceeded again until the early 2nd millennium during the High Middle Ages.
"An important aspect of this study is that we had some idea from archaeology of where the Roman-era mining and smelting sites were located," McConnell said. "Using leading-edge atmospheric modeling, we were then able to determine with reasonable accuracy how exposure levels varied around the empire. Note that archaeological and historical evidence suggests that the Roman Empire was the only large source of lead emissions in the Western Hemisphere during this period."
Based on modern epidemiological studies, the researchers estimated that these levels would have led to an increase in the quantity of lead in the blood of young children during the Roman era. Considering the size of this increase, the authors suggest that childhood lead exposure would have led to widespread cognitive declines of roughly 2.5-3 IQ points throughout the Empire.
To place this into context, the IQ declines that the study reports were about one-third of those seen in the U.S. during the late 1970s soon after the Clean Air Act was implemented, and about double those during the early 2010s.
Blood lead levels and health impacts near modern smelters are much greater nearby and drop off almost exponentially with distance from the smelters. And the same would have been true during the Roman era, according to the researchers.
"The bottom line in my view is that detailed ice core records of Arctic lead pollution, together with sophisticated atmospheric modeling and modern epidemiology, indicate that human industrial activities were measurably damaging human health more than 2,000 years ago," McConnell said.
Lead exposure has been associated with a range of health impacts, with even relatively low levels known to affect the cognitive development of children.
Immune system impairment has also been linked to low-level exposure. So it is "intriguing" that the destructive Antonine plague—the first great epidemic of the Roman Empire that killed an estimated 5-10 million people, or roughly 10 percent of the population—immediately followed roughly two centuries of highly elevated atmospheric lead emissions, the authors wrote in the study.
"While our findings suggest that lead poisoning from background air pollution was not acute especially compared to other exposure routes such as drinking water from lead pipes, lead table wares, cosmetics, and intentional ingestion, it was sufficient to reduce IQ throughout the empire in elites and non-elites in cities and rural areas alike," McConnell said.
"No one could escape exposure to, and the health effects of, air lead pollution. I leave it to epidemiologists and historians to assess the potential effects on history."
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Update 01/07/25, 8:01 a.m. ET: This article was updated with additional information and comment from Joseph McConnell.
Reference
McConnell, J. R., Chellman, N. J., Plach, A., Wensman, S. M., Plunkett, G., Stohl, A., Smith, N.-K., Vinther, B. M., Dahl-Jensen, D., Steffensen, J. P., Fritzsche, D., Camara-Brugger, S. O., McDonald, B. T., & Wilson, A. I. (2024). Pan-European atmospheric lead pollution, enhanced blood lead levels, and cognitive decline from Roman-era mining and smelting. PNAS, 121(0), e2419630121. https://doi.org/10.1073/pnas.2419630121
